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Conductor ampacity

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If you have a nameplate full load current of a motor, when do you use the nameplate value vs looking up from hp in table 430.250 to calculate ampacity for choosing the proper conductor size?  

 

The solution for proguide 78 test says use the table to get 26 amps, but I thought since 30 amps was given, to take 125% of that?

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Hello - hope all is well.

For Minimum Motor Conductor
Ampacity, lookup the motors FLC using HP and voltage rating from NEC Tables 430.247-250. Do not use the motors nameplate FLC. If conductor serves only one motor that is continuous duty, multiply FLC (found on tables) by 125%.

Hope this helps.

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The way to remember this is

FLC - C means code value

FLA- A means actual values, name plate values. 

For sizing conductor, use the FLC values, the reason being if you change the motor etc, you may not change the conductor, so that has to be the worst case value. Due to efficiency, different manufactures, the name plate value will vary. 

For setting the overload etc, one need to use the actual value. 

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On 9/7/2019 at 1:23 PM, Wow! said:

If you have a nameplate full load current of a motor, when do you use the nameplate value vs looking up from hp in table 430.250 to calculate ampacity for choosing the proper conductor size?  

 

The solution for proguide 78 test says use the table to get 26 amps, but I thought since 30 amps was given, to take 125% of that?

Hi @Wow! The NEC® is pretty clear on when to use the name plate amps, and when to use the 430.250 table values.

NEC® 430.6(A)(1) instructs the use of table values instead of "the actual current rating marked on the motor nameplate" for the ampacity of conductors, ratings of switches (any device that makes or breaks a circuit), branch circuit short circuit and ground fault protection. This applies to all motors except those "built for low speeds, high torque, or multi speed motors".  

NEC® 430.6(A)(2) instructs the use of "motor nameplate current rating" for separate overload protection. 

 

PM me your email address if you'd like a copy of our four page PDF motor cheat sheet that we give out to our students after our live class on rotating machines:

 

image.png.c1e5751efa3717affd53881ba7bfc433.png

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On 9/8/2019 at 1:02 PM, Zach Stone, P.E. said:

Hi @Wow! The NEC® is pretty clear on when to use the name plate amps, and when to use the 430.250 table values.

NEC® 430.6(A)(1) instructs the use of table values instead of "the actual current rating marked on the motor nameplate" for the ampacity of conductors, ratings of switches (any device that makes or breaks a circuit), branch circuit short circuit and ground fault protection. This applies to all motors except those "built for low speeds, high torque, or multi speed motors".  

NEC® 430.6(A)(2) instructs the use of "motor nameplate current rating" for separate overload protection. 

 

PM me your email address if you'd like a copy of our four page PDF motor cheat sheet that we give out to our students after our live class on rotating machines:

 

image.png.c1e5751efa3717affd53881ba7bfc433.png

Please forward to vallyawa@gmail.com

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