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Home > Home Wiring USA Archive: NEC 1999 > Motor Design > Motor Design (NEC 1999)

Motor Design (NEC 1999)

By Warren Goodrich
Motor Design (NEC 1999)

Motor Designing Step 1;

Scope of this Motor Design section does not Include the Following;

Hermetic Motors [ air conditioning and / or Refrigeration equipment ]. Found in NEC Article 440.

Generators and / or Roto-Phase Generators, and / or Static-Phase Generators. Found in NEC Article 455.


FLC = Full Load Current.

Definition of Full Load Current is the current {in amps} that is required to run a motor at it’s full capacity.

To Find F.L.C. = For use in a design formula you must obtain the F.L.C. rating from the motor or motors from the name plate rating information attached to the motor.

To Find F.L.C. = [ SECOND CHOICE ] = If the motor or motor’s name plate is missing then you must refer to the charts in NEC Table 430/148 for single phase motors, and NEC Table 430/150 for three phase. You must know the horse power rating and the voltage rating of the motor to finding the F.L.C. in the NEC 430 Charts.

How to use the charts in the NEC Table 430/148 and NEC TABLE 430/150. You must know special methods discussed as follows;

To use the single phase chart in NEC Table 430/148 you need only to know the horse power and voltage that the motor is rated.

To use the 3 phase chart in NEC Table 430/150 you need to know whether it is a synchronous type or induction type motor. If you are in question use induction type motors for your calculations. Induction type motors are more commonly used. Synchronous motors are more of a specialty application.

Special Notes: The synchronous type is a specialty motor requiring less amperage to run, but more amperage to start {Impact load as applied in the NEC} and are usually known as high efficiency motors.

To find the F.L.C. of a motor rated more than one motor H/P listed and less than next Larger Motor H/P listed, you must interpolate the two sizes and calculate what that motor F.L.C. would be.


The F.L.C. of a 230 volt - single phase - 4 H /P motor would be calculated as follows; The F.L.C. of a 5 H/P motor is 28 amps. The F.L.C. of a 3 H/P motor is 17 amps. Therefore the difference between the two sizes in amps would be 11 amps. The difference in the horse power of a 5 H/P and the 3 H/P motors would be 2 H/P. Therefore you divide the F.L.C. difference 11 amps by the difference in H/P ratings of the two listed H/P motors “2 H/P”, which would equal 5.5 amps difference per H/P. (11 amps divided by 2 H/P = 5.5 amps) You may now discover the F.L.C. of your 4 H/P motor by adding the 5.5 amps {you received in that division of F.L.C. in amps}, to the F.L.C. rating found in the chart for a 3 H/P. (3 H/P=17 amps + 5.5 amps) {found to be the difference in amps per H/P found in your calculation}, and you will find the F.L.C. of a 4 H/P motor is 22.5 amps.

You could also find the same answer by subtracting the difference in F.L.C. amps per H/P 5.5 from the F.L.C. amp rating for a 5 H/P motor 28 amp - 5.5 amps = 22.5 amps for your 4 H/P motor.

You could adjust up from the smaller motor rated, or down from the larger motor rated, after you know the difference in F.L.C. amps per H/P listed. You must calculate each F.L.C. listed above and below the motor F.L.C. amp rating you are working with, that is not listed, because the difference in F.L.C. amps per H/P is not always the same as you change H/P ratings.

Motor Designing Step 2


Overload = Heater or Thermal cut out protection.

Special Notes: All continuous duty motors rated more than one horse power must have some type of an approved overload device. NEC 430/32 An overload shall be installed on each conductor that controls the running of the motor rated more than one horsepower. NEC 430/37 plus the grounded leg of a three phase grounded system must contain an overload also. This Grounded leg of a three phase system is the only time you may install an overload or over - current device on a grounded conductor that is supplying a motor.

To Find the motor running overload protection size that is required, you must multiply the F.L.C. [full load current] with the minimum or the maximum percentage ratings as follows;

Special Notes: Definition of an overload protection would be a device that thermally protects a given motor from damage due to heat when loaded too heavy with work.

Maximum overload = F.L.C. [full load current of a motor] multiplied by the percentage allowed for the maximum setting of an overload, 130% for other motors, found in NEC Article 430/34. Do not overlook the increase of 5% allowed if the marked temperature rise is not over 40 degrees or the marked service factor is not less than 1.15. Again these are specialty motors, and unless you know that your motor meets the requirements of a heavy duty motor load allowing the 5% increase don’t use it.

Minimum Overload = F.L.C. [full load current of a motor] multiplied by the percentage allowed for the minimum setting of an overload, 115% for other motors found in

NEC Article 430/32/B/1. Do not overlook the increase of 10% allowed to 125% if the marked temperature rise is not over 40 degrees or the marked service factor is not less than 1.15. Again these are specialty motors, and unless you know that your motor meets the requirements of a heavy duty motor load allowing the 10% increase to 125% don’t use it.

Special Notes: The minimum we refer to is not really the minimum overload. You may use as small of an overload as you want. I know not the reason that the NEC Article 430/32/A/1 is referred to as a minimum but it is accepted practice. The NEC stands mute as to an actual minimum rating of the required overload protection. I believe that the way the CODE sets up these requirements referring to both NEC Article 430/32/A/1 and NEC Article 430/34 in the CODE book is an invitation to refer these articles as to the minimums and maximums.

Special Notes: The intent of these minimum and maximum settings for overload protection is to prompt you to use the lowest setting that will allow the motor to run correctly without overheating the motor. Use the smallest overload you can.

Special Notes: Most single phase motors have a thermal overload cut out installed in the motor design and installed within the motor. This thermal overload cut out may be either automatic or manually reset. These overload cut outs built into the motor will take the place of and overload required n your wiring design, and additional overload will no be required.

Special Notes: A motor rated one horsepower or less that is non-automatically started and is within sight of a controller shall not require an overload protection. NEC 430/32/B

In Sight = That which is able to be seen when standing at the controller and the motor can be seen. The distance can not be more than

50 feet away. NEC Article 100 Definitions

Special Notes: You may use the maximum overload if the load that the motor it is driving does not allow the minimum overload to facilitate the starting of the motor. NEC Article 430/34

Special Notes: You must use an overload on a 3 phase motor that has a grounded leg [Three phase Grounded Delta System] This would not be a grounded neutral wire. NEC Article 430/37 and 430/36

Special Notes: A fused disconnect may be used as a controller if the disconnect has a horse power rating marked on the disconnect NEC Article 430/83 and if the disconnect contains enough disconnect-able contacts that will stop the motor from trying to run. NEC Article 430/84 This disconnect must be capable of interrupting the locked rotor current of the motor it controls. NEC Article 430/82A This Disconnect may be used as an overload protection if it is fused with the minimum number of overload devices required in NEC Table 430/37. [1 for 120 volt single phase / 2 for 220 volt single phase, and 3 for three phase motors]. NEC Article 430/39 This disconnect that is used as an overload device must be capable of and contain fuses rated within the 115% and 135% of the F.L.C rated for that motor. NEC Article 430/22

Motor Designing Step 3


To find the minimum ampacity of a branch circuit conductor you must multiply the F.L.C. [full load current] by 125% for continuous use. NEC Article 430/22

Special Notes: When referring to the 125% for conductors, the 25% increase is to protect from the overloading of the conductor during continuous use regardless if it is rated or used as continuous.

Special Notes: NEC Article 100 Definition of continuous load = A load where the maximum current is expected to continue for three hours or more.

Special Notes: NEC Article 100 Definition of Ampacity = That which the current in amperes a conductor can carry continuously under the conditions or use without exceeding its temperature rating.

Special Notes: When finding the minimum conductor ampacity of a branch circuit for a motor, you must use & Table 430/150 instead of the name plate rating of the motor being used.

Special Notes: The obelisks in NEC Table 310/16 Through Table 310/19 does not apply to the world of motors. # 14 AWG. wire may be used on a continuous basis in a commercial setting carrying the ampacity figured. Starting with # 14 AWG. = 25 Amp for THHN, # 12 AWG. = 30 Amp for THHN, and # 10 AWG. =40 Amp for THHN. The obelisk note below the tables do not apply to motors. NEC Article 300/1/B

Motor Designing Step 4


Special Notes: The branch circuit over - current device of a motor supply wiring design does not intend to protect the conductor from overheating but only to protect the branch circuit short - circuit and ground = fault factors. Not necessarily the conductor itself. The conductor itself is designed to be protected by the overload [heater / or / thermal] device.

To find the maximum branch circuit over - current device [breaker or fuse];

Special Notes: This calculation is to discover the maximum over - current device. You must use the lowest over - current device rating that will effectively perform without trouble or jeopardizing minimum safety as required and is reasonable.

Special Notes: The word polyphase refers to 3 phase motors.

Special Notes: A Squirrel cage motor is your most common light or heavy duty motor. A squirrel cage motor is not normally [if at all ] weather proof.

Special Notes: A wound rotor motor is an a/c motor used in speed control type applications.

Special Notes: A non - time delay fuse will hold 5 times its rated current for 1/4 of a second only.

Special Notes: A time delay [ dual element ] fuse will hold 5 times its rated current for 10 seconds approximately.

Special Notes: A circuit breaker will hold around 3 times its rated current for different time frames depending on its breaker frame size [ bolt in versus light duty ] and voltage rating will change the time frame also.

Special Notes: An over - current device when not meeting a standard size rated for a standard over - current device as found in NEC Article 240/6. You may adjust up to the next higher standard rating found in NEC Article 240/6.

To Find the Branch Circuit Maximum Over - current Device you must refer to NEC Article 430/152.

To use NEC Article 430/152 you will find this table much simplified in the 1996 Version on the NEC. You must first look for the type of phase the motor is designed for

[Single Phase versus Polyphase { Three Phase }]. Then look for whether your motor is A/C or D/C. Then look for the type of motor. If it is a single phase motor, type does not apply. There is a section in the NEC Table 430 -152 that applies to all single phase motors as one category. Direct Current motors are also only referred to in one category found in the NEC Table 430 - 152. Polyphase {three phase} motors may be of a type squirrel cage or synchronous or wound rotor. polyphase motors of the squirrel cage type are separated into two types. Design E and other than Design E motors.

Special Notes: Design E motors are special energy Efficient motors that are at least above an 80% power factor or efficiency rating of above 80%. Design E motors have a history of a high impact of amp load upon start but return to a lower running amp under normal operations.

Now you must look for the type of over - current device you are intending to use. You have 4 choices. Non - Time Delay { 1/4 second at 5 times the load rated } - Time Delay {10 seconds at 5 times the load rated} - Instantaneous Trip Breaker {Heavy duty specially designed for motors} - and Inverse time breakers {most of the breakers found being normally used} - and capable of holding around 3 times the load rated and at a time frame that changes with breaker styles and frame size.

Now find the percentage of increase you are allowed to adjust up to. Multiply the motor F.L.C. times the percentage allowed found in NEC Table 430/152 and this is the maximum size over - current device allowed, unless your answer lands between the standard amp rated in NEC 240/6. You may then adjust up to the next higher over - current device. that is a standard size.

You may only adjust the branch circuit over - current device to the next higher size on a branch circuit. Not on a feeder [feeders must adjust down to the next lower size feeder over - current device].


Special Notes: Feeder = A conductor supplying power. A feeder shall be found between the service equipment / a transformer / or a generator / or phase converter and the final branch circuit overcurrent device.

To find the feeder conductor size you must multiply the largest motor F.L.C., found on that feeder, by 125%. This will increase the largest motor by 25%, and then add the F.L.C. of all of the other motors on that feeder to that calculation. The answer is the minimum feeder conductor size.

Special Notes: Do not forget single phase motors may not necessarily be on all three phases. This may cause an imbalance between the phases if not intentionally balanced between phases.

{Three phase feeder conductors }

A three phase motor uses Line 1 / Line 2 / Line 3 to run the motor.

A single phase 240 / or / 208 volt motor uses only 2 of the three phases to run the motor.

A single phase 120 volt motor uses only 1 of the three phases to run the motor.

Special Notes: The facts above may cause an unbalanced load on any one of the three phase feeders of a three phase system. Therefore you must balance the load of a three phase system when single phase motor loads are on the three phase system.

This document is based on the 1999 national electrical code and is designed to give you an option, as a self-help, that should pass minimum code requirements. While extreme care has been implemented in the preparation of this self-help document, the author and/or providers of this document assumes no responsibility for errors or omissions, nor is any liability assumed from the use of the information, contained in this document, by the author and / or provider.

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