All electrical systems have one thing in common—they must be properly connected. Schematic diagrams and wiring plans are needed to properly connect and maintain electrical systems. As an electrician, you should be able to identify and understand the common wiring terms and symbols used in these diagrams and plans. Although most of the branch circuits discussed in this study unit are based on residential use, they may also apply to many commercial and industrial applications.
When you complete this study unit, you’ll be able to · Differentiate between feeder and branch circuits
· Identify the correct type of general or special-purpose circuit when given a list of circuit descriptions
· Describe how wiring is installed for branch circuits in a residence under particular situations · Differentiate between portable, fixed, and stationary appliances and describe how each
type is wired
· Identify the components needed for an electrical circuit · Calculate the current in a neutral conductor
· Calculate the size of service-entrance conductor needed for a residence
Preview
ELECTRICAL SYSTEM FUNDAMENTALS . . . 1
Parts of an Electrical System Electrical Circuits
Types of Circuits
INSTALLING SERVICE-ENTRANCE COMPONENTS . . . 16
Sizing and Installing Service-Entrance Conductors Installing the Meter Base
Sizing and Mounting the Service Panel
Grounding and Bonding of Service and Equipment Subpanels
LOCATING RESIDENTIAL DEVICES . . . 40
Wiring Layout for a Small House Location of Receptacles Location of Switches
Location of Lighting Outlets Listing of Residential Outlets Area Requirements
GENERAL CIRCUIT WIRING CONSIDERATIONS . . . 60
General-Purpose Circuits Small-Appliance Circuits Special-Purpose Circuits Circuit Protection Circuit Grounding Other Circuits SELF-CHECK ANSWERS . . . 83 EXAMINATION . . . 85
Contents
ELECTRICAL SYSTEM FUNDAMENTALS
An electrical system may be compared to a tree. As the tree roots support the tree trunk and branches, the electrical system, or service, supports the service-entrance equipment and branch circuits. The type of electrical service is determined by the public utility supplying the electric power. Most electric utilities produce and distribute alternating current (AC) power to their customers. (The distribution of direct current or DC power by electric utilities has largely stopped.) While an industrial maintenance electrician may still have to work on DC circuits, the vast majority of residential circuits are AC powered. Therefore, this study unit will deal mainly with AC power.
The typical residential service is 120/240 V (volts) single-phase AC power. The slash between 120 and 240 means that both voltages are available to the customer. The service-entrance equipment brings the electric power into the building and then controls it before distributing it to the branch circuits. The branch circuits are the circuits that supply the loads. The electrical system within a building consists of many components such as wires, fuses, circuit breakers, switches, and lamps or other loads.
Figure 1shows an electrical system for a residence. Industrial and commercial systems use similar components in the same way. For convenience, the explanations given in this study unit will be for a residence, although the same rules apply to the complex wiring system of a large industrial plant.
Parts of an Electrical System
Service equipment and wiring circuits can best be explained by studying the wiring layout inFigure 1. Let’s start by looking at the service entrance. It includes the service drop, service-entrance cables, watt-hour meter, service-disconnecting means, and grounding conductor. Let’s look at each individual part of the service entrance shown inFigure 1A.
· Service Drop. The utility company installs the service drop. Service-drop conductors are shown coming from a utility pole to the first attachment point on the building. Should the conductors come from either a manhole or a pad-mounted transformer to the building, they’re referred to as service lateral conductors.Figure 1Bshows a typical underground-service lateral system. The number of conductors depends on the type of distribution system.
Wiring Electrical Circuits
· Service-entrance cable. The service-entrance cable continues from the point of attachment to the building through the watt-hour
meter. Underground service might not require that you install service-entrance conductors or cables. The reason is the service lateral conductors, which run from the street mains (main electrical distribution lines) to the building, are terminated by the Power Company directly at the meter base.
· Watt-hour meter. The watt-hour meter is the device that measures the energy used by the consumer. The utility company generally specifies its location. The meter is installed between the service drop or service lateral and the service-disconnecting means.
· Service-disconnecting means. The service-disconnecting means is shown inFigure 1A. In this case, the disconnecting means is a circuit breaker. Switches or fuses may also be used as disconnecting means. The service-disconnecting device must be manually operated. It must provide a visual means to tell whether it’s in the open or closed position. In devices where a circuit breaker handle operates vertically, the UP position shall be the ON position. The service-disconnecting device must be mounted in an accessible location near the service entrance. A service-overcurrent device may also be contained within the service-disconnecting device. Each ungrounded service-entrance conductor must have either a fuse or circuit breaker in series with it. This prevents excessive current draw caused by faults, overload of the building’s wiring, or an excessive supply of electrical power delivered to the service entrance.
FIGURE 1B—This drawing shows an alternative (underground) service lateral configuration. Remember that a system also includes loads, like the range and lamp, as well as devices such as the receptacle, light switch, lamp holder, and circuit breakers.
· Grounding conductor. The grounding conductor is a low-resistance conductor connected between the ground and the identified neutral wire of the alternating current service conductors or the housing for the disconnecting means or both. In a typical installation, the neutral wire is identified by its white or natural gray insulation and the grounding conductor is either a bare conductor or green in color. Let’s summarize what’s just been described about the service entrance. It begins at the junction of the service drop and service-entrance cable; it ends where the grounding conductor is grounded. Included in the service entrance are the insulators and other materials used to support the building end of the service drop. Service-entrance installations are covered in detail in other texts.
To understand how power is fed into the home, let’s look at the right side of the diagram inFigure 1A.
· Feeder cable. Feeder conductors extend the electrical system. The feeder(s) is/are a cable or conductors between the load side of the service disconnect and the branch circuit panelboard. Generally, no loads are connected between those two points.
· Distribution panel board. The branch circuits are tapped off of the feeders at the distribution panel board. Such panels contain fuses or circuit breakers that protect the branch circuits. Although separate distribution panel boards are shown inFigure 1, such a center may be enclosed in the same cabinet as the service-disconnecting means. Circuit breakers are shown for protecting the branch circuits. If fuses were used as service-overcurrent devices, then the branch circuits would normally also be protected by fuses. However, both fuses and circuit breakers may be used in the same installation. · Subpanel feeder cable. The subpanel feeder cable is an extension of the
main feeder. The conductors in this cable must be protected by a circuit breaker or fuses in the main distribution panel. In Figure 1, it’s shown as a cable between the two distribution panel boards.
· Branch circuit cables.Figure 1Ashows only four branch circuit
ca-bles extending from the distribution panel boards. This means that
only four of the 15 branch circuits are supplying loads. One of the eight breakers shown in the first panel is protecting the subpanel feeders. These four branch circuits are 120-V circuits that are sup-plying a duplex receptacle, or convenience outlet, and a lamp that’s controlled by a switch. Although only single loads are shown for each circuit, the typical circuit would supply more than one load or device of the same type, such as one circuit that supplies current to several lamps in an area of the building.
The electric range uses a separate circuit breaker within the equipment panel board. In this example, the range circuit is a 240-V circuit that’s protected by a two-pole circuit breaker. In this study unit, you’ll learn about the many types of branch circuits found in a modern residence, and how to identify and install the required circuit components.
Electrical Circuits
Definition of a Circuit
According to the NEC, the term electrical equipment means any material, fitting, device, appliance, or apparatus used as part of, or in connection with, an electrical installation. The words equipment and component are used interchangeably because each piece of equipment is a component of the electrical installation. If electrical components are connected together properly, they form an electrical circuit. Simply put, an electrical circuit is the complete path followed by electric current.
A branch circuit consists of the conductors and devices installed between the branch-circuit protection device (breaker or fuse) and the receptacle or connection point of the load. Once an appliance or load is either plugged into or connected to the receptacle or connection point, those components used to plug in or connect to the branch circuit become part of the circuit. These components are sometimes referred to as the “load circuit,” although the electrical trade doesn’t recognize this phrase in defining circuits.
Circuit Components
Figure 2shows the five basic types of components in a simple electrical circuit. All electrical circuits include, at the very least, the following three components—the power source, load, and conductors that join them. InFigure 2, a transformer is used as a power source and conductors join the transformer to a lamp. This is the electrical load or destination. Even without the fuse and switch, a complete circuit would still exist. The power source may originate from a panel board, transformer, battery, or generator. Most circuits contain at least one or more electrical devices such as switches, which are control devices, or fuses, which are protective devices. Many circuits also have circuit breakers, devices that both protect and control the circuit. A receptacle is a convenience device, used with a mating plug. A plug makes it easy to connect a conductor between the power source and load.
Types of Circuits
Electric circuits have names such as series circuits, branch circuits, 120-V circuits, high-voltage circuits, and remote-control circuits. The different names can be very confusing, especially when the same circuit is called by more than one name. Often, two or more names are combined to give a complete and accurate description of the circuit.
The following list shows how circuits are described in terms of their characteristics. FUSE CONDUCTORS SOURCE LOAD L2 L1 120V SWITCH
FIGURE 2—A simple circuit will always contain at least three of the five different components shown here—the power source, load, and conductors.
Circuit Characteristics Common Descriptive Terms
Wiring connection Series, parallel, compound, delta, wye Number of wires Two-wire, three-wire, four-wire
Number of phases Single-phase, polyphase, three-phase
Type of grounding Grounded neutral, ungrounded neutral, with ground
Electrical variables High-voltage, low-voltage, high-ampacity, wattage, 60-Hz (hertz) frequency, low-capacitance, high-impedance
Circuits can also be classed in terms of the type of customer, area served, section of the total installation, type of load, function, and even the method used to control them. The following list shows common descriptive terms for these classes.
Often there’s no clear-cut category for a circuit. The name of the circuit used at any one time depends on the circuit characteristics being discussed at that time.
Series and Parallel Circuits
When several parts or devices are used in a wiring system, their circuits may be interconnected in various ways. The two basic connections of electric circuits are series and parallel.
In a series circuit, all parts are connected end-to-end, like the links in a chain. This connection forms a closed-path circuit as inFigure 3A. A basic rule for a series circuit is that the same current flows through each part. If 10 A (amperes) flows through the switch, then 10 A will flow through the fuse and through each of the heaters. However, the voltage drop across each part in a series circuit is different and depends on the resistance of that part and the amount of current flowing through it. In the parallel circuit shown inFigure 3B, two or more parts are connected across the same voltage source. The two heaters and fan are connected in parallel or in shunt with each other and the power source. The basic rule for a parallel circuit is that the same voltage exists across each of the parallel-connected parts. The current branches off and a portion flows through each of the parallel branches. The total of the currents in the branches if added will be equal to the total current if measured at the source. This means that the higher the resistance of a branch, the lower the current through that branch. If a series circuit is broken (or opened) at one point, the entire circuit’s dead. No current flows in any part of it. If one of the branches in a parallel circuit is disconnected (or opened), the current in the other branches continues to flow. Therefore, an open circuit in one branch of a parallel circuit doesn’t stop current flow in other branches.
Classes of Circuits Common Descriptive Terms
Customer type Residential, commercial, industrial
Area served Hazardous, outdoor, weatherproof, raintight Section of installation Service entrance, feeder, branch
Type of load General-purpose, lighting, small appliance Function Power, communications, alarm, control Method of control Manual, automatic, remote control
Combination Circuits
Parts of a wiring system may be connected in a combination of series and parallel circuits.Figure 4shows two diagrams of a heater circuit. In the schematic view, the heater and switch are connected in series with each other. The fan motor is connected in parallel with them. The circuit has two parallel branches. One branch has only one part, the fan motor; the other branch contains two parts in series, the heater and the switch. In this heater diagram, the switch controls only the heating element, not the fan. This is a series-parallel combination, or compound circuit.
FIGURE 3—When a series circuit is opened, no current flows. In a parallel circuit, one branch can be opened, and current will still flow through the others.
M FAN MOTOR WHITE COMMON WIRE RED SWITCH BLACK HEATER WHITE SOURCE TERMINALS BLACK SOURCE TERMINALS WHITE WHITE WHITE RED FAN SWITCH BLACK BLACK HEATER LINE CORD MOTOR
The pictorial diagram inFigure 4labels the colors of the wires. Note that one side of each branch has a white wire. These white wires are connected to the common white wire, which leads to a source terminal. The red wire from the switch and the red wire from the fan motor are connected to the black wire. The black wire then leads to the other source terminal. Remember that inFigure 4, the common wire is white and all white wires are connected together.
Remember that the same voltage exists across each branch of a parallel circuit. For that reason, electrical loads are designed to operate at certain standard voltages. All loads in a typical residence that require alternating current operate at either 120 or 240 V. Industrial and some commercial equipment are designed to operate at these or higher standard voltages. No matter what the voltage is, all loads will be connected in parallel with the voltage source.
Electrical devices that protect and control an entire circuit or branch of a circuit will be connected in series with the portion of the circuit wiring or the load that the devices protect or control. For that reason, electrical devices such as fuses, circuit breakers, and switches are rated based on the amount of current they must handle.
Circuit Variables
Most circuits you’ll be dealing with are parallel circuits. These circuits have the same voltage for all the parallel loads. Suppose you’re going to connect a new electrical load in a parallel circuit. You’ll need to know the circuit’s operating voltage and the branch circuit conductor’s current-carrying capacity, or ampacity. Suppose the existing circuit is rated at 120 V and 20 amperes; the new load will be connected to the existing circuit to put it in parallel with the circuit. It’s critical that the sum of the currents of all the parallel loads, including the newly added load, doesn’t exceed 20 amperes.
Two- and Three-Wire Circuits
The circuits discussed so far have been two-wire circuits consisting of an ungrounded hot wire and a grounded neutral wire.Figure 5shows that two such circuits running near each other have a total of four wires. Note that lines a and b are hot (ungrounded) conductors. The remaining lines n are the neutrals. These neutrals are connected together at the neutral bus in the panel board. If lines a and b are supplied by opposite sides of the service entrance, the current will flow as shown by the arrows. “Opposite sides of the service entrance” means that if line a is supplied by the black service-entrance wire, line b will be supplied by the red service-entrance wire. In this configuration, the voltage between lines a and n (or b and n) is 120 V, while the voltage between lines a and b is 240 V.
Since the neutral conductors are connected together at the panel board, they’re one-wire. Why use two wires when one will do? It’s important to note that a three-wire 120/240-V circuit will do the work of two, two-wire 120-V circuits. However, opposite phase conductors sometimes share the same neutral as in the three-wire (multi-wire) circuit shown in Figure 5. If the neutral is disconnected or interrupted, the circuit would become a series circuit with a nominal voltage of 240 V! The load, if not rated for the higher voltage, would most likely be damaged or destroyed by the higher voltage. For this reason, it’s very important to be cautious in disconnecting neutral conductors in a panel.
Assume that equal loads of 20 A each are present on the multi-wire circuit (which in this case is a three-wire circuit) inFigure 5. Then 20 A flows in lines a and b, but the two arrows on the neutral n are in opposite directions. Thus, the currents cancel and no current flows in the neutral. The result is no voltage drop in the neutral and less voltage drop in each circuit.
When the currents are unequal there’s less current in the neutral than in either hot conductor. As an example, if line a is carrying 20 A and line b is carrying 15 A, then the neutral n is carrying 5 A (20 – 15 = 5 A).
Although the neutral current now is no longer zero, it’s still much less than either line current. This example remains there even when the sin-gle loads inFigure 5are replaced by several smaller loads located where needed. However, this is true only if their current is drawn equally from each of the two sides of the circuit.
FIGURE 5—This shows examples of two- and three-wire circuits.
Grounded Circuits
You’ll remember that the neutral wire in a two-wire system is grounded or connected to the earth. When the neutral wire is connected to the earth, it’s a grounded conductor because it carries current during normal circuit operation. Don’t confuse the grounded conductor with the grounding conductor. The grounded conductor is part of the current-carrying electrical circuit whose function is to provide a circuit path and stability to the level of voltage. A grounding conductor, on the other hand, isn’t a current-carrying conductor. Its function is to provide safety and protection to both personnel and equipment with a low impedance path to ground in case of a short in the electrical system.
The grounded conductor or neutral wire must be kept continuous. In residential wiring, the neutral wire is never interrupted by a fuse, circuit breaker, switch, or other device. In industrial wiring, the neutral wire may be interrupted, but only if the ungrounded wires and the neutral wire are interrupted at the same time.
Circuit Description by Load Type
A circuit is often named after or described by the equipment (or load) to which it delivers power. Here’s an example. Some residential circuits are called small-appliance, general-purpose, electric range, and electric dryer circuits. Each of these circuits has its own basic characteristics. Given the type of load, an experienced electrician could identify many of the circuit characteristics. The electrician would know such features as voltage, ampacity, number of phases, and number and size of conductors. Industrial branch circuits aren’t as standardized as residential circuits. However, given the load (such as a motor or lighting equipment), many of the circuit characteristics and much of the circuit equipment can often be determined.
Circuit Description by Function
So far, you’ve learned about the standard types of circuits used in electrical wiring systems. There are, however, many special circuits and auxiliary circuits. These circuits are also described by the function they serve or by the method by which they’re controlled. These circuits may include emergency power, hazardous-area, alarm, communication, and control circuits. The following briefly describes the first four of these circuits. Control circuits will be discussed in a later section.
Emergency power can be distributed in one of two ways. Either the emergency power source, such as a diesel-powered generator or battery supply, can be switched directly into the main feeder and branch circuits, or the emergency power system may be equipped with its own separate feeders and branch circuits.
Another type of circuit is the hazardous-area circuit. It’s unique in that the circuit requires special explosion-proof devices and fittings. The
electrical loads on these circuits, such as motors and lamps, must be of special construction.
Other unique circuits include alarm circuits, doorbell or chime circuits, fire alarm circuits, and control circuits. Some of these circuits operate on low voltages such as 12 V, 16 V, or 24 V.Figure 6shows how a door chime circuit might be connected. The transformer shown inFigure 6 could be replaced by a battery in an emergency situation without affecting the chime’s operation. Control circuits are often found in industrial facilities and are used to transfer or transmit electrical control signals from one location to another.
Control Circuits
Control circuits are the next most common circuits an electrician has to
work on beside branch and feeder circuits. That’s why you, the electrician, must have a thorough understanding of basic electricity and be able to read and understand control drawings. Control circuits are commonly used to regulate or control the supply of electrical power to a load. They may either switch the power on or off or may adjust the power to a desired level. Some control circuits are very simple while others are quite complex.
Many major home appliances and a majority of the electrical equipment in an industrial setting have control circuits.Figure 7shows a simplified schematic for a home air conditioner. The compressor and fan motor are the two obvious main loads in the circuit. The other circuit devices—switch, temperature control, and the two capacitors—are control devices. These devices determine when and how long the electrical loads are operated.
TRANSFORMER TWO-DOORCHIME
FRONT-DOOR SWITCH BACK-DOOR SWITCH T L S-1 S-2
BLOCK DIAGRAM FOR A TWO-DOOR CHIME CIRCUIT
FRONT BACK
FIGURE 6—The two-door chime can be wired to sound a double note for the front door and a single note for the back door.
In the control drawing for the air conditioner, the heavy black dots in the mode switch indicate which connections are energized for the various modes of operation. In the HI FAN position, notice that line terminal L is connected only to terminal 1, which goes to the fan motor.
Review
Circuits may be referred to by different names but all circuits are basically common to one another in that each circuit or group of circuits has three components—a source of power, conductors, and an electrical load. Most circuits will also likely contain protective and control devices. As you gain more experience as an electrician, you should be able to describe the types of circuits in this study unit and their characteristics.
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Self-Check 1
At the end of each section of Wiring Electrical Circuits, you’ll be asked to check your under-standing of what you’ve just read by completing a “Self-Check.” Writing the answers to these questions will help you review what you’ve learned so far. Please complete Self-Check 1 now.
1. The conductors that run in the air from a utility pole to the first point of attachment on a building are called the ________ ________.
2. The two components inFigure 2that provide the necessary circuit protection and control are the fuse and the _______.
3. Protective and control devices are connected in _______ with the load. 4. Electricity is distributed by most electric utilities as _______ current.
5. In a three-wire 120/240-V circuit, if the current in line a is 17 A and the current in line b is 8 A, the current in the neutral is _______ A.
6. A _______ conductor is connected to the earth and doesn’t carry current during normal operation.
7. When you call a circuit an electric heater circuit, or a motor circuit, you’re referring to it by the type of _______ it supplies.
8. Name two types of special or auxiliary circuits.
_______________________________________________________________________________ 9. If you’re installing an electrical system in a hazardous area, you must be sure to use
_______ devices and fittings.
INSTALLING SERVICE-ENTRANCE COMPONENTS
Now that you’ve reviewed circuit theory and components let’s consider placing and sizing the actual components that make up a typical electrical system. Keep in mind that the typical residential electrical system includes service drop or lateral feed, service-entrance conductors, weather head, watt-hour meter, service panel, grounding electrode, grounding conductor, feeders, branch circuits, and various devices. You’ve also learned how to route conductors for branch circuits and how to select the various electrical devices most commonly installed. Later in this study unit, you’ll learn to properly design branch circuits and specify the correct number of devices for each part of the dwelling. First, however, electricians should understand how the service-entrance components are sized and installed. This section provides the information needed to properly lay out and install a residential electrical system that’s safe, convenient, and code compliant.
Sizing and Installing Service-Entrance Conductors
As learned earlier, the service-entrance conductors provide the path by which power moves from the service drop, to the watt-hour meter, and to the service-entrance panel. These conductors are sometimes part of the service-entrance cable (SE cable). In other installations, they’re individual conductors that run inside conduit. Electricians should follow standard wire-ampacity guidelines to size the service-entrance conductor to match the maximum-amperage rating of the service. For instance, if the main disconnecting switch and panel board are sized for 200 A, then 2/0 or 3/0 copper wire may be used for the service-entrance conductor. In this particular case, while the NEC specifies that a 2/0 copper wire has a maximum ampacity of 200 amps, the electrician may install the next largest conductor size for safety and future additions.
However, the local code may dictate that the service load be calculated using established methods. One such method is the optional calculations for dwelling units described in Article 220-30 of the NEC. The following steps show how to compute the estimated load on the hot and neutral service-entrance conductors using this method. A sample calculation will follow.
Follow Steps 1 through 6 to calculate the service conductor load. Step 1: Calculate the volt-ampere (VA) load for the general lighting
and receptacles. Find this by multiplying the total square footage of the building by 3 VA as stated in Article 220-30(b)(2) of the
NEC. (Note that to convert VA to kVA you divide by 1000.)
Step 2: Calculate the volt-ampere loads of the kitchen and laundry branch circuits by adding together the total number of 2-wire, 20-ampere, small-appliance branch circuits in the kitchen and each laundry branch circuit. Multiply this number by 1500 VA as discussed in Article 220-30(b)(1) of the NEC. Save this result for a later calculation.
Step 3: Add together all the volt-ampere load ratings (stamped on nameplates) of appliances that are secured in place (except air-conditioning and heating units) as discussed in Article 220-30(b)(3) of the NEC.
Step 4: Calculate the total volt-ampere demand load from the last three steps by applying the demand factors listed in Table 220-30 in the NEC to the calculated total.
Step 5: Find the larger of the heating or air-conditioning load rating (not both) and apply the demand factors as listed in Article 220-30(c). Air-conditioning or heat pumps are calculated at 100%, while space heating is calculated at 65% for three or less units, and 40% for four or more units. Add this number to the total demand load found in Step 4.
Step 6: Divide the total found in Step 5 by the system voltage, usually 240 volts. The result is the amperage rating for the service conductors as covered in the NEC Article 310-16.
Follow Steps 7 through 8 to calculate the neutral conductor load.
Step 7: Add the general load found in the first step with the kitchen and laundry loads in the second step. Begin the neutral con-ductor calculation by counting the first 3000 VA at 100%, or 3000VA. Combine this with 35% of the remaining VA from steps 1 and 2. Now add 100% of the dishwasher VA, and 70% of the range and the dryer VA.
Step 8: Divide this total by the system voltage (240 volts) and the answer is the amperage. According to Article 310-16 of the NEC, you’ll use this number to determine the required neutral conductor type and size for this residence.
Example: Find the total estimated load on the hot and neutral
load service-entrance conductors for a dwelling with a total of 2500 square feet. The dwelling contains a 3 kVA or 3000 VA water heater, 1.5 kVA or a 1500 VA dishwasher, and a 5 kVA or 5000 VA air conditioner. It also contains four or more combined space heaters of 15 kVA or 15000 VA, a 5.5 kVA or 5500 VA dryer, and a 12 kVA or 12000 VA range.
Step 1: Calculate the lighting and receptacle load. 2500 sq ft ´ 3 VA/sq ft = 7500 VA (7.5 kVA)
Step 2: Add together the two NEC-required small-appliance kitchen circuits, and one laundry branch circuit.
3 ´ 1500 VA = 4500 VA (4.5 kVA)
Step 3: Determine rating total of all secured appliances except heating and air conditioning.
Range 12000 V
dryer 5500 VA
water heater 3000 VA
dishwasher 1500 VA
Total 22000 VA
Step 4: Total the figures from Steps 1 through 3. 7500 + 4500 + 22000 = 34000 VA
Apply this to the demand factors based on the optional method found in the NEC.
100% of the first 10 kVA 10,000
40% of the remaining 24 kVA (34000 – 10000 = 24000) 24000 ´ .40 = 9600
Total: 10000 VA + 9600 VA = 19600 VA
Step 5: Add 40% of the larger of the two heating and air conditioning loads to the total from Step 4.
Total from Step 4 19600 VA
40 % of the heating (15000 ´ .4) +6000 VA
Total 25600 VA
Step 6: Divide the total in Step 5 by the provided voltage. 25600 VA ÷ 240 V = 106.6 A
Keep in mind that conductors should be sized so that the esti-mated amperage load doesn’t exceed 85% of the conductor’s rated capacity. For a system with an estimated load of 130 A, the NEC (Table 310-16) requires the service conductors to be equal to or greater than the diameter of #2 AWG wire. Note that 85% of 130 A is 110.5 A, which is close to 106.6 amperes. However, the service conductors in most situations will be sized in accordance to the standard rating of the service equip-ment. In this situation, the service equipment will most likely be rated at 200 amperes since 130 amperes isn’t a common rating
for service equipment and, therefore, the service conductors must be large enough to handle 200 amperes.
Step 7: To determine the service feeder neutral load, apply the appropriate demand factors to all of the loads:
100% of the first 3000 VA 3000
(Step 1 + Step 2 – 3000 = 9000)
35% of the remaining load (.35 ´ 9000) 3,150
100% of the dishwasher 1,500
70% of the range (12000 ´ 0.7) 8,400
70% of the dryer ( 5,500 ´ 0.7) 3,850
Total 19,900
Step 8. Divide 240 V into the total neutral demand found in Step 7. 19 900 240 82 9 , . dVA dV = A
Answer: A minimum of a #4 AWG copper conductor is
required for the service neutral conductor.
Note that the power company must approve the selected location of the meter base. The electrician will supply the power company with the service-entrance cable hookup point (covered by a weatherhead or similar device), then install a meter base and service panel (Figure 8A). The electrician will then run the service-entrance cable from the meter base to the hookup point and from the load side of the meter base to the service panel, making sure the grounding electrode and grounding conductor are installed. Only then will the power company hook up their cable, splice their incoming line to the installed service-entrance cable, and install and seal the watt-hour meter in the meter base (Figure 8B). Electricians must follow the NEC and local codes closely when placing the service-entrance cable. The minimum height of the power line above pedestrians or vehicle traffic, the size of the conduit required (when conduit is used), and the space between cable clamps (when service-entrance cable is used) are all closely specified by the NEC (Figure 9). Local codes sometimes expand on the requirements of the NEC but never reduce the requirements. Always check local codes and ordinances before installation to assure compliance. When calculating clearance heights and conductor lengths, remember to account for the amount of conductor the power company will require for a drip loop and splicing. It’s typically acceptable to leave a minimum of two feet of excess cable beyond the weatherhead. The local power company sometimes specifies the length of excess cable to be extended beyond the weatherhead.
FIGURE 8—(A) shows a typical service-drop installation while (B) shows how the power company uses splices to connect the service drop to the service-entrance cable the electrician has installed.
Installing the Meter Base
As you learned earlier, the power company installs the watt-hour meter in a residential application. The power company also supplies the meter base (or meter socket) but the electrician must install it. Likewise, the power company must approve the location of the meter base. However, it’s important that the electrician follow local codes and/or utility regu-lations that often govern the meter base’s exact placement. Ordinances may include its height off the ground, and how power is run from the meter to the service panel. In addition, the meter base capacity must match the rating of the system and the system configuration (above ground versus below ground). For example, the meter base for a 200 A service won’t suffice if a 400 A service is to be installed.
Sometimes the meter base will be located on the opposite side of the wall from the service panel, as illustrated inFigure 8A. When these two pieces of equipment are located back-to-back, the job of completing the service-entrance circuit is much easier and less expensive. However, the electrician usually doesn’t decide on the location of the service.
CLAMPS WEATHERHEAD NO MORE THAN 30" BETWEEN CLAMPS (DEPENDING ON LOCAL CODES) NO MORE THAN 12" METER BASE FIGURE 9—This shows
service-entrance cable support requirements.
If the meter base and service panel aren’t located back-to-back, a longer run of service-entrance conductor will be needed from the meter base to the panel. This means that a service disconnecting means will be needed to control the power within that additional run of service cable in case of a short circuit. If the meter base and service panel are mounted back-to-back, routing the wires from the base to the panel is simple. First, to mount the meter base on the outside of the house, remove the knockouts from the back of the base (designed to allow conductors to run from the base to the panel box). Hold the meter base level against the building with the correct side facing up. Trace the shape of the box opening onto the wall, remove the box, and cut a hole slightly larger than the one traced. Obviously, don’t cut through any wall studs because they’re designed to support the structure.
Now attach any required conduit fittings to the meter base. Conduit fittings attached may be those that accept the service-entrance cable from the drip loop or ones that route the cable onto the panel box. Remember that for an overhead service the connector on the top of the meter base must be watertight and matched to the size conduit or service-entrance cable being installed. The service-entrance cable from an underground service will always be installed in conduit and fed into the bottom of the meter base. In this case, the conduit will be joined to the meter base with a standard conduit fitting.
Next, coat the wall around the outside of the hole with a heavy bead of sealant. Make sure the meter base is level and fasten the meter base to the wall using screws or appropriate anchors. To provide additional sealing against moisture, place an additional bead of sealant where the top and two sides of the meter base meet the wall. Don’t seal where the bottom of the meter base meets the wall because moisture that seeps in from the top or sides needs to exit the space behind the meter base.
Connecting the Service-Entrance Conductors to the
Meter Base
The next step in completing the electrical service is to install the service entrance conductors from the weatherhead (for service drops) and terminate them on the line side (top) terminals of the meter base. If sheathed cable (SE) is installed inside conduit, be sure to remove only enough sheathing so that the sheathing extends through the weatherproof connector on the top of the meter base. You won’t normally need to install the service entrance conductors for lateral feed as the power com-pany will usually do this. Strip enough insulation from the end of the conductors for the connections (normally3
4to 1 inch). Connect the incom-ing service-entrance conductors to the meter base. If the service-entrance cable is aluminum, the stripped portion of the conductor must be coated with an antioxidant compound before the conductors are connected to the meter base.
When hooking up the meter base, remember that the incoming power line is attached to the top (line side) terminals, while the conductors that exit the meter base and feed the service panel will connect to the bottom
(load side) terminals. This is shown inFigure 10. In higher-amperage services where two parallel service-entrance cables are used, the hookup procedure is the same except the two conductors per phase are attached to a special double terminal inside the meter base. When cables are installed in parallel, it’s very important that service conductors of like phases be kept together. This requires marking the conductors in the meter base and at the weatherhead (where the power company will hook up) so that the opposite phase conductors aren’t connected together.
NEUTRAL CONDUCTOR TWO HOT CONDUCTORS NEUTRAL CONDUCTOR TWO HOT CONDUCTORS L1 L2
FIGURE 10—This shows upper and lower terminals connected to the base.
Wiring from the Meter to the Service Panel
If the meter base and service panel are back-to-back, run a section of conduit through the wall from the back of the meter base so that it enters the back of the panel as was shown earlier inFigure 8A. The conduit will often be a presized galvanized rigid-conduit nipple. The nipple will be installed with locknuts on both sides of the meter base’s and panel board’s sheet metal wall. The locknuts ensure that the conduit will pro-vide a good ground path from the base to the panel box. In addition to the locknuts, the electrician should always install one conduit bushing on the inside of the meter base, and one conduit bushing on the inside of the panel. This provides a smooth contact edge for the conductors to en-ter and exit the conduit nipple.
It’s more likely that the service panel won’t be located directly on the other side of the wall from the meter base. If not, the service-entrance cable will exit the bottom of the meter base, and may enter the dwelling as shown inFigure 11. INSIDE WALL METER BASE SE CABLE SERVICE DISCONNECT PANEL BOX MAIN FLOOR OF HOUSE BASEMENT OUTSIDE WALL FIGURE 11—In this example,
a service-entrance cable exits the bottom of the meter base, enters the basement, is routed through a main disconnect mounted nearest the point of
basement entrance, and enters the panel.
Before finishing your final systems design and certainly before purchasing materials or submitting a material list or bid, the electrician should lay out the complete service entrance on paper. One erroneous dimension could result in several improperly placed systems components. Both
NEC requirements and local code requirements must be reviewed
prior to installing any components to assure compliance. Consider easily forgotten components such as grounding rods, grounding conductors, clamps, connectors, anchors, fittings, anti-oxidant (for aluminum conductors) and other miscellaneous hardware when preparing a material list or bid.
Sizing and Mounting the Service Panel
It’s critical that you install a service panel with enough capacity to handle a reasonable amount of future expansion in the electrical system. It’s very likely that the homeowner will some day want to modify the dwelling in a way that requires additional branch circuits and/or in-creased load capacity. Although a load calculation may be done prior to installation, it’s always best to size the system larger to accommodate later expansion or additions. If load calculations conclude that the system will require 150 amps, it’s good practice to install a 200-amp panel. In fact, electricians should probably install at least a 200 ampere-rated service in most modern-day residential systems. The NEC requires the service to be rated no less than 100 amperes for a one-family residence (Article 230-79[c]).
Of course, some dwellings will need more than a 200-amp service. If your load calculations or other factors indicate a need for a higher am-perage service, 250 amps for instance, install at least a 300-amp service. Based on material and labor cost factors, you or someone else will need to decide whether it’s more economical to install a single 300-amp panel or 200- and 100-amp panels side-by-side.
Panels are also rated based on the number of breakers they hold. The largest-capacity 200-amp panels may hold up to forty-two circuit breakers. In most cases, you should select the panel within the target amperage range that’s capable of holding the most breakers.
Finally, remember that the electrician is ultimately the person who must hook up the panel. When working with different panel designs, try to pre-evaluate them for ease of assembly and growth. The location of neutral buses, the arrangement of breaker installation, and several other features contribute to making the panel either easier or more difficult to wire.
Sizing and Installing the Main Breaker
In some residential installations, the service disconnecting means will be the main breaker in the service panel. When this is the case, the main breaker controls and protects two of the four conductors entering the
panel, as shown inFigure 12. The switched conductors are always the two hot conductors. When the main breaker in the service panel serves as the main disconnecting device, it must be labeled “Service Discon-nect.” The other two conductors entering the panel (neutral and ground) are connected to the neutral bus and grounding bar (not shown) respec-tively. The neutral bar must be bonded to the panel enclosure using a bonding screw that’s normally supplied with the panel enclosure. You’ll learn more about bonding in a later section.
If the service disconnecting means is installed indoors but not in the service panel, it should be located as closely as possible to the point where electrical power enters the building. Sometimes this requires it to be located in its own separate enclosure, in line with the service-entrance circuit, and ahead of the service panel.
Installing Circuit Breakers
Later in this study unit, you’ll learn how branch circuits are laid out in a typical dwelling, and why the components in one branch circuit must be separated electrically from other branch circuit components. For now, however, assume that each branch circuit has been determined and the various loads in the dwelling are connected to these branch circuits. You may have twelve general lighting circuits (15 A), two small-appliance (20 A) circuits serving the kitchen, one 20 A branch laundry circuit, one 20 A branch workshop circuit, and several dedicated branch circuits serving the dryer, water heater, range, and electric heat pump. As you
FIGURE 12—This shows a panel enclosure with a main-breaker switch.
now know, each of these branch circuits requires its own breaker and in many cases, these breakers will be quite different from one another. In this section of the study unit, we’ll discuss the various types of breakers you’ll encounter and explain how they’re used in a modern residential service panel.
You’ve already learned that circuit breakers are installed in a panel enclosure. However, you should be aware that there are several types of breakers, each with its own application. The most commonly encountered standard breaker (if there is such a thing) is the full-size breaker. It’s single-poled, designed to protect a single branch circuit, and is usually rated at either 15 or 20 amps. Electricians refer to this type of breaker by the number of poles it contains, as well as its amperage rating. In this case, you would refer to the breaker as a single-pole 15-amp (or 20-amp) breaker.
Dual or piggyback breakers can usually be inserted only into panels that
provide slots for their installation. They’re designed to protect two indi-vidually separate branch circuits. The dual breaker is normally the same width as a standard breaker but contains two half-thickness breakers, each with its own handle (Figure 13). A half breaker is simply one-half of a dual breaker and can only be connected to one branch circuit. It’s some-times referred to in the trade as a “thin” breaker. As more dual and half breakers are used, the number of branch circuits connected in a panel increases, as does the ambient temperature in the panel enclosure. Therefore, the allowable number of dual and half breakers is limited. Half and dual breakers can only be installed in slots indicated by the panel box manufacturer. These panels incorporate a special groove designed to accept a tab that’s formed into the bottom of the breaker. Only panels that are equipped with the groove can accept these special breakers.
FIGURE 13—Dual and half breakers are designed to fit only grooved slots in a panel enclosure.
Double-pole breakers connect to both legs of the hot bus. These breakers
normally protect and control branch circuits that supply 240 volts. Double-pole breakers protecting residential branch circuits usually range from 15 to 70 amps, with 30-amp breakers protecting most clothes dryer and water heater circuits. The 40- or 50-amp breakers are normally used to protect the electric kitchen range and/or oven circuits.
Balancing Circuit Loads
As you’ve already learned, a standard 120/240 service includes two hot wires and a neutral. Each conductor is connected to a specific “bus,” which distributes the function of the conductor. The hot buses distribute the hot lines to the breakers, which in turn distribute the hot lines to the loads. The neutral bus distributes the grounded neutral to the branch-circuit neutral conductors, and the grounding bus distributes the equipment ground to the grounding conductors. The two hot conductors are 180 degrees out of phase with each other. This means that when the alternating current in one hot bus reaches its maximum positive peak, the current in the other bus reaches its maximum negative peak. Normally, the panel-board is designed so that half of the circuit breaker positions are arranged to draw current from one of the hot buses while the other half draws from the other bus. The panel manufacturers accomplish this by “staggering” the connecting points from top to bottom in the panel. In other words, one breaker will connect to “a” phase while the next breaker will connect to “b” phase. This staggering goes from top to bottom in the panel. This isn’t information that you’ll need in your daily functions as an electrician, but it does help you to better understand why certain components are designed the way they are.
A balanced circuit load occurs when the current through each hot bus is equal. When this happens, the positive peak current cancels out the negative peak current and no current flows through the neutral conductor. Of course, it’s not likely that the two hot conductor currents will be equal. Hence, when the current levels aren’t equal, the neutral conductor carries the difference in currents back to the panel. As an example, if branch circuits attached to one bus draw 65 amps collectively, while those attached to the other bus draw 75 amps collectively, the neutral conductor will carry 10 amps (the difference between 65 and 75 amps) back to the neutral bus in the panel.
To better balance a circuit, distribute the loads equally between the two hot buses as much as possible. One method you may use is splitting the two small-appliance circuits to the kitchen between the two hot buses.
Installing a Grounding Electrode
As you know, every electrical service panel must include a grounding conductor. If possible, this conductor should be attached to a grounding
electrode. In some localities, it’s allowable to ground the electrical system
to the water main, but this type of grounding must be supplemented by an additional grounding electrode as specified in the NEC Article 250-50 and Article 250-52. Grounding electrodes must be at least 8 feet long and
driven into the ground. They should maintain a continuous low-resistance conductive contact with the soil. Rods made of ferrous (steel or iron) material must be at least5
8inch in diameter. Nonferrous rods (copper clad) must be at least1
2inch in diameter. The top of the rod to which you’ll attach the grounding conductor should be flush with or just below the surface. If the presence of rock makes it impossible to fully drive the rod into the earth, it may be buried horizontally in a trench that’s at least 212-feet deep, or driven at an angle not exceeding 45 degrees. You may never shorten the 8-foot length of a grounding rod by cutting portions from the rod. It must always remain 8 feet in length. Depending on lightning hazards, you may need to install more than one grounding rod. The NEC and local codes specify many of the procedures associated with the installation of the grounding system. The NEC also specifies that the conductor used to connect the grounding rod to the service panel be free from interruptions or splices.
Grounding and Bonding of Service and Equipment
The word bonding is defined as the coming together of all metal parts in the system so that no potential difference exists between them.
Remember that grounding is the interconnection of all metal components with the grounding conductor to provide a low-impedance path for fault-current flow should one of the energized conductors come into contact with the metal components.
When installing any electrical service, always bond the neutral bar to the panel enclosure. If any fault current develops on the system it will flow across the equipment, through the bonding screw and onto the service grounded conductor. This will cause the overcurrent device to open, thus eliminating dangerous voltages on the equipment.
The neutral bar may be bonded in any one of the following ways. · Using a bonding screw. In 100 and 200 ampere panels the bonding
screw is normally a 10/32-type screw that’s generally green in color.
· Attaching a bonding strap. The thickness of the strap may vary depending on the rated loads of the panel.
· Installing a bonding conductor (or jumper). ConsultTable 1for the size of bonding conductor needed to bond a neutral bar. Note that this table, which is taken from the NEC, is for grounding electrode conductors. That’s because Article 250-102(c) states that bonding jumpers can’t be smaller than the grounding electrode conductor.
All raceways for the service entrance shall be bonded together. Again, Table 1indicates the size of the bonding jumper needed. If the conductors within the conduit or the conduits themselves are larger than 1100 kcmil copper or 1750 kcmil aluminum, the bonding jumper shall have an area not less than 121
2percent of the area of the largest phase conductor. In the case of parallel conductors, the largest area of a phase conductor is considered to be the sum of the areas of the conductors paralleled in one phase.
Table 1
GROUNDING ELECTRODE CONDUCTOR FOR ALTERNATING-CURRENT SYSTEMS
Size of Largest Service-Entrance Conductor or
Equivalent Area for Parallel Conductors1 Size of Grounding Electrode Conductor
Copper Copper-Clad AluminumAluminum or Copper Copper-Clad AluminumAluminum or 2
2 or smaller 1/0 or smaller 8 6
1 or 1/0 2/0 or 3/0 6 4
2/0 or 3/0 4/0 or 250 kcmil 4 2
Over 3/0 through
350 kcmil Over 250 kcmilthrough 500 kcmil 2 1/0
Over 350 kcmil
through 600 kcmil Over 500 kcmilthrough 900 kcmil 1/0 3/0 Over 600 kcmil
through 1100 kcmil Over 900 kcmilthrough 1750 kcmil 2/0 4/0
Over 1100 kcmil Over 1750 kcmil 3/0 250 kcmil
Notes:
(a) Where multiple sets of service-entrance conductors are used as permitted in Section 230-40, Exception No. 2, the equivalent size of the largest service-entrance conductor shall be determined by the largest sum of the areas of the corresponding conductors of each set.
(b) Where there are no service-entrance conductors, the grounding electrode conductor size shall be determined by the equivalent size of the largest service-entrance conductor required for the load to be served.
1This table also applies to the derived conductors of separately derived AC systems. 2See installation restrictions in Section 250-64(a) of the NEC
(Reprinted with permission from NFPA 70-1999, the National Electrical Code®, Copyright© 1998, National Fire Protection Association, on the referenced subject which is represented only by the standard in its entirety. National Electrical Code® and NEC® are registered trademarks of the National Fire Protection Association, Inc., Quincy, MA 02269)
Let’s work through several example problems usingTable 1.
Problem 1:
What size of copper-bonding jumper is required to bond a metal conduit with three 500-kcmil THWN Cu conductors and one neutral conductor?
Solution:
Figure 14shows that we have only one 500 kcmil per phase. Refer to
Table 1under service-entrance conductors in the Copper column. A
500 kcmil conductor falls under the “Over 350 kcmil through 600 kcmil” category. Therefore, 1/0-size copper wire is needed to bond the metal conduit properly.
FIGURE 14—This shows the bonding of a raceway with three conductors.
Problem 2:
What size copper-bonding jumper is required to bond three raceways using one bonding jumper, where each metal conduit contains three 600-kcmil Cu conductors that are parallel per phase?Figure 15illustrates this example.
Solution:
The first step is to add up the conductors to find the total kcmil per phase (600 ´ 3 = 1800 kcmil). Now that we know we have an area of 1800 kcmil per phase, we can checkTable 1for the size of the bonding jumper needed.
SinceTable 1only goes up to 1100 kcmil Cu, we’re now required to take 121
2% of the largest phase conductor. (The largest phase conductor is considered to be the total area of the parallel conductors, or 1800 kcmil.)
1800 kcmil ´ 0.125 = 225 kcmil
FIGURE 15—This shows the bonding of three raceways in a series with three parallel-phase conductors.
Rounding off, we find that 250 kcmil bonding jumper is required to bond all three metal conduits properly.
Let’s take this example one step further. Everything remains the same, except, instead of having all three conduits bonded in a series, each conduit will be bonded individually. SeeFigure 16.
UsingTable 1, locate the largest phase conductor in the conduit (as explained in note [a]). This would be 600 kcmil Cu. Then refer to the “Over 350 kcmil through 600 kcmil” column inTable 1. A 1/0-size conductor is the minimum size that would be required to bond each conduit properly.
FIGURE 16—One bonding jumper to each conduit requires only a 1/0 Cu when bonding individually.
This example shows that if the situations permits, it’s more cost-effective (material and labor) to bond each conduit individually in a paralleled system.
The interior metal piping through a building must also be bonded.Table 1 also sizes the bonding conductor for the metal piping. Refer toFigure 17 that shows the proper bonding of the interior metal water pipe. Keep in mind that it serves no purpose to bond non-metallic water piping such as PVC.
Should you decide or be required to run the bonding conductor in a metal conduit, you then must also bond the conduit itself to the waterline. SeeFigures 18and19.
The following items should be bonded together to make up the grounding electrode system if they’re available on the premises: (a) metal under-ground water pipe (10 feet or more of metal pipe in direct contact with the earth); (b) metal frame of building (where building is intentionally grounded to the earth); (c) concrete-encased electrode; and (d) grounding ring (constructed by burying at least 20 feet of #2 or larger bare copper wire in a trench 21
2feet deep or more, encircling the building or structure requiring the grounding system). The size of the grounding conductor is found inTable 1unless otherwise noted on the plans or drawings. Should none of these items be available, you would then be required to install grounding rods, pipe electrodes (minimum34-inch trade size iron or steel, metal coated to prevent corrosion), or electrode plates (minimum14-inch thick iron or steel plate or minimum .06 -inch thick nonferrous plate, with at least 2 square feet of plate surface exposed to the soil).
If we installed a 400-A three-phase service and had access to all of these items, the installation would look much like the installation inFigure 20. Bonding and grounding protects against the unpredictable ground faults and shorts which may develop in any electrical system. Proper bonding and grounding of a system won’t only lessen personnel exposure to high voltages and potential damage to conductors and equipment, but should also open affected overcurrent devices.
FIGURE 19—This shows a close-up view of the bonding jumper area in Figure 18.
Subpanels
There may be instances in a residential wiring plan that call for a subpanel installation. One may be that when a large number of major appliances and similar heavy loads are located a long distance from the service panel, a subpanel may be needed to supply these appliances or loads. Subpanels are also used when an addition is added onto a house. An addition usually requires several branch circuits and is generally located quite far from the original service panel. Subpanels are sometimes installed when adding equipment such as room air conditioners, dishwashers, etc. because the existing service panel has no more room for additional breakers.
In new installations, installing subpanels may reduce the amount of conductor-routing work by permitting the electrician to install only one large feeder cable from the service panel to the subpanel. The subpanel can then be located much closer to the locations of the loads. A subpanel resembles and functions much like a regular panel, with some exceptions. First, because the subpanel is fed through the main panel, there’s no need for a main breaker in the subpanel (although you may still install one if you wish). Secondly, the neutral and ground buses in the subpanel must be completely isolated from one another. This means that they can’t be directly connected to one another and that the neutral bus must be isolated from contacting the subpanel enclosure by mounting it in the enclosure using some type of insulating material. This is usually accomplished with plastic separators between the neutral bus and the enclosure. As just learned, components such as panels, subpanels, and other enclosures must be connected to equipment ground. Because of the required separation between the neutral and ground buses, the equipment-bonding device may only be connected to the ground bus and not the neutral bus.
Power is supplied to the subpanel directly from the service panel, normally using a four-conductor service-entrance cable.
Now take a few moments to review what you’ve learned by completing
✔
Self-Check 2
1. True or False? If possible, it’s best to ground each conductor individually. 2. Bonding screws are used to bond the _______ bar.
3. The main breaker can be used as the service _______.
4. The drip loop of the SEC attached to the building must be at least _______ feet above the ground where only pedestrian traffic is a concern.
5. True or False? Residential wiring must always have a subpanel installed. 6. The neutral bus and subpanel are normally isolated by _______.
7. Heating and burning of conductors caused by short circuits can be reduced by bonding and _______.
LOCATING RESIDENTIAL DEVICES
Wiring Layout for a Small House
Quite often, the electrician won’t receive complete and adequate plans for small residential dwellings. An experienced electrician can use a building plan as a basis for designing an electrical layout that complies with the National Electrical Code and any local codes.
Figure 21shows a wiring layout plan for a three-bedroom ranch-type house with a basement. Assume that the laundry facilities and the service panel are in the basement. The basement plan with its wiring layout isn’t shown in this drawing. The outlets are indicated using the standard electrical symbols you should be familiar with by now. All duplex receptacles are grounding-type receptacles.
The arrowheads on the circuits indicate the home runs, which are the cable runs to the distribution panel where the branch-circuit protective devices are located. The number of 2- and 3-wire circuits can be found by counting the arrowheads. The home runs for each circuit normally begin at the outlet nearest the panel. The ideal location for the panel is where the load is concentrated, which is in the kitchen and laundry. The location of the home runs inFigure 21isn’t typical of a house because the home runs shown here are scattered without regard to the possible panel location. Branch circuits normally end at lighting outlets or receptacle outlets. The light gray lines connecting the outlets inFigure 21represent runs of cable. Broken lines are also used sometimes to indicate exposed wiring in the basement, but the basement isn’t shown in this wiring layout. In the kitchen, receptacles have been provided for the refrigerator, clock, iron, can opener, toaster, and other small appliances. A special outlet S is provided for an ironing station. It’s equipped with a switch and pilot lamp so that the homeowner will know whether the iron is on or not. A special receptacle (R) is provided for the range. Several special outlets are represented in the wiring plan, including one for a clock (C), dishwasher (DW), garbage disposal (GD), and range hood (RH). Each special outlet is identified on the drawing to indicate its use. Many range hoods contain both a fan and a lamp so separate fan and lamp holder outlets aren’t shown. Note that the kitchen and dining room share two small-appliance circuits and these circuits don’t enter other rooms. The
NEC doesn’t permit these circuits to supply power to any other rooms
except breakfast nooks.
A sufficient number of receptacles are installed in the other rooms. They’re spaced approximately equal distances apart. The distance between adjacent receptacles in the same room (excluding kitchen and bathroom) should always be less than 12 feet according to the NEC. Each bedroom is equipped with a combination switch and a receptacle outlet as well as a ceiling light for general lighting. Each closet has an enclosed lamp fixture controlled by a pull-chain switch. The receptacle at the entrance door is conveniently located for connecting a vacuum cleaner or other small appliances.
The bathroom has a ceiling light for general lighting and special lights at the mirror. Electric heating is often used in the bathroom to supplement the regular heating. All receptacles in a bathroom shall be ground-fault protected. The bathroom circuit may not supply any other room other than another bathroom.
A split-wired duplex receptacle is shown near the front door in the corner of the living room. The top half of the receptacle is wired so that it’s con-trolled by two three-way switches. The bottom half is always energized. This permits a lamp to be plugged into the top half and controlled from the front door and the hall, while an appliance such as a clock may be plugged into the bottom half. The terrace is equipped with two ground-fault-type weatherproof (WP) receptacles for portable lamps, decorative lighting, or tools. Because two receptacles are installed, there’s no need to pass cords over the doorway, thus reducing the chance of
damaging cord insulation. Switches are often used to control weatherproof outlets, which allow the outlets to be used more easily.
Three-way switches are used in the hall, kitchen, and living room to reduce the need of retracing steps when a person moves from one part of the house to the other. Another convenient feature is the push-button door-chime switches located at both exterior doors to control the chime located in the hall.
Location of Receptacles
There isn’t a required height for mounting receptacles but a convenient recommended height for duplex receptacles is 16 inches (in.) above the floor. At that height, the outlet is more accessible and more adaptable to appliance cords. In the kitchen, bathroom, laundry, and garage, a recommended height for receptacles is 48 inches above the floor. In the kitchen, that height equates to approximately 12 inches above the countertops.
Present practice is to provide enough receptacles so that no point in a room (except kitchen and bathroom) is more than 6 feet from a receptacle. Thus, the distance between receptacles is always less than 12 feet. Any wall space greater than two feet in length shall require a receptacle. Sliding panels, such as sliding glass doors, aren’t counted as wall space according to the NEC Article 210-52(a)(2)(d). The receptacles should be approximately equally spaced. However, the spacing may be changed somewhat to address anticipated placement of furniture. In the kitchen, receptacles installed on a countertop shall be installed so that no point along the counter (measured horizontally) is more than 24 inches from a receptacle outlet. A receptacle outlet should also be installed to serve each counter space 12 inches wide or wider. The receptacle for the refrig-erator should be hidden from view when the refrigrefrig-erator is in place. Weatherproof GFCI (ground-fault circuit interrupter) receptacles should be installed at convenient outdoor locations, front and back, for supplying decorative lighting and power tools. These receptacles should be kept at least 18 inches above ground level for protection of the receptacle and ease of accessibility. Special receptacles with key locks are available for use where vandalism is a problem. For extra convenience, wall-mounted switches may control these outdoor receptacles.
Location of Switches
A convenient height for light switches is approximately 48 inches above the finished floor, on the lock side of a door, and within 6 inches of the door frame. Switch locations should be carefully planned to accommodate the residents by following the normal course of passage from room to room that a resident may normally take. For example, upon entering the house a person should be able to turn on a light without taking many
