Small Motor FLC Determination

[xptpcrewx]

Let's say we need to determine the FLC of a 1/4 H.P. motor (three-phase, 480-V). Some question that come to mind:

1. How would you guys go about determining the FLC given Table 430.250 stops at 1/2 H.P?

2. Is interpolation possible based on 430.6(A)(1) even if the motor is marked in H.P.?

3. Since interpolation is the process of estimating unknown values that fall between two known points, how can it be done accurately for endpoints?

4. Can Table 430.250 be calculated or is it based on empirical data alone? (Applying the standard motor formula seems to produce larger deviations for lower H.P.)

 

[petersonra]

I don't think I have ever seen a 1/4 HP 480 V 3 phase motor.

If I had to use such a thing, I would probably use half the value of the 1/2 HP motor for calculation purposes (after considering what the NP said) and not worry about it all that much.

 

[GeorgeB]

I don't think I have ever seen a 1/4 HP 480 V 3 phase motor.

If I had to use such a thing, I would probably use half the value of the 1/2 HP motor for calculation purposes (after considering what the NP said) and not worry about it all that much.

SEW Eurodrive makes powered reducers with 0.25 HP motors; I just went through their online config page; their motor is 0.52A rated current (1.04A at 230V)

 

[xptpcrewx]

The most logical thing I can think of is to just use the nameplate FLA. Provided you stay within short-circuit motor branch protection limits, can anyone think of any technical reason why this would be inadequate or prohibited by the code?

 

[augie47]

The most logical thing I can think of is to just use the nameplate FLA. Provided you stay within short-circuit motor branch protection limits, can anyone think of any technical reason why this would be inadequate or prohibited by the code?

Considering you should be using nameplate for OL and when you you look at the SCGF protection vs the "standard" fuse sizes, I see no reason not to use nameplate for both.