Point of Service

[Coachmike]

We have a 2000-amp service on an assisted living facility. We are installing a 500KW genset and 2000-amp ATS on the outside of the facility. The existing service is fed by 4- 4" parallel PVC UG conduits with 4- 750 MCM CU conductors. There is an existing spare/ empty 4" conduit that we would like to run our new EGC in. The AHJ is requiring a full size EGC- 250 CU in each of the 4 existing feeders. We would like to pull one full 250 CU in the spare, empty, parallel conduit or at the very least decrease the size of the ground wire for parallel circuits. Is this acceptable?

 

[augie47]

300.3 will require the install as the inspector states.

 

[infinity]

The EGC has to be full sized in each parallel raceway. As Augie stated you cannot run a single EGC in the empty conduit.

 

[Coachmike]

UGH. Thanks everyone for the disappointing confirmation.
But can anyone explain the logic by requiring the fault current capacity of four parallel grounds to not be reduced. Yet the current carrying conductors are additive? 250 MCM CU is rated for 2000 amps. Why would 8000-amps of protection be necessary?

 

[tom baker]

Because in a parallel conductors fault current can be fed from each end, and the egc must be close to the ungrounded conductors (reduces impedance)
This rule comes up frequently here

 

[electrofelon]

Your conductors appear to be undersized and not compliant.

 

[don_resqcapt19]

UGH. Thanks everyone for the disappointing confirmation.
But can anyone explain the logic by requiring the fault current capacity of four parallel grounds to not be reduced. Yet the current carrying conductors are additive? 250 MCM CU is rated for 2000 amps. Why would 8000-amps of protection be necessary?

The First Draft Report for the 2026 NEC, shows new language in 250.122(F) that will say that with parallel conduits the EGC will not have to be larger than the largest ungrounded conductor in the conduit.

There are a number of Public Comments to delete this. The substantiations for these Public Comments have some detailed technical information as to why a full sized EGC is required in each of the parallel raceways.

You can go to nfpa.org/70, click on "next edition" and then click on "view public comments" to read these technical substantiations.

 

[kingpb]

Could it be because in an unbalanced three phase system, the zero sequence current on the ground is additive and thus the unbalanced current could be as high as 40% of the phase current, then you could have 120% of the phase current on the ground.

 

[Lioneye]

Your conductors appear to be undersized and not compliant.

Agreed. Can't round up over 800A.

 

[electrofelon]

Agreed. Can't round up over 800A.

.......Unless they Branch out to serve multiple service disconnects, then they can be sized to the load.

 

[Coachmike]

The state is mandating Assisted Living Facility's to have full load generators installed. This ALF pulls 260 amps per phase regularly on a 500KVA utility transformer. We are adding a 500KW generator and a 2000 amp ATS. All seems as killing a fly with a sledgehammer. AND the original life safety ATS must remain or be replaced, yet it is fed by new generator. Why? Application of a little common sense and first principles needed IMO. Thanks all. Love this forum.

 

[electrofelon]

The state is mandating Assisted Living Facility's to have full load generators installed. This ALF pulls 260 amps per phase regularly on a 500KVA utility transformer. We are adding a 500KW generator and a 2000 amp ATS. All seems as killing a fly with a sledgehammer. AND the original life safety ATS must remain or be replaced, yet it is fed by new generator. Why? Application of a little common sense and first principles needed IMO. Thanks all. Love this forum.

Using 220.87 can result in a drastically smaller service. I know it seems super strange to downgrade a 2000A service to a 400/600/800 whatever, but of that's what the historical usage is and there is no reason to keep extra capacity, I would have to trouble doing it. I guess we would need to know who is doing the design and what their reasons are.