EGC sizing for motor circuits

[infinity]

Is table 250.122 used to size a motor circuit EGC even though the OCPD may be considerably larger than the ampacity of the circuit conductors?

For example a 125 amp motor feeder (100 amps*125%) with an dual element time delay fuse providing short circuit and ground fault protection at 175% (218.75 amps) would require a EGC sized according to 125 amps or (218.75 amps next higher), 225 amps?

This would be the difference between using a #8 EGC or a #6 EGC.

 

[celtic]

I was told that ANYTIME you increase the conductor size, the EGC must be increased proportionally.

I'm sure you saw that post here, too

 

[infinity]

I was told that ANYTIME you increase the conductor size, the EGC must be increased proportionally.

I'm sure you saw that post here, too

Yes, I agree, but here we're increasing the OCPD ahead of the feeder so we end up with two values, one the ampacity of the feeder conductors and two the ampacity of the oversized OCPD. So which is used to size the feeder EGC?

My guess is that since the EGC is used for ground fault protection you would size the EGC according to the OCPD size. But I can't seem to find the reference in the NEC.

 

[Michael15956]

Trevor,

See if this is right?
PartXIII of Art.430.
430.245
250.122
T250.122 based on the Rating of Overcurretn Device in Circuit Ahead of Equip.

 

[infinity]

Yes, the heading table 250.122 should have been clear enough. I was side tracked into thinking that it couldn't be that simple. Then I went on the think about using a instantaneous trip CB which would be sized at 700% of the motor ampacity. That got me thinking that I wouldn't need to use an EGC sized for the 700% or would I. Then I read 250.122(D) and it became clear that my original thinking was rather cloudy.:roll:

 

[Michael15956]

Yes, the heading table 250.122 should have been clear enough. I was side tracked into thinking that it couldn't be that simple. Then I went on the think about using a instantaneous trip CB which would be sized at 700% of the motor ampacity. That got me thinking that I wouldn't need to use an EGC sized for the 700% or would I.

I agree. The EGC can be based on the motor overload protective device, but not smaller than listed in T250.122.