MWD
METROPOLITAN WATER DISTRICT OF SOUTHERN CALIFORNIA ENGINEERING SERVICES SECTION
ELECTRICAL
CHAPTER TITLE PAGE 1 INTRODUCTION... 1-1
1.1 OBJECTIVE... 1-1 1.2 RESPONSIBILITIES OF THE ELECTRICAL
DESIGNER... 1-1 1.3 DESIGN TASKS ... 1-2 1.3.1 Study Phase ... 1-2 1.3.2 Preliminary Design Tasks ... 1-2 1.3.3 Final Design Tasks (30%)...1-2 1.3.4 Final Design Tasks (60%)...1-3 1.3.5 Final Design Tasks (90%)...1-3 1.3.6 Final Design Tasks (100%)...1-4 1.4 DOCUMENT CONTROL ... 1-4
2 PROJECT DESIGN ELEMENTS ... 2-1
2.1 GENERAL APPROACH ... 2-1 2.1.1 Design Criteria ... 2-1 2.1.2 Drawings... 2-1 2.1.3 Specifications... 2-1 2.2 BASIC ELECTRICAL ENGINEERING FORMULAS ... 2-2 2.2.1 List of Symbols ... 2-2 2.2.2 Direct Current (dc) Formulas ... 2-2 2.2.3 Alternating Current (ac), Single Phase ... 2-3 2.2.4 Alternating Current (ac), Three Phase ... 2-3 2.2.5 Motors... 2-4 2.2.6 Power Factor Correction ... 2-4 2.3 DESIGN CALCULATIONS ... 2-4 2.3.1 General ... 2-4 2.3.2 Load... 2-5 2.3.3 Conductor Size, General ... 2-5 2.3.4 Conduit Size and Fill ... 2-8 2.3.5 Motor Branch Circuit ... 2-9 2.3.6 Power Factor Correction Capacitors... 2-14 2.3.7 Transformer Primary and Secondary Conductors 2-17 2.3.8 Voltage Drop... 2-20 2.3.9 Short Circuit ... 2-23 2.3.10 Lighting ... 2-27
TABLE OF CONTENTS MWD Electrical Design Manual
CHAPTER TITLE PAGE
2.3.11 Grounding ... 2-36 2.4 DRAWINGS... 2-36 2.4.1 General ... 2-34 2.4.2 Organization ... 2-34 2.4.3 Legend... 2-37 2.4.4 Abbreviations ... 2-37 2.4.5 Site Plan(s) ... 2-37 2.4.6 One-Line Diagrams... 2-38 2.4.7 Floor Plans... 2-39 2.4.8 Grounding Plan... 2-40 2.4.9 Equipment Elevations ... 2-40 2.4.10 Control Schematic Diagrams ... 2-40 2.4.11 Installation Details...2-40 2.4.12 Electrical Schedules ...2-40
2.5 PROJECT FILES ... 2-44
3 STANDARD ELECTRICAL DESIGN PROCEDURES... 3-1
3.1 GENERAL APPROACH ... 3-1 3.1.1 Types of Electrical Systems... 3-1 3.1.2 References ... 3-1 3.1.3 Plant Distribution Systems ... 3-2 3.1.4 Voltage Considerations... 3-7 3.1.5 Voltage Selection... 3-8 3.1.6 Voltage Rating ...3-8 3.1.7 Protection/Coordination Philosophy ... 3-8 3.1.8 Equipment Heat Dissipation Data ... 3-13 3.2 LOCATING ELECTRICAL EQUIPMENT ... 3-13 3.2.1 Equipment Rooms and Buildings... 3-13 3.2.2 Equipment Enclosures ... 3-14 3.3 SWITCHGEAR ... 3-15 3.3.1 Low Voltage ... 3-15 3.3.2 Medium Voltage (4.16 kV through 13.8 kV) ... 3-17 3.4 TRANSFORMERS... 3-17 3.4.1 Pad-Mounted ... 3-18 3.4.2 Unit Substations... 3-18 3.4.3 Equipment Selection ... 3-20 3.5 MOTOR CONTROL EQUIPMENT ... 3-20 3.5.1 Low Voltage ... 3-20 3.5.2 Medium Voltage... 3-25 3.5.3 Adjustable Speed Drives... 3-27 3.5.4 Power Factor Correction ... 3-32
CHAPTER TITLE PAGE
3.5.5 Control Circuit Devices ... 3-32 3.6 MOTORS... 3-33 3.6.1 Basic Motor Types ... 3-33 3.6.2 Design Considerations... 3-33 3.6.3 Low-Voltage Single-Phase Induction Motors ... 3-39 3.6.4 Low-Voltage Three-Phase Induction
Motors... 3-39 3.6.5 Medium-Voltage Induction Motors ... 3-39 3.6.6 Synchronous Motors... 3-40 3.6.7 Direct Current Motors ... 3-40 3.7 RACEWAY SYSTEMS ... 3-41 3.7.1 Conduit System ... 3-41 3.7.2 Conduit Identification ... 3-42 3.7.3 Wireway ... 3-42 3.7.4 Cable Tray System ... 3-42 3.7.5 Trench System... 3-43 3.7.6 Ductbank System... 3-43 3.8 CONDUCTORS ... 3-44 3.8.1 Low-Voltage Wiring
Systems (600 Volts and Below) ... 3-44 3.8.2 Medium and High Voltage Conductors
(Above 600 Volts) ... 3-47 3.8.3 Splices and Terminations... 3-47 3.8.4 Conductor Identification ... 3-48 3.8.5 Conductor Installation ... 3-48 3.9 JUNCTION BOXES AND PULL BOXES ... 3-49 3.9.1 Indoor Locations ... 3-49 3.9.2 Outdoor Locations ... 3-50 3.9.3 Corrosive Locations ... 3-50 3.9.4 Hazardous Locations ... 3-50 3.9.5 Terminal Junction Boxes... 3-50 3.10 MANHOLES AND HANDHOLES... 3-51 3.10.1 Handholes... 3-51 3.10.2 Manholes ... 3-52 3.11 LIGHTING SYSTEMS... 3-52 3.11.1 General Illumination ... 3-53 3.11.2 Recommended Illumination Levels ... 3-54 3.11.3 Lighting System Design ... 3-54 3.11.4 Luminaries ...3-54 3.11.5 Emergency/Standby Lighting ... 3-57 3.11.6 Exit Signs... 3-58 3.11.7 Controls ... 3-58
TABLE OF CONTENTS MWD Electrical Design Manual
CHAPTER TITLE PAGE
3.12 LOW VOLTAGE POWER DISTRIBUTION... 3-59 3.12.1 Voltage Selection... 3-59 3.12.2 Panelboards... 3-59 3.12.3 Convenience Receptacles ... 3-60 3.12.4 Hazardous Area Receptacles ... 3-61 3.12.5 Power Receptacles ... 3-61 3.13 GROUNDING ... 3-64
3.13.1 General ... 3-64 3.13.2 System Grounding ... 3-64 3.13.3 Grounding Electrode Systems and
Grounding Grids ... 3-65 3.13.4 Equipment Grounding ... 3-67 3.13.5 Instrumentation and Computer Grounding... 3-67 3.13.6 Lightning Protection System Grounding ... 3-67 3.14 EMERGENCY AND STANDBY POWER SYSTEMS.... 3-67 3.14.1 General ... 3-67 3.14.2 Emergency Power Systems... 3-68 3.14.3 Legally Required Standby Power System... 3-68 3.14.4 Optional Standby Systems... 3-69 3.14.5 Engine Generators... 3-69 3.14.6 Unit Equipment ... 3-70 3.14.7 Computer Power Systems ... 3-71 3.15 SPECIAL SYSTEMS ... 3-71 3.15.1 Plant Communication System... 3-71 3.15.2 Fire Alarm System ... 3-74 3.16 ELECTRICAL TESTING ... 3-76 3.16.1 General Requirements... 3-76 3.16.2 Plant Electrical System ... 3-77 3.16.3 Medium and Low Voltage Equipment ... 3-78 3.16.4 Conductors ... 3-80 3.16.5 Emergency/Standby Generators... 3-80 3.16.6 Grounding ... 3-81
4 CONTROL SYSTEM DESIGN PROCEDURES ... 4-1
4.1 CONTROL PANELS ... 4-1 4.1.1 NEMA Standards ... 4-1 4.1.2 Panel Design ... 4-1 4.1.3 Indicating Devices... 4-2 4.1.4 Switches, Pushbuttons, and Lights ... 4-2 4.1.5 Annunciators... 4-3 4.1.6 Relays and Timers... 4-4
CHAPTER TITLE PAGE
4.1.7 Control Panel Layout ... 4-5 4.1.8 Wiring and Terminations ... 4-6 4.1.9 Nameplates... 4-8 4.1.10 Installation... 4-8 4.1.11 Seismic Design Requirements... 4-8 4.2 FIELD WIRING ... 4-9 4.2.1 Field Signal Wiring ... 4-9 4.2.2 Conduit ... 4-13 4.2.3 Spare Conductors... 4-14 4.3 CONTROL DEVICE INTERFACING... 4-14 4.3.1 Remote Terminal Unit Outputs ... 4-15 4.3.2 Control Panels ... 4-15 4.3.3 Status Monitoring... 4-16 4.3.4 Signal Convertors ... 4-17
APPENDIX TITLE PAGE
A REFERENCES ... A-1 B ABBREVIATIONS... B-1 C SAMPLE ELECTRICAL DESIGN CRITERIA MEMO ... C-1 D ENCLOSURE TYPES ... D-1 E MOTOR ENCLOSURE TYPES ... E-1 F MOTOR DESIGN TYPES...F-1 G MOTOR TORQUE DEFINITIONS ... G-1 H STANDARD SPECIFICATIONS FOR THE IDENTIFICATION OF
ELECTRICAL CURRENT CARRYING CONDUCTORS... H-1
FIGURE TITLE PAGE
2-1 Relation Between kVA, kW, and kvar ... 2-14 2-2 Impedance Diagram ... 2-23 2-3 Zonal Cavity Calculations ... 2-29 2-4 Calculation of Task Illumination ... 2-32 2-5 Example Panel Drawing ... 2-40 3-1 Example Control Station Wiring... 3-25 3-2 NEMA Configurations of General Purpose Nonlocking Plugs
and Receptacles... 3-52 3-3 Additional NEMA Configurations ... 3-53 4-1 Constant Speed Motor Control ... 4-17 4-2 Reversing Motor Control... 4-18 4-3 Two-Speed Motor Control ... 4-19
TABLE OF CONTENTS MWD Electrical Design Manual
CHAPTER TITLE PAGE
4-4 Incremental Valve Control ... 4-20 4-5 Open/Close Valve Control, Electric Motor Applicator ... 4-21 4-6 Open/Close Valve Control, Hydraulic/Pneumatic Operator ... 4-22 4-7 Variable Speed Motor Control-Single Phase ...4-23 4-8 Variable Speed Motor Control-Three Phase...4-24 F-1 Examples of Power Feeder Cable Identification for
Water Treatment Plant Section ...F-5 F-2 Examples of Control and Instrumentation Cable
Identification for Water Treatment Plant Section ...F-6 F-3 Cable Identification ...F-8 F-4 Identification for a Multi-Conductor Cable...F-9 F-5 Identification for a Single-Conductor Cable ...F-9 F-6 Typical Box Identification ...F-11 F-7 Typical Duck Bank Identification...F-12
TABLE TITLE PAGE
2-1 Motor Circuit Design Data--480 Volt, Three-Phase Motors ... 2-12 2-2 480-Volt Lighting Transformer Circuit Design Chart (75o C) ... 2-17 2-3 Three-Phase Line-to-Line Voltage Drop for 600 V
Single-Conductor Cable per 10,000 A-ft ... 2-21 2-4 Coefficient of Utilization Zonal Cavity Method ... 2-27 2-5 Candlepower Distribution Curve ... 2-31 3-1 Losses in Electrical Equipment... 3-8 3-2 Recommended Illumination Levels... 3-45 3-3 Requirements for Fire Alarm and Detection Devices... 3-65 4-1 Annunciator Sequences ... 4-4 A Conductor Voltage Level Color Codes ... F-7
INTRODUCTION
MWD Electrical Design ManualŮŮŮŮŮŮŮŮŮŮŮŮŮŮŮŮŮŮŮŮŮŮŮŮŮŮŮŮŮŮŮŮŮŮŮŮŮŮŮŮŮŮŮŮŮŮŮŮŮŮŮŮŮŮŮŮŮŮŮŮŮŮŮŮŮŮŮŮŮŮŮŮŮŮŮŮŮŮŮŮŮŮŮŮŮŮŮŮŮŮŮŮŮŮŮŮŮŮŮŮŮŮŮŮŮŮŮŮŮŮ
1.1 OBJECTIVE
The objective of these electrical design standards is to provide a guide that can be used for Metropolitan Water District of Southern California's (Metropolitan) electrical practice. Anticipated users of this manual include the engineer/designer with limited experience, management staff, and the more experienced engineer/designer. The senior staff may find the manual useful as a training tool for subordinates. The information
contained herein has been assembled from a number of sources; a list of the readily available sources is contained in Appendix A, References. These electrical design standards shall be used as the basis for all designs prepared for Metropolitan. Outlined within these standards are procedures for preparing design instructions, procedures for making most of the calculations that will be required for a design, a data table that can be used in making those calculations, drawing presentation formats, standard legend items and abbreviations, descriptions of materials to be used, and a number of informative memos. This information, used with engineering judgment in conjunction with appropriate codes, national standards, and other reference information, will provide electrical systems that are safe and electrically suited for the intended application.
1.2 RESPONSIBILITIES OF THE ELECTRICAL DESIGNER
The electrical engineer/designer is responsible for all facets of a project that are related to:
x x x x x x
Electrical energy for equipment located on the project site; Adequate illumination in all areas;
Special electrical systems;
Conduits and conductors for power distribution and instrumentation and control (I&C) systems;
Protective and safety alarm systems;
INTRODUCTION MWD Electrical Design Manual
x Communication systems; x Emergency power systems;
The engineer/designer must take an active role in consulting with other members of the project team to identify the needs of his or her design and the needs of other design groups.
1.3 Design Tasks
The following is a partial list of design tasks that the electrical engineer/ designer must assume responsibility for during the course of the design. 1.3.1 Study Phase
x Provide electrical support for preparation of draft study. x Review electrical elements of project description in draft
study.
1.3.2 Preliminary Design Tasks
x Prepare the written electrical design criteria that are specific to the needs of the project.
x Provide input to preparation of preliminary design report (PDR).
x Define the method of electrical service. Contact the utility that will serve the site to define the interface required between the utility’s system and the site’s electrical
distribution system. Obtain a copy of the electrical rates that will apply to the service.
x Identify and talk to the electrical inspection authority having jurisdiction at the project site and obtain copies of any special ordinances or codes that may apply to the electrical design.
x Work with the process design staff and mechanical
engineers as well as other concerned design disciplines to define the electrical load that will be required on the project site and identify the electrical equipment.
x Develop a preliminary one-line diagram and written narrative that describes the proposed electrical distribution system. x Prepare a preliminary electrical site plan showing the
location of all major electrical equipment such as switchgear, transformers, electrical ductbanks, etc.
x Perform preliminary calculations to size major electrical equipment.
x Update the electrical one-line diagram. x Update the electrical site plan and sections.
x Update calculations for sizing of electrical equipment. x Complete the power system study.
x Update the electrical equipment list
x Prepare draft electrical equipment specifications Table of Contents.
x Prepare the electrical drawings required to define the electrical system to be constructed (See paragraph 2.4, Drawings).
x Prepare draft schedules for panelboards, lighting fixtures, electrical boxes, manholes, conduit, cable,etc.
1.3.4 Final Design Tasks (60%)
x Update the electrical one-line diagram. x Update the electrical site plan and sections. x Complete and stamp all electrical calculations. x Update the electrical drawings required to define the
electrical system to be constructed such as the power plan, lighting plan, grounding plan, communication systems, fire alarm systems, etc.
x Prepare the text electrical specifications required to define the electrical system to be constructed (See paragraph 2.1.3, Specifications).
x Update all schedules for panelboards, lighting fixtures, electrical boxes, manholes, conduit, cable, etc.
x Prepare draft control schematics and wiring diagrams. x Review the Instrumentation and Control System Diagrams
(I&CS) to verify that all equipment on the project site that must be interfaced with the electrical system has been accounted for. In addition, the I&CSs should be consulted when the control diagrams are being prepared because they define the relationships that exist between the electrical control equipment, the instrumentation system, and many of the equipment items supplied in other divisions of the text specifications.
x Prepare the ladder diagrams required for all panels that will be provided by the I&C supplier. These ladder diagrams should be used during the preparation of the process plans to determine the conduit and conductor requirements of the discrete control systems.
INTRODUCTION MWD Electrical Design Manual
x Complete the electrical one-line diagram. x Complete the electrical site plan and sections.
x Complete all electrical drawings required to define the electrical system to be constructed such as the power plan, lighting plan, grounding plan, communication systems, fire alarm systems, etc.
x Complete all electrical equipment lists.
x Complete all text specifications for electrical equipment. x Complete all control schematics and wiring diagrams. x Complete all schedules.
x Complete the coordination of process control schematic diagrams with Mechanical, I&C and SCADA design. x Complete all protection relay settings.
1.3.6 Final Design Tasks (100%)
x Signoff of electrical plans and specifications.
1.4 DOCUMENT CONTROL
This manual is intended to be (1) the primary technical reference resource for new employees in this discipline, and (2) the only reference guide for engineering consultants who will augment Metropolitan engineering staff. It is important that this manual be updated to keep it current and maintain its usefulness. To propose changes to this manual, follow the change control system procedure, located in ESD-171, Engineering Administration
PROJECT DESIGN ELEMENTS
MWD Electrical Design ManualŮŮŮŮŮŮŮŮŮŮŮŮŮŮŮŮŮŮŮŮŮŮŮŮŮŮŮŮŮŮŮŮŮŮŮŮŮŮŮŮŮŮŮŮŮŮŮŮŮŮŮŮŮŮŮŮŮŮŮŮŮŮŮŮŮŮŮŮŮŮŮŮŮŮŮŮŮŮŮŮŮŮŮŮŮŮŮŮŮŮŮŮŮŮŮŮŮŮŮŮŮŮŮŮŮŮŮŮŮŮ
2.1 GENERAL APPROACH
A design project can be broken down into a number of specific elements that are prepared during several phases of the project. The two project phases that are being covered by this design manual are the preliminary design and final design phases. During the preliminary design phase, the needs of the project must be evaluated, a preliminary one-line diagram and electrical site plan prepared, the needs of the project outlined in a brief report, and the design criteria for the project prepared. The electrical drawings and text specifications are then prepared during final design using the information prepared during preliminary design as a basis for that design. All of the major decisions should be made during preliminary design. Final design is an implementation of those decisions.
2.1.1 Design Criteria
The Electrical Design Criteria is a compilation of general information, specific requirements that are applicable to the project, and design instructions that shall be used by all of the design team members to assure a complete and consistent product. An example Electrical Design Criteria memo is presented in Appendix C.
2.1.2 Drawings
The purpose of a design is to develop a set of instructions and rules that a contractor can use to bid the project and, if awarded the contract, build what the designer had in mind. The drawings are a part of that installation instruction set and describe the location and quantity of materials and equipment needed for the project; the text specifications describe the type and quality of materials and equipment and the quality of workmanship. See paragraph 2.4, Drawings, for a description of the drawings to be included in a construction package.
2.1.3 Specifications
The text specifications shall describe the materials to be furnished by the contractor and the requirements for the products themselves, the
requirements for installing the products, and the quality control measures that will be used to check the products and the execution of construction. Moreover, the text specifications provide these descriptions in one place for the general contractor's comprehension and use. As an electrical engineer/designer, one may think that the electrical text specifications are written for the electrical contractor, subcontractor, or equipment supplier, but this is not the case. The text specifications are addressed to the
PROJECT DESIGN ELEMENTS MWD Electrical Design Manual
2.1.3.1 Organization. The electrical text specifications will be
prepared in Construction Specifications Institute (CSI) narrative format in the indicative mood. The standard electrical text specifications will consist of sections organized as shown in Metropolitan's ESD-135, Standard
Specifications Sections Catalog.
2.1.3.2 Standard Specifications. The Standard Master Specifications
have been prepared to cover all normal projects that are expected to be designed for or by Metropolitan. It is intended that the engineer will select only those text specification sections that are applicable to the project and then use those sections without changes.
2.1.3.3 Project Specifications. The engineer shall prepare project
specifications in the CSI narrative format for any additional requirements not covered by the Standard Master Specifications. These specifications shall also be prepared in the indicative mood. Only three parts will be provided for in each technical section:
x Part 1--General; x Part 2--Products; x Part 3--Execution.
2.2 BASIC ELECTRICAL ENGINEERING FORMULAS
2.2.1 List of Symbols E = voltage (volts) I = current (amps) R = resistance (ohms) X = reactance (ohms) Z = impedance (ohms) P = power (watts) VA = voltampere W = watt
ș = angle whose cosine is the power factor ĭ = phase
Eff = efficiency
2.2.2 Direct Current (dc) Formulas
Basic formulas for dc current include:
Voltage (E) = Current (I) x Resistance (R) (Eq. 2-1) Power (P) = E2/R = EI (Eq. 2-2)
P = I2 x R (Eq. 2-3)
2.2.3 Alternating Current (ac), Single Phase
Basic formulas for ac current, single phase, include:
Voltage (E) = Current (I) x Impedance (Z) (Eq. 2-4) Power factor (PF) = cosș (Eq. 2-5) Apparent Power (VA) = E x I (Eq. 2-6)
Reactive Power (vars) = E x I x sinș (Eq. 2-7) Real Power (Watts) = E x I x PF (Eq. 2-8)
ș = arctan (vars/Watts) (Eq. 2-9) PF = Watts/(E x I) = Watts/VA (Eq. 2-10) The voltage drop formula is:
Ed = 2 x (I x R x cosș) + (I x X x sinș) (Eq. 2-11) where:
Ed = voltage drop in circuit sin ș = load reactive factor
X = line reactance for one conductor, in ohms
2.2.4 Alternating Current (AC), Three Phase
Basic formulas for ac current, three phase, include:
Line Voltage (E) = 31/2 x Eĭ (Wye-connected) (Eq. 2-12) Current (I) = 31/2 x Iĭ (Delta-connected) (Eq. 2-13) Apparent Power (kVA) = (31/2 x E x I)/1000 (Eq. 2-14)
Real Power (kW) = kVA x cosș (Eq. 2-15) Reactive Power (kvar) = kVA x sinș (Eq. 2-16) ș = arctan (kvar/kW) (Eq. 2-17)
Power Factor (PF) = cosș = kW/kVA (Eq. 2-18) PF = kW/((E x I x 31/2)/ 1000) (Eq. 2-19) The voltage drop formula is:
Ed = 31/2 x (I x R x cosș + I x X x sinș) (Eq. 2-20) where:
PROJECT DESIGN ELEMENTS MWD Electrical Design Manual
X = line reactance for one conductor in ohms R = line resistance for one conductor in ohms
2.2.5 Motors
Motor (general) formulas include:
1 horsepower (hp) = 746 Watts (Eq. 2-21) Torque (ft-lb) = (hp x 5250)/rpm (Eq. 2-22) Fan hp = (cfm x Pressure)/(33,000 x Eff) (Eq. 2-23) Pump hp = (gpm x Head x
Specific Gravity)/(3960 x Eff) (Eq. 2-24) Motor (single phase) formula is:
Horsepower = (E x I x Eff x PF)/746 (Eq. 2-25) Motor (three phase) formulas include:
Synchronous Speed: ns =
(120)(Frequency)/(# Poles) (Eq. 2-26) Horsepower = (E x I x 31/2 x Eff x PF)/746 (Eq. 2-27)
2.2.6 Power Factor Correction
The size of the capacitor needed to increase the power factor from PF1 to PF2 with the initial kVA given is:
kvar = kVA([1-(PF1)2]1/2 - PF1/PF2[1-(PF2)2]1/2) (Eq. 2-28)
2.3 DESIGN CALCULATIONS
2.3.1 General
Electrical calculations shall be made for all projects and filed in the project notebook. They may be made either manually or by computer programs approved by Metropolitan. As a minimum, the following types of
calculations shall be made where applicable and submitted to Metropolitan for review:
x Load calculations; x Conductor sizing; x Conduit sizing;
x Power factor improvement;
x Transformer primary and secondary circuit sizing; x Voltage drop;
x Motor starting voltage dip; x Short circuit analysis; x Lighting levels;
x Grounding in substations.
Note: All references to the National Electrical code (NEC) for calculations shown in this design manual are based on the 2005 Edition of the NEC.
If computer programs are used to make the calculations, the name and version of the software, along with all input and output data, shall be included in the submittal to Metropolitan. All calculations shall be certified by the signature and stamp of a registered professional electrical
engineer.
2.3.2 Load
Load calculations shall be made using applicable sections of Articles 220, 430, and other sections of the NEC. The following load calculations will be used for sizing:
x Feeder conductors and protective devices; x Transformers;
x Panelboard and switchboard main busses; x Motor control center components;
x Service entrance devices and conductors.
Load calculations must include all loads and should be made by summing all of the loads, using appropriate diversity factors as allowed by NEC Article 220, that are connected to each panelboard, switchboard, and motor control center. The loads for each branch of the distribution system can then be summed back to the service entrance equipment.
PROJECT DESIGN ELEMENTS MWD Electrical Design Manual
conductors and feeder conductors in accordance with the requirements of NEC Article 220, the size of service entrance conductors as covered in NEC Article 230, the size of motor branch circuit conductors as covered in NEC Article 430, the size of air conditioning equipment branch circuit conductors as covered in NEC Article 440, the size of generator
conductors as covered in NEC Article 445, the size of transformer primary and secondary conductors as covered by NEC Article 450, and the size of conductors to capacitors as covered in NEC Article 460. In this section we will look at the general requirements for sizing conductors once the
calculated load current is known.
Paragraphs 210.19 and 215.2 of the NEC require that branch circuit and feeder conductors have an ampacity not less than the load to be served. NEC Paragraph 220.18 contains additional information relative to branch circuit loads. Once branch circuit and feeder loads have been determined using applicable sections of NEC Article 230 and other applicable articles, conductor sizes shall then be determined using Tables 310.16 through 310.20 of the NEC for conductors zero through 2,000 volts and
Tables 310.67 through 310.86 of the NEC for conductors rated above 2,000 volts. The four examples presented below are based on the ampacities presented in NEC Table 310.16 as modified by the applicable correction factors for temperature and conduit fill.
2.3.3.1 Example No. 1. Conditions: Continuous load rated 37 amps
served by a conduit containing only the conductors for the load, running through an area having an ambient temperature of 38o C. Conductors shall be copper with type TW insulation.
Required ampacity per NEC Paragraphs 210.19 and 210.18: Ampacity required = continuous load x 125%
= 37 x 1.25 or 46.25 amps
A No. 6 AWG copper conductor having an ampacity of 55 amps would appear to be the correct choice.
Where the ambient temperatures exceed the 30o C ambient that NEC Table 310.16 is based on, the allowable ampacity of the conductor must be corrected using the correction factors at the bottom of Table 310.16 as required by NEC Paragraph 310.10. Corrected ampacity of No. 6 conductor = 55 x correction factor (0.82)
or
Because an ampacity of 46.25 amps is required, this conductor is not adequate and the next larger size (or a conductor with different insulation) will need to be used.
2.3.3.2 Example No. 2. Conditions: The same load and ambient
temperature as above but with six phase conductors in the same conduit. Assume that the conductors used above were No. 6 copper with RHW insulation.
Corrected ampacity of No. 6 RHW = 65 x 0.88 = 57.2 amps Where more than three current carrying conductors are
contained in the same raceway, the ampacity of the conductors must also be derated by the ampacity adjustment factors contained in NEC Table 310.15(B)(2)(a).
Corrected ampacity of No. 6 RHW conductor = 57.2 (ampacity corrected for temperature) x 0.8 (ampacity adjustment factor) = 45.7amps
Because an ampacity of 46.25 amps is required, this conductor size is not satisfactory for this application. A larger conductor or a different configuration must be used.
2.3.3.3 Example No 3. Conditions: A feeder with 200 amps of
noncontinuous load and 65 amps of continuous load to be installed in conduit in a wet area with an ambient temperature of 30o C or less.
Required ampacity per NEC Paragraph 215.2 = noncontinuous load + 1.25 x continuous load or 200 + 1.25 x 65 = 281.25 amps
The feeder overcurrent device would be sized at 300 amps since that is the next largest standard rating (see Article 240 of the NEC).
The conductor ampacity requirement can be met by either one 300 kCMIL conductor or two 1/0 conductors with RHW
insulation per phase. Because the ampacity of one 300 kCMIL RHW conductor is only 285 amps, NEC Paragraph 240.4(B) must be invoked.
PROJECT DESIGN ELEMENTS MWD Electrical Design Manual
2.3.3.4 Example No. 4. Conditions: The same load as used in
example No. 3 but the conduit is to be installed in a dry area with an ambient temperature of 38o C.
Required ampacity calculated above = 281.25 amps. Ampacity of one 300 kCMIL RHH conductor is 320 in a dry location.
Correction factor for 90o C conductors in a 38o C ambient = 0.91.
Corrected ampacity = 320 amps x 0.91 = 291.2 amps The results are the same as for example No. 3, so NEC Paragraph 240.4(B) must be invoked.
2.3.4 Conduit Size and Fill
Where conductors are installed in conduit, the conduit shall be sized in accordance with Tables C.1 through C.12(A) in Annex C of the NEC, and all associated notes. Following are two examples of how conduits can be sized under different circumstances.
2.3.4.1 Example No. 1. Conditions: Three 4/0 AWG conductors with
RHH/RHW insulation installed in rigid steel conduit (no separate ground conductor).
See NEC Table 3C.8 for conduit size required for three 4/0 AWG conductors with RHH/RHW insulation.
NEC Table 3C8 would allow three conductors to be installed in a 2-inch conduit.
2.3.4.2 Example No. 2. Conditions: Three No. 4/0 AWG phase
conductors, one No. 1/0 AWG neutral and one No. 2 AWG equipment ground conductor to be installed in rigid steel conduit. Phase and neutral conductor insulation will be RHH/RHW and the ground conductor will have TW insulation.
Because NEC Table C.8 is for situations where all conductors in a conduit are the same size, they cannot be used for this
example. Table 4 in Chapter 9 of the NEC, using appropriate conduction areas from Table 5 in Chapter 9 of the NEC, must
then be used. Total conductor area:
Conductor size Area 4/0 RHH/RHW 0.4754 1/0 RHH/RHW 0.3039 # 2 TW 0.1333
Total Area = 3(0.4754) + 0.3039 + 0.1333 = 1.8634 sq.in. Conduit size required:
Because more than two conductors that are not lead covered are being installed, the column for 40 percent fill in Table 4 in Chapter 9 of the NEC can be used.
Select conduit with a usable area greater than 1.8634 square inches; therefore, conduit size = 2-1/2 inch (40 percent of total area = 1. 946 sq.in.)
2.3.5 Motor Branch Circuit
NEC Article 430, Motors, Motor Circuits, and Controllers, covers the provisions for motors, motor circuits, and controllers. NEC Article 430 includes tables for motor full-load currents, which are the minimum values that can be used in determining sizes of motor branch circuits, motor feeders, short circuit and overcurrent device sizes and settings, and miscellaneous load calculations. Actual nameplate currents should be used if they are known and must be used if they are larger than the minimum. The full load current to be used for motors with speeds less than 1,200 rpm should be obtained from the motor manufacturer. NEC Article 440 contains special provisions that apply to the installation of air-conditioning and refrigeration equipment and should be referred to for these applications.
The following calculations and the accompanying table are based on the applicable provisions of NEC Article 430 and are provided as a guide for performing motor branch circuit and feeder calculations and for sizing components for motor branch circuits as part of a design. The typical calculations that are required are demonstrated by the following examples.
PROJECT DESIGN ELEMENTS MWD Electrical Design Manual
2.3.5.1 Example No. 1. Conditions: Induction motor is rated 60 hp,
460 volts, three-phase, 1,800 rpm continuous, and will be powered by a combination motor starter through a conduit system. All equipment and the conduit system is located in areas with ambient temperatures of 30o C or less.
In NEC Table 430.250, the motor full-load current that must be used in the calculations is 77 amps. Using this value we can size the motor branch circuit and ground fault protection device, the branch circuit conductors, and the motor disconnecting means.
Motor branch circuit and ground fault protection devices are to be sized as outlined in Part IV of NEC Article 430 with maximum settings as provided in NEC Table 430.52. Actual settings should reflect the recommendation of the manufacturer of the motor control equipment that will be provided.
For example, the following are General Electric's recommendations: Device type Rating
Magnetic only circuit breaker 100 amp Thermal magnetic breaker 125 amp Time delay fuses 90 amp
Branch circuit conductors shall be sized in accordance with the requirements of Part II of NEC Article 430. NEC Paragraph 430.22 requires that conductors supplying a motor must have an ampacity not less than 125 percent of the full-load current of the motor. A special exception is made for motors that are operated intermittently for short periods of time.
Motor branch circuit ampacity shall be equal to or greater than: 77 amps x 1.25 = 96.25 amps
Conductor size to be No. 1 AWG copper with RHH insulation No. 1 AWG = 110 amps at 60o C
Note: 60o C ampacity rating of conductors No. 1 AWG and smaller must be used unless the engineer is sure that all terminals are rated for use at 75o C--see the Underwriters Laboratories, Inc. General Information Directory for more details on this subject.
Motor disconnecting means shall be sized in accordance with the requirements of Part IX of NEC Article 430. The discon-necting means for motor circuits rated 600 volts, nominal, or less, shall have an ampere rating of at least 115 percent of the full-load current rating of the motor.
Motor disconnecting means shall be sized greater than: 77 amps x 1.15 = 88.5
Disconnect to be rated 100 amps
See Table 2-1 for the conduit and conductor requirements for motors typically found in design projects.
2.3.5.2 Example No. 2. Conditions: Determine the size of the feeder
conductors and thermal magnetic circuit breaker feeding a motor control center that has a total connected motor load of 215 amps with the uppermost 60-hp motor being the largest motor. In addition, there are 45 amps of continuous load and 65 amps of noncontinuous load.
Conductors shall be copper with type RHH/RHW insulation installed in an area where the ambient temperature is less than 30o C. Assume all motors are 460 Volt, 3 phase and 1800 rpm.
Motor feeder conductors shall be sized in accordance with applicable portions of Part II of NEC Article 430 and feeder breakers shall be sized in accordance with applicable portions of Part V of NEC Article 430. NEC paragraph 430.24 requires that the conductors supplying the motor control center have an ampacity not less than 125 percent of the full-load current rating of the highest rated motor plus the sum of the full-load current ratings of all other motors in the group, as determined by Paragraph 430.6(A), plus the ampacity required for the other loads. The required ampacity of the conductors shall be calculated as follows:
Total motor load + 25% of largest motor FLA + noncontinuous load + 125% of continuous load
or 215 + (.25 x 77) + 65 + (1.25 x 45) = 355 amps
Conductors may be either one 500 kCMIL or two No. 3/0 AWG per phase (one 500 kCMIL = 380 amps, two No. 3/0 =
PROJECT DESIGN ELEMENTS MWD Electrical Design Manual
NEC Paragraph 430.62 covers the requirements for sizing the motor feeder short-circuit and ground-vault protection.
NEC Paragraph 430.63 covers the requirements for sizing the feeder protection when the feeder supplies a motor load and other power and lighting loads.
Table 2-1. Motor Circuit Design Data 480 Volt, Three-Phase Motors HP Mcp Size Starter Size FLA FLA *1.50 Conductor Size Conduit Size Max. Dist. 1/2 3/M 1 1 1.25 3#12,1#12G 3/4” 5,333 3/4 3/M 1 1.4 1.75 3#12,1#12G 3/4” 3,810 1 3/M 1 1.8 2.25 3#12,1#12G 3/4” 2,963 1-1/2 7/M 1 2.6 3.25 3#12,1#12G 3/4” 2,051 2 7/M 1 3.4 4.25 3#12,1#12G 3/4” 1,569 3 7/M 1 4.8 6.00 3#12,1#12G 3/4” 1,111 5 15/M 1 7.6 9.50 3#12,1#12G 3/4” 701 7.5 15/M 1 11 13.75 3#12,1#12G 3/4” 485 10 30/M 1 14 17.50 3#12,1#12G 3/4” 381 15 30/M 2 21 26.25 3#10,1#10G 3/4” 403 20 50/M 2 27 33.75 3#8,1#10G 1” 485 25 50/M 2 34 42.50 3#6,1#10G 1-1/4” 580 30 100/M 3 40 50.00 3#6,1#10G 1-1/4” 493 40 100/M 3 52 65.00 3#4,1#8G 1-1/4” 577 50 100/M 3 65 81.25 3#3,1#6G 1/1/2” 554 60 250/M 4 77 96.25 3#1,1#6G 2” 719 75 250/M 4 96 120 3#1,1#6G 2” 577 100 250/M 4 124 155 3#2/0,1#12G 2” 611 125 250/M 5 156 195 3#3/0,1#12G 2-1/2” 577 150 300 5 180 225 3#4/0,1#12G 2-1/2” 615 200 400 5 240 300 3#350Kcm,1#3G 3” 600
Notes:1) Conductor ampacity is based on 60 C through size No. 1 AWG and on 75 C above size No. 1 AWG.
2) Use thermal/magnetic circuit breakers in all autotransformer type starters.
3) Conduit size is based on NEC Table 4 and 5, and areas are based on conductor insulation Type RHH/RHW.
4) Conductor size is based on 125% of motor full load current.
5) Maximum distance is based on an allowed voltage drop of 3%. These distances are calculated using Table 2-3 assuming copper conductors in rigid metal conduit and a PF of 80%.
PROJECT DESIGN ELEMENTS MWD Electrical Design Manual
For the above example, a 400-amp device would be selected. For the 400-amp device to be used to protect the 500-kcm conductors, NEC Paragraph 240.4(B) needs to be invoked.
2.3.6 Power Factor Correction Capacitors
Power factor correction capacitors are installed for either one of the following reasons:
x To increase the measured power factor at the serving utilities meter and reduce the power factor penalty being imposed by the utility. Power factor correction for this reason cannot be justified unless the serving utility actually has a power factor penalty in their rate schedule.
x To release additional capacity in existing feeder conductors. For example, a three-phase load of 200 kW would be equal to 301 amps at 480 volts if the power factor were 80 percent, but would be only 254 amps if the power factor were raised to 95 percent. This would release 47 amps of capacity for additional loads.
Article 460 of the NEC covers the installation of capacitors on electric circuits. In this section those calculations needed to determine the size of the capacitor required and the size of conductors required to connect the capacitors to their electric power supply will be discussed. Following are several examples to illustrate the required calculations:
2.3.6.1 Example No. 1. Conditions: A load of 200 kVA exists at
480 volts with a power factor of 80 percent. Determine the amount of capacitors required to improve the power factor to 95 percent.
Power factor = Real power (kW) y
Apparent power (kVA) (Eq. 2-29) Figure 2-1 is provided to show the relation that exists between apparent power, real power, and reactive power (kvar). By definition, the power factor is the cosine of the angle that exists between the real power and apparent power phasors.
The calculation to determine the amount of capacitance (measured in kvar) shall be made as follows:
RESULTANT REACTIVE POWER (kV AR) RESULTANT POWER SUPPLIED BY CAPACITATORS (kV AR) REAL POWER (KW) REQUIRED REACTIVE POWER (kV AR) APPARENT POWER (kVA) T2 T1
PROJECT DESIGN ELEMENTS MWD Electrical Design Manual
kvar @ initial power factor = [(kVA)2- (kW)2]1/2 (Eq. 2-30)
or [(kVA)2 - (kVA X PF)2]1/2 (Eq. 2-31) kvar = [(200kVA)2- (160kW)2]1/2
kvar = (40,000 - 25,600)1/2 = 120 kvar
Because the real power of a load is not changed when the power factor is improved, we can use the known real power and desired power factor to calculate the new kvar value in the phasor triangle.
kvar @ 95% power factor = [( kW y PF)2 - (kW)2] 1/2 (Eq. 2-32) kvar = [(160 y .95)2 - (160)2]1/2
kvar = (28,366 - 25,600)1/2 = 52.6
Required kvar for correction = 120 - 52.6 = 67.4 kvar
Similar calculations can be made to determine the size of the capacitor required to improve the power factor of a single motor to a higher power factor, but tables are available from capacitor manufacturers to simplify the selection of these capacitors. Capacitors larger than the maximum size recommended by motor manufacturers must not be installed.
2.3.6.2 Example No. 2. Conditions: Load is a 60-hp, 1,800-rpm motor
operating at 480 volts, three-phase. Capacitors rated 15 kvar at 480 volts are being installed to improve the power factor. Determine the size of the conductor needed to meet the requirements of the NEC.
NEC Paragraph 460.8 contains two criteria that must be met when sizing branch circuit conductors to capacitors. First, the ampacity of the
conductors must be at least 135 percent of the rated current of the
capacitors. Second, if the capacitors are connected to a motor circuit, the conductors to the capacitor shall have an ampacity not less than one third of the ampacity of the motor branch circuit conductors.
Capacitor rated amps = 15 y (0.48 x 1.73) = 18 Branch circuit amps = 18 x 1.35 = 24.4 minimum Motor branch circuit amps = 1.25 x 77 = 96.25
Need to use No. 1 AWG at 110 amps (60o C ampacity) Capacitor branch circuit amps as one-third of motor branch circuit conductor ampacity = 110 y 3 = 36.7 amps
Therefore, the branch circuit conductors to the capacitor must have an ampacity of 37 amps or greater.
Refer to the Industrial Power Systems Handbook by Beeman or Electrical
Systems Analysis and Design for Industrial Plants by Lazar for additional
formulas related to the application of capacitors on electrical systems.
2.3.7 Transformer Primary and Secondary Conductors
Article 450 of the NEC, Transformers and Transformer Vaults, covers the installation of all transformers. Article 450 deals with transformers over 600 volts nominal and transformers 600 volts, nominal, and less. The calculations most often made during an electrical system design are for a transformer 600 volts, nominal, or less with both primary and secondary protection.
The following calculations and Table 2-2 are based on the provisions of NEC Paragraph 450.3(B). Primary conductors and feeder overcurrent and ground fault protection devices (feeder breakers) are sized for the next larger device above 150 percent of the transformer full-load amps to minimize the possibility of the feeder breaker tripping on transformer inrush (NEC would allow breaker to be sized up to 250 percent of primary full load amps). The secondary conductors and secondary breaker are sized at the standard rating that is nearest to 125 percent of the calculated secondary full load current as required by the NEC. Note 1 to Table 450.3(B) of the NEC allows moving up to the next higher standard rating. Following are two examples to show the calculations that are required for three-phase and single-phase transformers.
2.3.7.1 Example No. 1. Conditions: Assume a 45-kVA 3-phase
transformer with a 480-volt primary and a 208Y/120-volt secondary. Calculate primary full-load amps:
45 kVA y [(480 volts x 1.73) y 1000] = 54.2 amps
Calculate required feeder breaker and conductor ampacity: 54.2 amps x 1.5 = 81 amps
Table 2-2. 480-Volt Lighting Transformer Circuit Design Chart (75 q C) Transfo rmer Primary Circuit Seconda ry Circuit KVA Phase Volts Amps Ckt. Amp s Ckt. Condui t & W ir e Ckt. B reak er Volts Amps Ckt. Amp s Ckt. Condui t & W ir e Ckt. B reaker 5 1 480 10 15.00 3/4" C-2# 12 ,1#8G 20A/2P 240 21 26.25 3 /4" C-3# 10 ,1#8G 30A/2P 7.5 1 480 16 24.00 3/4" C-2# 10 ,1#8G 25A/2P 240 31 38.75 1" C-3# 8 ,1#8G 40A/2P 10 1 480 21 31.50 1" C-2# 8 ,1#8G 30A/2P 240 42 52.50 1 1/2" C-3# 6,1#8 G 60A/2P 15 1 480 31 46.50 1 1/4" C-2# 6,1#8 G 50A/2P 240 63 78.75 1 1/2" C-3# 4,1#8 G 80A/2P 25 1 480 52 78.00 1 1/4" C-2# 3,1#8 G 80A/2P 240 104 130.00 2" C-3# 1 ,1#6G 150A/2P 37.5 1 480 78 117.00 1 1/2" C-2# 1,1#6 G 125A/2P 240 156 175.00 2”6,3#3 /0 ,1#4G 200A/2P 50 1 480 104 156.00 2" C-2# 2/0 ,1#6G 175A/2P 240 208 260.00 2 1/2" C-3# 4/0 ,1#2 G 250A/2P 9 3 480 11 16.50 3/4" C-3# 12 ,1#8G 20A/3P 208 25 31.25 1 " C-4# 10 ,1#8G 35A/3P 15 3 480 18 27.00 3/4" C-3# 10 ,1#8G 30A/3P 208 42 52.50 1 1/2" C-4# 6,1#8 G 60A/3P 30 3 480 36 54.00 1" C-3# 6 ,1#8G 60A/3P 208 83 103.75 1 1/2" C-4# 2,1#6 G 110A/3P 45 3 480 54 81.00 1 1/2" C-3# 3,1#8 G 90A/3P 208 125 156.25 2 1/2" C-4# 2/0 ,1#4 G 175A/3P 75 3 480 90 135.00 2" C-3# 1/0 ,1#6G 150A/3P 208 208 260.00 2 1/2" C-4# 4/0 ,1#2 G 250A/3P _ ________ ____ _________ ____ _ _ _______ ____ _________ ____ _________ ____ _________ ____ _________ ____ _________ ____ _________ ____ _________ ____ _________ ___ Rules Used: 1)
Feeder circuit breaker at ne
xt size larger tha
n 1.5 times primary am ps (NEC 450.3 (b ) allow s up t o 25 0% of p rima ry amp s) . 2) Panel main breaker sized at
next size larger t
han 1.25 times secondar
y amps. (NEC 450. 3(B) allow s up to n e xt l a rger th an 125 % of sec. Amps)
3) All conductors No.1 AWG
a
nd smaller sized
based on 60
q
C
ampacities, larger conductor sizes based on 75
q
C
ampacities. (Conductors sized per
NEC 240 -4 including exceptions. 4) Minimum g round conducto r sized at #8; Table 250.122 used f o r other primar y
side grounds and
Table 250.66
used for secon
dar
y side grounds.
5) Con
duit size based on NEC
Chapter 9 Table 3C. _ ________ ____ _________ ____ _________ ____ _________ ____ _________ ____ _________ ____ _________ ____ _________ ____ _________ ____ _________ __ _ _ _________ ___
Use a 90-amp breaker and No. 3 AWG copper conductors* Calculate secondary full-load amps:
45 kVA y [(208 volts x 1.73) y 1000] = 125.06 amps Calculate required secondary breaker size and conductor ampacity: 125.06 amps x 1.25 = 156.3 amps
Use a 150-amp breaker and No. 1/0 copper conductors* Note: This selection limits the continuous load that can be supplied by the transformer to 43.2 kVA ((80% x 208 volts x 150 amps x 1.73) y 1000).
The ground conductors for the above circuits shall be sized in accordance with NEC Tables 250.66 and 250.122. The ground conductor in the feeder to the primary shall be sized as an equipment ground in accordance with NEC Table 250.122. The grounding electrode conductor on the secondary of the trans-former shall be sized as required by NEC Paragraph 250.30 using Table 250.66.
2.3.7.2 Example No. 2. Conditions: Assume a 25-kVA single-phase
transformer with a 480-volt primary and a 120/240-volt secondary. Calculate primary full-load amps:
25 kVA y (480 volts y 1000) = 52.1 amps
Calculate required feeder breaker size and conductor ampacity: 52.1 amps x 1.5 = 78.1 amps
Use an 80-amp breaker and No. 3 AWG copper conductors* Calculate secondary full-load amps:
25 kVA y (240 volts y 1000) = 104.2 amps
Calculate required secondary breaker size and conductor ampacity: 104.2 amps x 1.25 = 130 amps
*
Conductor sizes for examples No. 1 and No. 2 are based on the use of 60q C wire for sizes Nos. 14 through 1 AWG and 75q C wire for sizes No. 1/0 and larger as required by the General
PROJECT DESIGN ELEMENTS MWD Electrical Design Manual
Use a 125-amp breaker and No. 1 AWG copper conductors*.
2.3.8 Voltage Drop
2.3.8.1 Feeder and Branch Circuits.
Fine-print note No. 4 to NEC Paragraph 210.19 says that branch circuit conductors must be sized so that voltage drop on the branch circuit does not exceed 3 percent. Furthermore, it states that the total voltage drop on feeder conductors plus branch circuit conductors must not exceed
5 percent. Fine-print note No. 2 to NEC Paragraph 215.2(A) would allow the voltage drop on a feeder to be 3 percent as long as the total voltage drop to the load is 5 percent or less.
Steady-state voltage drops are caused by current flowing through an impedance. To calculate steady-state voltage drop, the circuit impedance, circuit current, and power factor of that current relative to some voltage must be known. Rigorous methods of calculating voltage drop can be very involved and complicated and for purposes of ordinary use in
designing power circuits for industrial projects, approximate methods are generally satisfactory. IEEE Standard 141 (Red Book) gives the
approximate formula for voltage drop as:
V = IR cos ș + IX sin ș (Eq. 2-33) where:
V = voltage drop in circuit, line to neutral I = current flowing in conductor
R = line resistance for one conductor in ohms X = line reactance for one conductor in ohms ș = angle whose cosine is the load power factor cos ș = load power factor in decimals
sin ș = load reactive factor in decimals
The voltage drop calculated using this formula must be multiplied by 2 for single-phase circuits and 1.73 for three-phase circuits.
Calculations using the above formula are not required for most designs *
Conductor sizes for examples No. 1 and No. 2 are based on the use of 60q C wire for sizes Nos. 14 through 1 AWG and 75q C wire for sizes No. 1/0 and larger as required by the General
Information Directory 1988, published by Underwriters Lab, Inc., because many items of equipment are still not rated with 75q C terminals in these sizes.
because the results obtained using published tables give satisfactory results. The following calculations were made using Table 2-3 which is a reproduction of Table 3-12 of IEEE STD 141-1993 and the procedure for making the calculations that accompanies Table 3-12. Similar results can be obtained using published tables and graphs available in other reference books and manufacturer's catalogs.
2.3.8.2 Example. Condition: No. 1 AWG copper conductors feeding a
motor rated 60 hp (77 amps full-load), three-phase, 460 volts through rigid metal conduit with a circuit length of 520 feet. Assume that the motor power factor is 85 percent.
Calculate voltage drop on a three-phase circuit from Table 2-3. The factor for No. 1 AWG copper conductors in magnetic conduit at 85 percent PF = 2.7 (need to interpolate between 0.8 and 0.9 PF)
Voltage drop = ((520 ft x 77 amps) y 10000) x 2.7 volts drop = 10.8 volts
Calculate percent voltage drop by dividing the calculated volts drop by the system voltage and then multiplying by 100:
(10.8 volts y 480 volts) x 100 = 2.25 % drop
Factors are provided at the bottom of Table 2-3 and are to be used to convert the calculated voltage drop to phase line-to-line and single-phase line-to-neutral values.
Table 2-3. Three-phas
e line-to-line volt
age drop for 600 V single-conductor cable
Per 10 000 A-ft (60
q C
conductor temperature, 60 Hz)
W
ire size (AW
G or ke m il) P o wer gin g 1 0 0 0 9 0 0 8 0 0 7 5 0 7 0 0 6 0 0 5 0 0 4 0 0 3 5 0 3 0 0 2 5 0 4 /0 3 /0 2 /0 1 /0 1246 8 * 1 0 * 1 2 * 1 4 * n 1: C o ppe r co nd uc to rs in m agne ti c co nd ui t 1. 0 0 0. 2 8 0. 3 1 0. 3 4 0. 3 5 0. 3 7 0. 4 2 0. 5 0 0. 6 0 0. 6 8 0. 7 8 0. 9 2 1. 1 1 .4 1. 7 2 .1 2. 6 3 .4 5. 3 8 .4 13 21 33 53 0. 9 5 0. 5 0 0. 5 2 0. 5 5 0. 5 7 0. 5 9 0. 6 4 0. 7 1 0. 8 1 0. 8 8 1. 0 1 .1 1. 3 1 .5 1. 9 2 .3 2. 8 3 .5 5. 3 8 .2 13 20 32 50 0. 9 0 0. 5 7 0. 5 9 0. 6 2 0. 6 4 0. 6 6 0. 7 1 0. 7 8 0. 8 8 0. 9 5 1. 1 1 .2 1. 3 1 .6 1. 9 2 .3 2. 8 3 .4 5. 2 8 .0 12 19 30 48 0. 8 0 0. 6 6 0. 6 8 0. 7 1 0. 7 3 0. 7 4 0. 8 0 0. 8 5 0. 9 5 1. 0 1 .1 1. 2 1 .4 1. 6 1 .9 2. 3 2 .6 3. 2 4 .8 7. 3 1 1 1 7 2 7 4 3 0. 7 0 0. 7 1 0. 7 3 0. 7 6 0. 7 8 0. 8 0 0. 8 3 0. 8 8 0. 9 7 1. 0 1 .1 1. 2 1 .3 1. 5 1 .8 2. 1 2 .5 3. 0 4 .4 6. 6 9 .9 15 24 38 n 2: C o ppe r co n d u ct o rs in no nm agn eti c co n d u it 1. 0 0 0. 2 3 0. 2 6 0. 2 8 0. 2 9 0. 3 3 0. 3 8 0. 4 5 0. 5 5 0. 6 2 0. 7 3 0. 8 8 1. 0 1 .3 1. 6 2 .1 2. 6 3 .3 5. 3 8 .4 13 21 33 53 0. 9 5 0. 4 0 0. 4 3 0. 4 5 0. 4 7 0. 5 0 0. 5 4 0. 6 2 0. 7 1 0. 8 0 0. 9 2 1. 0 1 .1 1. 5 1 .8 2. 2 2 .7 3. 4 5 .3 8. 2 1 3 2 0 3 2 5 0 0. 9 0 0. 4 7 0. 4 8 0. 5 2 0. 5 4 0. 5 5 0. 5 9 0. 6 8 0. 7 6 0. 8 5 0. 9 5 1. 1 1 .1 1. 5 1 .8 2. 2 2 .7 3. 3 5 .1 7. 9 1 2 1 9 3 0 4 8 0. 8 0 0. 5 4 0. 5 5 0. 5 7 0. 5 9 0. 6 2 0. 6 6 0. 7 3 0. 8 1 0. 8 8 0. 9 7 1. 1 1 .1 1. 4 1 .7 2. 1 2 .5 3. 1 4 .7 7. 2 1 1 1 7 2 7 4 3 0. 7 0 0. 5 7 0. 5 9 0. 6 2 0. 6 4 0. 6 6 0. 6 9 0. 7 4 0. 8 3 0. 8 8 0. 9 7 1. 1 1 .1 1. 4 1 .6 2. 0 2 .4 2. 8 4 .3 6. 4 9 .7 15 24 38 n 3: A lum inum con d u ct or s i n m agne tic c o nd u it 1. 0 0 0. 4 2 0. 4 5 0. 4 9 0. 5 2 0. 5 5 0. 6 3 0. 7 4 0. 9 1 1. 0 1 .2 1. 4 1 .7 2. 1 2 .6 3. 3 4 .2 5. 2 8 .4 13 21 33 52 --0. 9 5 0. 6 2 0. 6 5 0. 7 0 0. 7 3 0. 7 6 0. 8 3 0. 9 4 1. 1 1 .2 1. 4 1 .6 1. 8 2 .3 2. 7 3 .4 4. 2 5 .3 8. 2 1 3 2 0 3 2 5 0 --0. 9 0 0. 6 9 0. 7 2 0. 7 6 0. 7 9 0. 8 2 0. 8 8 0. 9 9 1. 2 1 .3 1. 4 1 .6 1. 9 2 .3 2. 7 3 .4 4. 1 5 .1 7. 9 1 2 1 9 3 0 4 8 --0. 8 0 0. 7 6 0. 8 0 0. 8 3 0. 8 5 0. 8 8 0. 9 5 1. 0 1 .2 1. 3 1 .4 1. 6 1 .8 2. 2 2 .6 3. 2 3 .9 4. 7 7 .3 11 17 27 43 --0. 7 0 0. 8 0 0. 8 3 0. 8 7 0. 8 9 0. 9 2 0. 9 8 1. 1 1 .2 1. 3 1 .4 1. 6 1 .7 2. 1 2 .4 2. 9 3 .6 4. 3 6 .5 10 15 24 37 --n 4: A lum inum con d u ct or s i n no nm agn et ic c o n d u it 1. 0 0 0. 3 6 0. 3 9 0. 4 4 0. 4 7 0. 5 1 0. 5 9 0. 7 0 0. 8 8 1. 0 1 .2 1. 4 1 .7 2. 1 2 .6 3. 3 4 .2 5. 2 8 .4 13 21 33 52 --0. 9 5 0. 5 2 0. 5 6 0. 6 0 0. 6 3 0. 6 7 0. 7 4 0. 8 5 1. 0 1 .1 1. 3 1 .5 1. 8 2 .2 2. 7 3 .4 4. 2 5 .2 8. 2 1 3 2 0 3 2 5 0 --0. 9 0 0. 5 7 0. 6 1 0. 6 5 0. 6 8 0. 7 1 0. 7 9 0. 8 9 1. 1 1 .2 1. 3 1 .5 1. 8 2 .2 2. 6 3 .3 4. 1 5 .0 7. 9 1 2 1 9 3 0 4 8 --0. 8 0 0. 6 3 0. 6 6 0. 7 1 0. 7 0 .7 6 0 .8 3 0 .9 2 1 .1 1. 2 1 .3 1. 5 1 .7 2. 1 2 .5 3. 1 3 .8 4. 6 7 .2 11 17 27 42 --0. 7 0 0. 6 6 0. 6 9 0. 7 3 0. 7 5 0. 7 8 0. 8 3 0. 9 2 1. 1 1 .1 1. 3 1 .4 1. 6 1 .7 2. 3 2 .8 3. 4 4 .2 6. 4 9 .9 15 24 37 --d c o nd uc to r. O ther c o n d u ct or s are st an d ard.
To convert voltage drop to
Multiply by Single-phase, three-wir e, line-to-line 1.15 Single-phase, three-wire , line-to-neutral 0.577 Three-phase, line-to-neutral 0.577 Reproduced fro m IEEE Std 141-1993 , IEEE Reco m m en
ded Practice for El
ectric Power
Distribution for Industrial Plants, ©
1994, by
the Institute of Electrical
an
d Electronics
Engineers, Inc., with
the perm
ission of the IEEE.
2.3.8.3 Motor Starting. The calculations required to determine the
voltage drop on an electrical system because of motor starting are too complex to be covered in this design guide. The Industrial Power Systems
Handbook by Beeman and Electrical Systems Analysis and Design for Industrial Plants by Lazar both have very complete sections on this
subject. These calculations are often done as part of a short circuit analysis using a computer program such as ETAP, because they are very complex and are based on much of the same information required to do the short circuit analysis. These calculations should be made based on the largest motor at each load center to determine if the voltage drop on motor starting is of such magnitude that it will cause adverse impacts on other equipment in the system. For instance, a 20 percent voltage dip could cause control relays to drop out since many of these are only designed to operate at voltage levels 10 percent below rated voltage.
2.3.9 Short Circuit
The proper selection of protective devices and coordination of their trip settings is based on short circuit calculations. The calculations required to complete a detailed short circuit analysis are very complex and beyond the scope of this design guide. The Industrial Power Systems Handbook by Beeman, Electrical Systems Analysis and Design for Industrial Plants by Lazar, the IEEE Std 141-1993, and many other references contain detailed procedures for performing short circuit analysis.
In those situations where an approximate value of short circuit current is needed for preliminary design purposes, the following abbreviated method can be used to determine a very conservative value. In every situation where this method is used, a detailed calculation, either made by hand or using an approved computer program, shall be made during final design. Calculations to determine an approximate value of symmetrical short circuit current in a power distribution system are shown in the following example:
2.3.9.1 Example. Conditions: The load will be served by a 1,500-kVA
transformer at 480 volts three-phase through a single motor control
center. The fault current available from the utility on the source side of the transformer is unknown, the transformer impedance is assumed to be 5.75 percent (based on published data), and the motor load on the transformer is approximately 75 percent of the rating of the transformer.
The current flowing during a fault at any point in an electrical system is limited by the impedance of the circuits and
equip-PROJECT DESIGN ELEMENTS MWD Electrical Design Manual
occurred. For these simplified calculations we will assume that the only sources are the transformer and the motors connected to the system. Figure 2-2 shows that the motors are connected in parallel with the transformer as impedance with an infinite bus as the source of the fault current.
The basic formula used to calculate short circuit currents is: Short circuit current = volts y total impedance
PROJECT DESIGN ELEMENTS MWD Electrical Design Manual
A point-to-point calculation of short circuit current available at any point can be made using this formula and it is the basic formula used in the per-unit method to calculate short circuit current values in electric power circuits. The reactance of the utility system must be assumed to be zero and the following simplification can be made to determine short circuit current let-through by a transformer:
Approximate transformer per-unit Z = (%Z)(base kVA) y [(100) (transformer kVA)] (Eq. 2-35) If we let base kVA = transformer kVA, then:
per-unit Z of the transformer = %Z / 100 (Eq. 2-36) The basic formula for calculating short circuit current when the per-unit method is used is:
Is.c. rms sym = base kVA y (1.73 x kV x (per-unit Z of the
transformer)) (Eq. 2-37) Because we have let base kVA = transformer kVA and transformer kVA y (1.73 x kV) = Transformer load current for three phase transformers, we can simplify the above formula to:
Is.c. rms sym = Transformer FLA y (%Z y 100) (Eq. 2-38)
Transformer FLA = 1,500 kVA y (0.48 x 1.73) or FLA = 1,806.4 amps (Eq. 2-39) The resulting short circuit current let through by the transformer in our example would be:
Is.c.rms sym= 1806.4 y (5.32*y 100) or Is.c. rms sym= 33,914 amps
*.Note: Published transformer impedances are subject to a ±7.5 percent tolerance. To be conservative in these calculations, the lower limit of 5.32 percent (5.75 - (0.075 x 5.75)) has been used.
The motor contribution to a fault by a single or group of low-voltage induction motors can be taken as approximately four times the motor full-load current since the reactance of a low-voltage induction motor,
including the leads, is approximately 25 percent. A point-to-point
calculation made as above for a transformer would result in a multiplier of 4.
Motor load of 75 percent of transformer rating was given; therefore, motor FLA would be 1,806 x .75 = 1,355 amps.
Is.c. rms sym = 1,355 x 4 = 5,420 amps
The total short circuit current available at the point of the fault would be the total of the contribution from the transformer plus the contribution for the motor load
or total Is.c. rms sym= 33,914 + 5,420 or 39,334 amps rms sym
Because neither the serving utilities' source impedance nor the imped-ances of the interconnecting conductors and equipment are included in this calculation, this value can be very conservative and must be used carefully.
2.3.10 Lighting
Lighting calculations shall be made using the recommended procedures established by the Illuminating Engineering Society and outlined in the IES
Lighting Handbook. Two methods are available for calculating the lighting
levels in a space. The first is the lumen or zonal cavity method and the second is the point-by-point method. The zonal cavity method is used to calculate the average footcandle level within the space and the point-by-point method is used to predict the illumination for a specific visual task. The following examples are provided to demonstrate these two calculation methods.
2.3.10.1 Example No. 1--Lumen or Zonal Cavity Method. Conditions:
Design a lighting system for a room 15 feet x 25 feet having an 11-foot ceiling that will be used for general office work. The ceiling will be lay-in ceiling tile and the walls will be painted an off-white. The luminaries will be cleaned regularly and lamps will be group-replaced when the first failures start to occur.
PROJECT DESIGN ELEMENTS MWD Electrical Design Manual
Basic equations:
Footcandles = total lumens striking area
square feet of area (Eq. 2-40) Footcandles = lamps x lumens per lamp x CU x LLF
area (Eq. 2-41) where:
CU = coefficient of utilization
LLF = light loss factor, which is made up of a number of factors. The ones to be included in most calculations are the LLD, LDD, and RSD
LLD = lamp lumen depreciation LDD = luminaire dirt depreciation RSD = room surface depreciation
The coefficient of utilization (CU) of a luminaire is calculated by the zonal cavity method and is a measure of how a specific luminaire distributes light into a given room. The CU takes into account luminaire efficiency, candlepower distribution of the luminaire, room size and shape, mounting height, and surface reflectances. The CU for a specific luminaire must be obtained from the manufacturer's catalog.
To determine the CU for a specific application, several values must be determined.
x Effective floor cavity reflectance; x Effective ceiling cavity reflectance; x Wall reflectance;
x RCR or room cavity ratio.
Most CU tables are based on a floor cavity ratio (pfc) of 20, so that figure will be used for this example (Table 2-4).
If the suspension length of the luminaire below the ceiling is zero, which it is for this example, the ceiling cavity ratio is equal to the ceiling
reflectance. If the luminaire is suspended, a ceiling cavity ratio must be calculated before the effective ceiling cavity reflectance can be
determined. Reflectance values for various surfaces are available in the
IES Lighting Handbook. For this example, 70 percent will be used.
Lighting Handbook. For this example, 70 percent will be used.
The room cavity ratio (RCR) must be calculated and it is equal to 2.5 times the area of the walls divided by the area of the work place.
RCR = (2.5) (room height - work plane height) x perimeter of walls y area (Eq. 2-42) The work plane height is the level at which most tasks will be performed and is assumed to be 30 inches for this example.
RCR = (2.5) (11-2.5) (2 (15+25)) = 4.53 (15) (25)
Table 2-3. Coefficient of Utilization Zonal Cavity Method
4-Lamp pfc 20 pcc 80 70 50 pw 70 50 30 70 50 30 50 30 RCR 0 76 76 76 74 74 74 70 70 1 70 68 66 69 66 64 64 62 2 65 61 57 64 60 56 58 55 3 60 55 50 59 54 50 52 49 4 56 49 45 55 49 44 47 43 5 51 44 39 50 44 39 42 38 6 48 40 35 46 39 35 38 34 7 44 36 31 43 36 31 35 30 8 40 32 27 40 32 27 31 27 9 37 29 24 36 29 24 28 24 10 35 26 21 34 26 21 25 21 Test No. 7834 S/MH = 1.3.
For 2-lamp: multiply above C.U.s by 1.16. For 3-lamp: multiply above C.U.s by 1.09.
From the table of coefficients of utilization, the resultant coefficient of utilization must be interpolated between 0.55 and 0.50.
PROJECT DESIGN ELEMENTS MWD Electrical Design Manual
performed, there are still several decisions that must be made. x Type of lamp to be used; this affects lumens per lamp.
Assume 3150.
x Lamps per luminaire; this affects the coefficient of utilization, which was calculated. For this calculation, assume four, which is the basis of the CU table.
x LLD must be determined. x LDD must be determined. x RSD must be determined.
x Footcandle level desired must be determined.
Values for LLD, LDD, RSD, and a number of other factors that cause light loss in the space can be found in the IES Lighting Handbook but for most calculations dealing with lighting in noncritical areas all of these factors can be combined into a single factor, which is often referred to as the light loss factor (LLF). For this calculation, an LLF of 0.75 has been assumed. Footcandle levels are recommended for a number of applications in the
IES Lighting Handbook. The recommended level for general office work
falls between 50 and 150 footcandles depending on the level of difficulty of the task. For this calculation, the level required is assumed to be
100 footcandles.
Put all of the numbers into a basic equation (Eq. 2-41) and solve it for the number of lamps required:
100 = lamps x 3150 x 0.523 x 0.75 15 x 25
No lamps = 30.4
At four lamps/fixture = 7.6 fixtures
Figure 2-3, Zonal Cavity Calculations, provides a form to be used in making lighting calculation. It, or something similar, shall be used to document all calculations.
Project Name: _____________________________________________________________________________ Date: _______________ Room Name: __________________ Reviewed By: _________________________
A. ROOM DATA B. CAVITY DATA C. FIXTURE DATA
1. Length ft 9. Height ft 17. Mft. 2. Width ft
Room
Cavity 10. Ratio
3. Floor area sq ft 11. Height ft
18. Cat. No. etc. Room
dimen.
4. Ceiling ht. ft 12. Ratio 19. Lamps per fixture 5. Ceiling %
Ceiling Cavity
13. Eff. Reflectance % 20. Lumens per fixture 6. Wall % 14. Height ft 21. Coeff. Of util (cu) Surface Reflect. 7. Floor % 15. Ratio 8. Fixture mounting ht. Floor Cavity 16. Eff reflectance %
22. Light loss factor (LLF)
D. FOOTCANDLES E. CALCULATING CAVITY RATIOS
No. of fixtures required to produce a give number of footcandles
5 x cavity height x (length + width) Cavity ratio =
Length x width 23. Desired lighting level fc ROOM:
24. No. of tootcandles produced by a given no. of fixtures
27. 5 x line 9 x (line 1 + line 2) = line 1 x line 2
5 x __x (__ + __) = __
__ x __ Option A 25. Fixtures fc CEILING:
Option B 26. Fixtures fc 28. 5 x line 11 x (line 1 + line 2) = line 1 x line 2
5 x __x (__ + __) = __
__ x __ FLOOR:
29. 5 x line 14 x (line 1 + line 2) = line 1 x line 2
5 x __x (__ + __) = __
__ x __ F. CALCULATING NUMBER OF FIXTURES
Floor are x desired footcandles 30. No. fixtures =
lamps per fixture x lumens per lamp x coeff. Of utilization x light loss factor 31. No. of fixtures = 5 x line 14 x (line 1 + line 2) =
line 1 x line 2
5 x __x (__ + __) = __
__ x __ G. CALCULATING FOOTCANDLES
32. Footcandles = no. of fixture x lamps per fixture x lumens per lamp x Coeff. Of utilization x maintenance factor floor area
33. Option A Line 25 x line 19 x line 21 x line 22 = line 3
___ x __x __ x __ = __
__ x __ 34. Option B Line 26 x line 19 x line 21 x line 22 =
line 3
___ x __x ___ x ___ = __
__ x __
PROJECT DESIGN ELEMENTS MWD Electrical Design Manual
The next task is to lay out the luminaries in the room to determine if they will fit in a logical arrangement. Since the luminaries are being installed in
a lay-in ceiling, spacing can only be in multiples of 2 feet.
For this example, installation of eight luminaries would require two rows of four luminaries each.
Spacing across room = 15 y 2 or 8 feet
Between rows and (15 - 8) y 2 or 3.5 feet between wall and closest luminaire (all dimensions are to centerline)
Spacing length of room = 25 y 4 or 6 feet
Between luminaries in the row and (25 - (3 x 6)) y 2 or 3.5 feet from the wall to the end luminaries
The footcandle level that results from the number of luminaries to be installed should then be checked:
Footcandles = (8 x 4) x 3150 x 0.523 x 35 = 105.4 (15) (25)
The maximum spacing of the luminaries shall also be checked against the mounting height above the work plane (S/MH ratio) to determine if it is within the ratio of the luminaire being used.
8 ft spacing y 8.5 ft mounting height = 0.94
This is well within the 1.3 S/MH ratio of the luminaire used in the example. If the luminaries required could not have been fit into the space in a
reasonable layout, or the footcandle levels that resulted from the selected layout were not acceptable, or the S/MH ratio calculated was not less than that of the luminaire being used, then the layout would need to be revised using a luminaire with a different number of lamps or different
characteristics.
The footcandle level calculated tells us the quantity of light that reaches the work surface. Other factors that affect visual comfort and ability to see include direct glare, indirect glare, reflected glare, and veiling reflections. In areas where seeing tasks are critical, these must also be evaluated. See the IES Lighting Handbook and other lighting design and application