Calculating Motor Full Load Rating

[slushin]

Can anyone tell me why the full load amps listed in the NEC do not equal the motor power rating divided by the motor voltage rating? For instance, a 10hp 230V single phase motor is rated at 50Amps according to NEC, but using I=V/P, you get 32Amps.

 

[davidbeach]

So that you can replace you 32FLA motor with any other 10hp motor and not run into trouble in terms of wire sizing or overcurrent protection.

 

[electricpete]

Don't forget to include power factor and efficiency in your calculations, too. If you assumed 0.8 pf and 0.9 efficiency that would get you up to 45A for a 10hp 230v motor.

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(2B)+(2B)' ?

 

[ScottyUK]

Cables heat up faster than motors too so if your load occasionally eats into the the overload margin but stays below the motor protection trip level, you don't want your cable being damaged. The motor should normally be the limiting factor, not the cable and controlgear ratings.


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If we learn from our mistakes I'm getting a great education!

 

[rbulsara]

In addition to what electricpete said, the pf of single phase motors are generally lower than 0.8 and efficiencies are also lower compared to 3 phase motors.

Try comparing NEC to the nameplate of an actual 10 HP, 1 Ph, 230V motor.

Rafiq Bulsara

http://www.srengineersct.com

 

[electricpete]

Good point Nick.
I looked at Reliance's site and I couldn't find any single-phase 10hp motor....
But here is a 5hp single phase 230vac motor:

http://www.reliance.com/pdf/pdf/aced/A00020-A-B001.pdf

FLA of the 5hp motor is 30.
If you doubled that for 10hp motor, you'd expect 60A.
If you allow for better efficiency and power factor on the 10hp than the 5hp, 50A might be reasonable.


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(2B)+(2B)' ?

 

[electricpete]

Sorry, I don't know where Nick came from... I mean Rafiq.

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(2B)+(2B)' ?