Parallel grounding conductors

[augie47]

This is spin-off from https://forums.mikeholt.com/showthread.php?t=197747
(I did not want to "muddy" that thread)

As I began to post in the above thread I was looking at 250.122. Assume we have a 4000 amp breaker with multiple conduits on the load side feeding a MLO panel.
Table 250.122 shoes a 750AL required for the equipment ground for the 4000 amp feeders and states each raceway must have that size conductor. 250.122(A) states that the EGC need not be larger than the circuit conductor supplying the equipment BUT in the case of parallel can you drop the EGC to the same size as the phase conductor ?

 

[LarryFine]

. . . BUT in the case of parallel can you drop the EGC to the same size as the phase conductor ?

Read the last line of 250.122(F)

 

[packersparky]

IMO, the size of the "circuit conductor" is the total area of all the paralleled conductors per phase. IMO, the EGC would have to be 750 AL in the install the you describe.

 

[charlie b]

250.122(A) states that the EGC need not be larger than the circuit conductor supplying the equipment. BUT in the case of parallel can you drop the EGC to the same size as the phase conductor ?

Well, yes, sort of. In your example (presuming this really is a feeder and not a service), and using the conductor sizes from the original post you cite, the rule is that the EGC in each conduit need not be larger than the collective cross sectional area of the 13 sets of 500 MCM that comprise the "circuit conductors supplying the equipment." In other words, you don't compare the size of the EGC in a conduit with the size of the ungrounded conductors in the same conduit. As Larry pointed out, you size the EGC in each conduit on the basis of the upstream OCPD.

But to reiterate what I have said in the other thread, if this is a service, then there is no conductor that has any form of the word "ground" in its title in the set of conductors between the service transformer secondary terminations and the location of the first downstream OCPD.

 

[infinity]

IMO, the size of the "circuit conductor" is the total area of all the paralleled conductors per phase. IMO, the EGC would have to be 750 AL in the install the you describe.

I agree. You size the EGC in each raceway according to the OCPD ahead of the feeder even if it's larger than the ungrounded conductors in each raceway. If you had a wireway with all of the ungrounded conductors in it and only one EGC you would not size it smaller than required by T250.122 because the ungrounded conductors are smaller than what's required by the table.