Locked Rotor Amperage...

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chevyx92

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When do you have to figure in the LRA when determining circuit conductor ampacity? Motors are not my forte. Here's what I have. Motor Nameplate: 25HP, 74 A, Volts 208, 3 Phase, Service Factor 1.15, Code F, Design B. Table 430.250 of the 2005 code gives me 74.8A. So I get 74.8 x 125%= 93.5 Amps to size my circuit conductors. Correct? And then for overcurrent protection (inverse time breaker) I get 74.8 x 250%= 187A, next higher setting is 200 Amp. Have I done this right? Do I need to worry about LRA in my calcs? Thanks.
 
chevyx92 said:
And then for overcurrent protection (inverse time breaker) I get 74.8 x 250%= 187A, next higher setting is 200 Amp.
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To be precise, that is sizing for short-circuit and ground-fault protection.

The OCPD for the conductors is the same as that for the motor.
 
Maximum OCP for Motors

Maximum OCP for Motors

Table 430.52 heading states "Maximum Rating or Setting..." and therefore does not allow the "next size up" breaker as Chevyx92 implies.

Use 175A breaker to comply.
 
PE-noy said:
Table 430.52 heading states "Maximum Rating or Setting..." and therefore does not allow the "next size up" breaker as Chevyx92 implies.

Use 175A breaker to comply.
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Take a look at 430.52(C)(1) Exception #1. It allows the next higher size breaker.
 
chevyx92 said:
Is there any article in the code that backs your thought up?
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Chevy look at 430.7 and table 430.7(B) maybe we can figure it out. I just remember way back in a class that the 125% was taking into account the LRA. I could be wrong just trying to help.
 
Take a look at Table 430.251 A and B of the 05

This table gives LRC conversions and it indicates that it's application is for selection of the disconnect. This could apply when a disconnect is not rated in horsepower. also refer to 430.110.

The LRC is rarely used to size the motor branch circuit and it's conductors.

430.52 is the motor branch circuit poirtection against shorts and faults to ground.

430.22 (for single motors) is to size the motor branch circuit conductors. These are sized 25% larger because all motors are considered as a continuious load unless the nature of the drive makes them intermmitent see the note to table 430.22E.

I think the op has selected his BCSC+GP proitection and the mbc conductors correctly.

keep in mind that the reference articles I gave apply generally there are exceptions etc.
 
Locked rotor current and Table 430.251(B) is used to figure the disconnect size for combination loads. See 430.110(C)
 
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