NEC 250.24(A)(5)

[david luchini]

I am working on a project where the owner has generators connected through a distribution switchboard that feeds multiple ATS's as a non-separately derived system. The existing ATS's are all 3 pole. The generators are reaching end-of-life and they would like to install new generators (building exterior) connected through a new generator distribution switchboard in an adjacent building, back-feed the existing generator distribution switchboard, and backup the adjacent building's service from the same generators.

As the existing ATS's in Building #1 are 3 pole, the new generators will not have a neutral-ground bond.

If the new ATS in Building #2 is also 3 pole, the neutrals of the 2 services would be ties together. Is this a violation of 250.24(A)(5)?

Can the new ATS in Building #2 be 4 pole with a switched neutral, making the distribution section of Building #2's service switchboard a separately derived system with the neutral-ground bond at Building #2's service disconnect when the ATS is in Normal position, and the neutral-ground bond at Building #1's service disconnect when the ATS is in Emergency position?

 

[wwhitney]

I am working on a project where the owner has generators connected through a distribution switchboard that feeds multiple ATS's as a non-separately derived system. The existing ATS's are all 3 pole.

Since your post subsequently mentions 4 pole ATSs, I infer that your generators and service are 3 phase, meaning the 3 pole ATSs do not switch the grounded conductor. Since you mention more than one such ATS, are the existing ATSs installed in a topology that complies with 300.3(B)?

E.g. if you have 4 separate enclosures, consisting of (A) the utility supply for each ATS, (B) the generator distribution equipment, (C) ATS1 and (D) ATS2, then suppose they are interconnected by 4 separate conduit runs: each of enclosures C and D gets a supply directly from each of enclosures A and B. With solid connections for the grounded conductor in each of the 4 enclosures, that creates a loop for the grounded conductor: A to C to B to D back to A. That means, for example, that the grounded conductor current from the loads on ATS1 to, say, the service would divide between the two separate paths around the loop--the intended path directly from C to A, and the other path from C to B to D to A. This would violate 300.3(B).

It is possible to avoid this, by using a bus topology, e.g. the 4 enclosures are interconnected by just 3 conduit runs, say A - C - D - B. The conduit A-C contains two feeders, one to ATS1 in C and one to ATS2 in D, and they may share a common grounded conductor per 215.4(A). Likewise for the conduit C-D and the conduit D-B.

Your one line diagram only shows one existing ATS, so it does not indicate if the existing installation complies with 300.3(B). If it does not, then that would be a reason to change (at least all but one of) the existing ATSs to 4 pole ATSs, which I believe would moot your further questions.

Cheers, Wayne

 

[wwhitney]

If the new ATS in Building #2 is also 3 pole, the neutrals of the 2 services would be ties together. Is this a violation of 250.24(A)(5)?

Yes. You can start at the grounded conductor on the load side of service disconnect #1, and then you have a path through the grounded conductor to service disconnect #2 and its MBJ and GES. As those connections are still on the load side of service disconnect #1, that violates 250.24(A)(5).

Also see below for another violation.

Can the new ATS in Building #2 be 4 pole with a switched neutral, making the distribution section of Building #2's service switchboard a separately derived system with the neutral-ground bond at Building #2's service disconnect when the ATS is in Normal position, and the neutral-ground bond at Building #1's service disconnect when the ATS is in Emergency position?

If the new generator is connected to a 3-pole ATS in Building #1, then it is never an SDS, as it has a solid connection of its grounded conductor to service #1's grounded conductor.

This configuration runs afoul of the wording in 250.32(B)(1), on buildings supplied by a feeder. It says in part "Any installed grounded conductor shall not be connected to the equipment grounding conductor or to the grounding electrode(s)." With solidly connected grounded conductors at the 3 pole ATS(s), the feeder grounded conductor would be connected to building #1's GES, violating that requirement.

So I guess that means that any building supplied by both a feeder and a service must use a transfer switch that switches the grounded conductor for any load that is to be powerable from either source.

Cheers, Wayne