What % do you use to determine OCPD of a fractional motor?

[egurdian3]

Guys, in trying to figure out OCP for fractional HP motors, what is the safe % to use once you find RLA?
Say that you have a .5HP, 120V motor, (746W)(.5)=373W/.8=466W. Now to find RLA, 466/120=3.88A.
Take 3.88 times 1.25 = 4.86A.
If you look it up on the SQD Motor Data Calculator you find that they list 9.8A, which is roughly 200% of RLA.
Alright you say, no big deal -but let's try 1HP for giggles.
1*746=746/.8=932.5W. 932.5/120=7.77A(1.25)=9.71A.
SQD says 16.0A, which is roughly 1.65%.
So, this is based on a "judgement call"?
Granted that I wouldn't recommend anything larger than .4HP @ 120V, -but the question remains: what % do you use?

 

[Smart $]

... what % do you use?

The NEC says you have to use Table 430.248... and its numbers are the same as what you quoted from SqD.

 

[augie47]

Just to be safe, let's verify what yoiu are sizing.
If you are trying to protect the motor from overload, then you need to refer to part III of Article 430 and select the correct device.
Note for fractional horsepower, it depends on the motor being automatically started {430.32(B)} or non-automatically started {430.32(D)}.

If you are to provide motor/conductor ground-fault/short circuit protection then you reference 430 Part IV.

You need to reference 430.6(A) to determine whether to use the Tables or the nameplate ratings as it varies depending on what type of protection you are providing (overload or short-circuit) and there are exceptions there for some fractional HP motors.

 

[templdl]

Guys, in trying to figure out OCP for fractional HP motors, what is the safe % to use once you find RLA?
Say that you have a .5HP, 120V motor, (746W)(.5)=373W/.8=466W. Now to find RLA, 466/120=3.88A.
Take 3.88 times 1.25 = 4.86A.
If you look it up on the SQD Motor Data Calculator you find that they list 9.8A, which is roughly 200% of RLA.
Alright you say, no big deal -but let's try 1HP for giggles.
1*746=746/.8=932.5W. 932.5/120=7.77A(1.25)=9.71A.
SQD says 16.0A, which is roughly 1.65%.
So, this is based on a "judgement call"?
Granted that I wouldn't recommend anything larger than .4HP @ 120V, -but the question remains: what % do you use?

One would like to think that the motor has an integral thermal overload built in' that the OCPD would be selected based upon protecting the conductor.

 

[egurdian3]

Thanks, guys.