Art. 250.122F

[les boyle]

Can someone explain the theory behind paralleled grounding conductors, all sized, to the overcurrent device. Eg. 4-4'' conduits with 4-400mcm and 1-4/0 ground in each raceway, with a 1200amp overcurrent device. This seems like over kill to me, with one 4/0 rated at 1600amps and a GFCI main set at 200amps.

 

[charlie b]

I think it has to do with a possible fault within a conduit (e.g., a fault from a phase conductor to the ground wire) You want the ground wire to be able to carry all of the fault current back to the source (so that it will trip the 1200 amp breaker), without itself failing open. Too much current through too small a wire might cause the wire to melt, and create an open circuit, before the breaker has a chance to trip. That is why you put a full sized EGC in each conduit.

 

[infinity]

I agree with Les, it sounds like overkill. Especially when using a metallic conduit.

 

[bdarnell]

Charlie is exactly correct in his explanation. Just because you have parallel conductors does not mean that the fault current will be divided among the parallel ground conductors. If you have a fault in one conduit, all of the available fault current will flow at that fault. In this case you have a GF main (not GFCI) which will theoretically open the device at the preset fault levels, but if this were a 208 volt service, you most likely would not have that protection. At least, you would not be required to have it.

Hope this helps explain the Code requirement.