SAMPLE ELECTRICAL DESIGN
CALCULATIONS REQUIRED
Calculations shall be done in an orderly manner either on a desktop PC computer or on computation paper. Each sheet shall have the date on which the computations were made, the project number, and the designers name. All information used in the preparation of the design shall be kept in a notebook with tabs to properly divide the different items such as telco memos, comps, letters, equipment data sheets, etc. Each computation shall clearly identify the facility for which the calculations are being made and the type of computation that is being performed. Copies of all calculations shall be sent to project lead electrical engineer as they are completed for his review and filing. At the completion of the project, all pertinent information shall be assembled in a single set of notebooks for inclusion in the project files. The computations listed below are the minimum that need to be documented. All calculations shall be reviewed before the related drawings are drafted.
SAMPLE ELECTRICAL DESIGN
CRITERIA MEMO
MWD Electrical Design ManualWire Sizing
Take into account wet/dry areas and ambient temperatures—see attached form for a guide. Show wiring sizing for all services, feeders, and large branch circuits.
Calculations shall include a summary of all loads to be served where there is more than one load. Calculations for feeders to panelboards shall reference the specific panelboard being supplied and a copy of the panel schedule with all loads indicated shall be included with the calculations.
Primary and secondary feeders to/from dry type transformers shall be sized in accordance with the attached transformer table and the proper sized main breaker shall be shown in the panelboard that is served from the transformer.
Dry Type Transformers
Include a list of all branch circuit panelboards to be supplied and the connected load on each panel. Demand and diversity factors that are allowed by the NEC may be used for sizing transformers that supply loads in areas that are not process related, but transformers in other areas shall be adequately sized to supply the total
connected load connected to the process.
Pad Mounted and Unit Substation Transformers
All power supply transformers shall be sized to supply the total load that is normally connected to the transformer's secondary bus without exceeding the air-cooled rating of the transformer. Where there are three MCC buses, it shall be assumed that the third MCC bus can be "normally connected" to either transformer. Each transformer shall also be able to carry the total load of the load center that would be expected to be operating during peak flow conditions without exceeding the
transformer's overload rating, assuming that one transformer has failed.
Voltage Drop
Prepare steady-state voltage drop calculations for all heavily loaded and/or long branch circuits and feeders using the attached "Voltage Drop Calculation Data." Base calculations for motor circuits on an 80 percent power factor. Motor starting voltage drop calculations shall be shown for all motors that exceed 20 percent of the rating of the serving transformer.
Steady state voltage drop shall be limited to the values listed in the Design Criteria with not more than 2 percent drop on feeder. Motor starting voltage drop shall be limited to 20 percent.
Appendix C
SAMPLE ELECTRICAL DESIGN
CRITERIA MEMO
MWD Electrical Design ManualBranch Circuits
Connected load and NEC requirements shall be used for sizing branch circuit breakers and conductors.
A minimum wire size of No. 12 AWG copper shall be used for lighting and receptacle branch circuits. No. 10 AWG shall be used where the first convenience receptacle is more than 75 feet from the panelboard.
In general, 120 volt lighting branch circuit load shall be used for up to 1800 voltamps.
120 volt lighting loads shall be connected to circuits separate from receptacles except in storage rooms where the lights may be connected to receptacle circuits or vice versa.
Branch circuit shall be limited to five duplex receptacles in process areas and six duplex receptacles in office areas. Special areas may require further reduction in number of receptacles per circuit.
Conduit Size
Calculations shall be included for sizing of all conduits that are not covered by the table of conduit sizes included hereinafter. Conduit fill shall not exceed that allowed by the NEC when all conductors, including the ground conductor, are included in the calculation assuming that ground conductors have TW insulation and phase
conductors have THW insulation.
Power Factor Correction
Calculations shall be made for the sizing of all power correction capacitors. The calculations shall include a statement showing all assumptions that are made to make the calculation. If tables are used, a copy of the table used with appropriate values marked, shall be included in the calculation section of the notebook.
Lighting
Calculations may be in any form. For small areas, a statement that "so many" lights of "such" a size will do the job, and is all that is required. For larger areas, use the "Zonal Cavity Calculations" form attached. The foot-candle level resulting from the actual fixtures to be installed shall be documented.
Fault Study and Coordination
This should take into account future loads and changed conditions. Presume the utility is an infinite bus unless better information can be obtained.
ENCLOSURE TYPES
MWD Electrical Design Manual NEMA Type 1, General Purpose. Enclosures intended for indoor use primarily to provide a degree of protection against limited amounts of falling dirt.NEMA Type 2, Dripproof. Enclosures intended for indoor use primarily to provide a degree of protection against limited amounts of failing water and dirt. Enclosures have provisions for drainage. If provision is made for the entrance of conduit at the top, it consists of a conduit hub or equivalent.
NEMA Type 3, Dusttight, Raintight, and Sleet- (Ice-) Resistant. Enclosures intended for outdoor use primarily to provide a degree of protection against rain, sleet, windblown dust and damage from external ice formation. Enclosures have conduit hubs or equivalent provision for watertight connection at the conduit entrance.
NEMA Type 3R, Rainproof and Sleet- (Ice-) Resistant. Enclosures intended for outdoor use primarily to provide a degree of protection against rain, sleet, and damage from external ice formation.
NEMA Type 3S, Dusttight, Raintight, and Sleetproof (Iceproof) Enclosures intended for outdoor use primarily to provide a degree of protection against rain, sleet, windblown dust and to provide for operation of external mechanisms when ice laden. Enclosures have conduit hubs or equivalent provision for watertight connection at the conduit entrance, mounting means external to the equipment cavity, and provision for locking. NEMA Type 4, Watertight, Dusttight, and Sleet-Resistant. Enclosures intended for indoor or outdoor use primarily to provide a degree of protection against windblown dust and rain, splashing water, hose-directed water and damage from external ice formation. Enclosures have conduit hubs or equivalent provision for watertight connection at the conduit entrance and mounting means external to the equipment cavity.
NEMA Type 4X, Watertight, Dusttight, Sleet- and Corrosion Resistant: Same provisions as Type 4 enclosure, and in addition, are corrosion-resistant.
NEMA Type 6, Submersible, Watertight, Dusttight, and Sleet- (Ice-) Resistant. Enclosures intended for indoor or outdoor use primarily to provide a degree of protection against hose-directed water, and the entry of water during occasional temporary submersion at a limited depth and damage from external ice formation. Enclosures have conduit hubs or equivalent provision for watertight connection at the conduit entrance and mounting means external to the equipment cavity.
NEMA Type 6P, Submersible, Watertight, Dusttight, and Sleet- (Ice-) Resistant. Same provisions as Type 6 enclosure except for protection against entry of water during prolonged submersion at a limited depth.
Appendix D
ENCLOSURE TYPES
MWD Electrical Design ManualNEMA Type 7, Class I, Group A, B, C, or D Hazardous Locations, Air-Break.
Enclosures intended for indoor use in locations classified as Class I, Division 1, Groups A, B, C or D hazardous locations as defined in the National Electric Code (NFPA 70). Enclosures are designed to withstand the pressures of an internal explosion and not ignite an explosive mixture outside the enclosure. Equipment within the enclosure shall be able to interrupt in a flammable atmosphere. Enclosures are commonly referred to as explosion-proof.
NEMA Type 8, Class I, Group A, B, C, or D Hazardous Locations, Oil-Immersed.
Enclosures intended for indoor or outdoor use in locations classified as Class I, Division 2, Groups A, B, C or D hazardous locations as defined in the National Electric Code (NFPA 70). Enclosures are designed such that the enclosed equipment is oil-immersed and can operate at rated voltage and most severe current conditions in the presence of flammable gas-air mixtures without igniting these mixtures. Enclosures are commonly referred to as oil immersed.
NEMA Type 9, Class II, Group E, F, or G Hazardous Locations, Air-Break. Enclosures intended for indoor use in locations classified as Class II, Division I, Groups E, F and G hazardous locations as defined in the National Electric Code (NFPA 70). Enclosures prevent the ingress of hazardous dust and are commonly referred to as dust-ignition proof.
NEMA Type 10. Nonventilated enclosures constructed for mine use and designed to meet the requirements of the Mine Safety and Health Administration.
NEMA Type 11, Corrosion-Resistant and Dripproof, Oil-Immersed. Enclosures intended for indoor use to protect the enclosed equipment against dripping, seepage, and
external condensation of corrosive liquids by providing for immersion of equipment in oil.
NEMA Type 12, Industrial Use, Dusttight, and Driptight. Enclosures intended for indoor use to protect the enclosed equipment against fibers, flyings, lint, dust, and external condensation of noncorrosive liquids. Enclosures have no holes, conduit knockouts or conduits openings, except that oiltight or dusttight mechanisms may be mounted through holes in the enclosure when provided with oil-resistant gaskets.
NEMA Type 13, Oiltight and Dusttight. Nonventilated enclosures intended for indoor use primarily to house control-circuit devices such as limit switches, foot switches, pushbutton, selector switches, and pilot lights and to protect these devices against lint and dust, seepage, external condensation, and spraying of water, oil, or coolant. All conduit openings have provisions for oiltight conduit entry.
MOTOR ENCLOSURE TYPES
MWD Electrical Design ManualOPEN MACHINE: ventilation openings permit passage of external cooling air over and
around the winding of the machine
DRIP-PROOF MACHINE: ventilation openings are so constructed that successful
operation is not interfered with when drops of liquid or solid particles strike or enter the enclosure at any angle from 0 to 15 degrees downward from the vertical.
SPLASH-PROOF MACHINE: ventilation openings are constructed so that successful
operation is not interfered with when drops of liquid or solid particles strike or enter the enclosure at any angle not greater than 100 degrees downward from the vertical.
SEMIGUARDED MACHINE: ventilation openings in the machine, usually in the top
half, are guarded as in the case of a "guarded machine," but the others are left open.
GUARDED MACHINE: all openings giving direct access to live metal or rotating parts
are limited in size by structural parts or by screens, baffles, grilles, expanded metal, or other means to prevent accidental contact with hazardous parts.
DRIP-PROOF GUARDED MACHINES: a drip-proof machine with guarded ventilation
openings.
OPEN EXTERNALLY VENTILATED MACHINE: ventilated by means of a separate
motor-driven blower mounted on the machine enclosure.
OPEN PIPE VENTILATED MACHINE: openings for the admission of the ventilation are
so arranged that inlet ducts or pipes can be connected to them. Machine shall be self-ventilated or force-ventilated, external from and not a part of the
machine.
WEATHER PROTECTED MACHINE TYPE I: ventilation passages constructed so as to
minimize the entrance of rain, snow, and airborne particles.
WEATHER PROTECTED MACHINE TYPE II: ventilation passages at both intake and
discharge are arranged so that high velocity air and airborne particles blown into the machine by storms or high winds can be discharged without entering the internal ventilating passages.
TOTALLY ENCLOSED MACHINE: enclosed to prevent the free exchange of air
Appendix E
MOTOR ENCLOSURE TYPES
MWD Electrical Design ManualTOTALLY ENCLOSED NONVENTILATED MACHINE: totally enclosed machine that is
not equipped for cooling by means external to the enclosed parts.
TOTALLY ENCLOSED FAN-COOLED MACHINE: equipped for exterior cooling by
means of a fan or fans integral with the machines but external to the enclosed parts.
EXPLOSION-PROOF MACHINE: enclosure designed and constructed to withstand an
explosion of a specified gas or vapor that may occur within the enclosure and to prevent ignition of the specified gas or vapor surrounding the machine by sparks, flashes, or explosions of the specified gas or vapor.
DUST IGNITION-PROOF MACHINE: enclosure designed and constructed in a manner
that will exclude ignitable amount of dust or amounts that might affect
performance or rating, and which will not permit arcs, sparks, or heat to cause ignition of exterior accumulations or atmospheric suspensions of a specific dust.
WATERPROOF MACHINE: constructed so that it will exclude water in the form of a
stream from a hose, except that leakage may occur around the shaft that provides for automatic drainage.
TOTALLY ENCLOSED PIPE VENTILATED: constructed with openings so arranged
that when inlet and outlet ducts or pipes are connected to them there is no free exchange of the internal air and the air outside the enclosure may be
self-ventilated or force-ventilated.
TOTALLY ENCLOSED WATER COOLED MACHINE: cooled by circulating water, with
the water or water conductors coming in direct contact with machine parts.
TOTALLY ENCLOSED WATER-AIR-COOLED MACHINE: cooled by circulating air
which, in turn, is cooled by circulating water. Machine is provided with a water-cooled heat exchanger for cooling the internal air and a fan or fans, either integral with the rotor shaft or separate, for circulating the internal air.
TOTALLY ENCLOSED AIR-TO-AIR COOLED MACHINE: cooled by circulating the
internal air through a heat exchanger which, in turn, is cooled by circulating external air. Machine is provided with an air-to-air heat exchanger for cooling the internal air and a fan or fans, either integral with the rotor shaft or
separate, for circulating the internal air and a fan or fans, either integral with the rotor shaft or separate but external to the enclosing part or parts, for circulating the external air.
MOTOR ENCLOSURE TYPES
MWD Electrical Design ManualTOTALLY ENCLOSED FAN-COOLED GUARDED MACHINE: all openings giving
direct access to the fan are limited by size or design of the structural parts, and have screens, grilles, expanded metal, etc. to prevent accidental contact with the fan.
TOTALLY ENCLOSED AIR-OVER MACHINE: intended for exterior cooling by
Appendix F
MOTOR DESIGN TYPES
MWD Electrical Design ManualThe polyphase induction motor shall be of either the squirrel-cage or the wound-rotor type. The squirrel-cage induction motor has been classified by National Electrical Manufacturers Association (NEMA) Tests and Performance-AC (MG1-1987) according to the following designs.
Design A. A Design A motor is a squirrel-cage motor designed to withstand full-voltage
starting and to develop locked-rotor torque. It has a breakdown torque as shown in Table MG1-12-39. It has a locked-rotor current higher than the value shown in Table MG1-12-35 and a slip at rated load of less than 5 percent.
Design A motors are usually used for applications where extremely high efficiency and extremely high full-load speed are required. Therefore, Design A motors tend to be special motors.
Design B. A Design B motor is a squirrel-cage induction motor designed to withstand
full-voltage starting, developing locked-rotor and breakdown torques adequate for general application as specified in Tables MG1-12-38.1 and MG1-12-39, drawing locked-rotor current not to exceed the values shown in Table MG1-12-35, and having a slip at rated load of less than 5 percent. Motors with 10 and more poles may have a slip slightly greater than 5 percent.
Design B motors are the standard general-purpose motors used where low locked-rotor current and moderate locked-rotor torque are required along with high full-load speed and efficiency.
Design C. A Design C motor is a squirrel-cage motor designed to withstand full-voltage
starting, developing locked-rotor torque for special high-torque applications up to the values shown in Table MG1-12-38.2, breakdown torque up to the values shown in Table MG1-12-39.2, with locked-rotor current not to exceed the values shown in Table
MG1-12-35, and having a slip at rated load of less than 5 percent.
Design D. A Design D motor is a squirrel-cage motor designed to withstand full-voltage
starting, developing high locked-rotor torque as shown in Table MG1-12-38.2, with locked-rotor current not greater than that shown in Table MG1-12-35, and having a slip at rated load of 5 percent or more.
Design F. A Design F motor is a squirrel-cage motor designed to withstand full-voltage
starting, developing low locked-rotor torque as shown in Table MG1-12-38.1 with breakdown torque as shown in Table MG1-12-38.2, with locked-rotor current not to exceed the values shown in Table MG1-12-35, and having a slip at rated load of less than 5 percent.
MOTOR DESIGN TYPES
MWD Electrical Design Manual The following figure shows typical speed-torque curves of NEMA design-class squirrel cage motors.Appendix G
MOTOR TORQUE DEFINITIONS
MWD Electrical Design ManualThe following terms are commonly used to describe motor torque.
Full load torque – Torque necessary to produce rated horsepower at full-load speed. Locked-rotor (starting) torque – Minimum torque which the motor will develop at rest
for all angular positions of the rotor, with rated voltage applied at rated frequency.
Pull-up torque – Minimum torque developed by the motor during the period of
acceleration from rest to the speed at which breakdown torque occurs. For motors which do not have a definite breakdown torque, the pull-up torque is the minimum torque developed up to rated speed.
Breakdown torque – Maximum torque which the motor will develop with rated voltage
applied at rated frequency, without an abrupt drop in speed.
Pull-out torque (synchronous motor) – Maximum sustained torque which the motor
will develop at synchronous speed with rated voltage applied at rated frequency and with normal excitation.
Pull-in torque (synchronous motor) – Maximum constant torque under which the
motor will pull its connected inertia load into synchronism, at rated voltage and frequency, when its field excitation is applied.