Motor grounding

[knoppdude]

Was asked this problem by an electrical project manager. Two 150 HP motors on the same starter, and breaker 800 amps. I told him that a 1/0 grounding conductor would have to be run to each motor per table 250-122. He said it did not make sense, and that he thought this could be reduced as the breaker was an instantaneous trip with adjustable settings. As I read section 250-122, there can be no reduction in the size of this equipment grounding conductor. Am I wrong, is there some part of article 430 that allows for a reduction? Section 250-122 seems to be very clear on this, and it makes sense that with two branch circuits on the same breaker, each would have to be capable of carrying the potential ground fault current, and as such should be sized using table 250-122. Thanks for all responses.

 

[infinity]

You did say instantaneous trip so did you check out 250.122(D)(2)? Also what's the motor voltage?

(D) Motor Circuits. Equipment grounding conductors for motor circuits shall be sized in accordance with (D)(1) or (D)(2).
(1) General. The equipment grounding conductor size shall not be smaller than determined by 250.122(A) based on the rating of the branch-circuit short-circuit and ground-fault protective device.
(2) Instantaneous-Trip Circuit Breaker and Motor Short-Circuit Protector. Where the overcurrent device is an instantaneous-trip circuit breaker or a motor short-circuit protector, the equipment grounding conductor shall be sized not smaller than that given by 250.122(A) using the maximum permitted rating of a dual element time-delay fuse selected for branch-circuit short-circuit and ground-fault protection in accordance with 430.52(C)(1), Exception No. 1.

 

[don_resqcapt19]

Maybe the engineer is looking at an older code. In the 2005 code the EGC for a motor circuit with an instantaneous trip breaker was permitted to be based on the rating of the overload device. This was changed to the rule that Rob quoted for the 2008 NEC.

 

[knoppdude]

You did say instantaneous trip so did you check out 250.122(D)(2)? Also what's the motor voltage?

The motors are 460 volt wound rotor motors. As both sets of conductors originate from the same lugs, it would seem that section 430-53(D)(1) would apply as the motors are more than 25 feet from the starter, and don't have individual controllers. This section states: No conductor to the motor shall have an ampacity less than that of the branch circuit conductors.
My question now is not in disagreement that a lesser sized grounding conductor can be used, but rather, how would you calculate the load to determine the size of the OCPD. I would think it would be acceptable to calculate this as you would a feeder OCPD, but what I have read so far doesn't seem to give any direction. Using the feeder method to calculate this, a #2 conductor can be used, if this method is applicable.