High-leg overcurrent sizing at service and possible code changes?

[derekk]

Hello,

I recently performed load calculations on a multi-unit residential building for a potential modernization and service upgrade. While doing the site survey, I noticed something which I had not encountered before. The service to this property is overhead fed, 240/120V, 3P, 4W, with a 600A inverse time Main Circuit Breaker. The only load on the high-leg is an elevator with FLA = 22A.
Assuming that this was a new design and I was 100% sure that there would never be any load on the high-leg besides the elevator, the procedure I would follow would be to size the conductor at a minimum to carry 1.25X the FLA, which is about 28A. The #8 minimum for CU per 230.31(B) would be greater than what is required to handle this current. Since the only load on this phase is the motor, we’d use table 430.52 and multiply 22A X 250% which is 55A. We can’t round up, but the min service would probably be 60A, so instead of 50A, I’d upsize the service conductor to #6 with 60A protection, probably with a fusible main or an MCB with an inline fuse. Note that there is a panelboard downstream of this point with a CB to feed the elevator.
What is actually installed on the high-leg is a #8 wire from the weatherhead landing directly into the 600A CB. The load side of the CB (also #8) goes into a gutter and eventually to a 3-phase meter/main with a 70A CB. I cannot find anything to justify this as being code compliant, but perhaps I’m missing something or was this installation once permitted but no longer?

 

[kwired]

If you never intend to supply additional circuits that utilize the high leg, I would likely install two service disconnecting means, one the 600 amp unit supplied with the two 120 volt legs and the other whatever sie is needed for the elevator, but all three phases.

 

[derekk]

That would be a good idea in many locations (this particular example included). In this city, that would be permitted for an "all residential" building, but if there were any commercial spaces (which is very common), our local code would require a single overall service disconnect. Even in this case, I think your idea is still possible if they installed a non-fusible disconnect to satisfy the single disconnect requirement along with two separate OCPD's immediately adjacent to the single disconnect (to comply with 230.91). You'd have a large 2-pole CB, fusible disc, etc. plus a much smaller 3-pole one just for the elevator and house loads. I'm not sure if two sets of OCPD's complies with the intent of 230.91 though.

Still, back to my original request. I believe the existing situation is a code violation, but I just wanted a few more brains to think about it and see if I'm maybe missing something specific to this situation (or that maybe used to be in the code to allow it ~40 years ago).

Thanks.

 

[kwired]

Many moons ago they also would have allowed a "delta breaker".

 

[Smart $]

...
Still, back to my original request. I believe the existing situation is a code violation...

I agree.

 

[kwired]

That would be a good idea in many locations (this particular example included). In this city, that would be permitted for an "all residential" building, but if there were any commercial spaces (which is very common), our local code would require a single overall service disconnect. Even in this case, I think your idea is still possible if they installed a non-fusible disconnect to satisfy the single disconnect requirement along with two separate OCPD's immediately adjacent to the single disconnect (to comply with 230.91). You'd have a large 2-pole CB, fusible disc, etc. plus a much smaller 3-pole one just for the elevator and house loads. I'm not sure if two sets of OCPD's complies with the intent of 230.91 though.

Still, back to my original request. I believe the existing situation is a code violation, but I just wanted a few more brains to think about it and see if I'm maybe missing something specific to this situation (or that maybe used to be in the code to allow it ~40 years ago).

Thanks.

Any idea why your city doesn't allow the multiple disconnects for the non residential? The six disconnect rule in NEC covers any occupancy type - even mixed types. NEC even allows them to not be grouped in multiple occupant types of applications, regardless of occupancy type.

You are even allowed more then six disconnecting means if you have more then one voltage, number of phases, frequency, etc. which this situation comes close to being allowed that distinction. Theoretically the single phase and three phase disconnecting means are still supplied by a single source and most would still see this as one service with two disconnecting means, but there may be some that see this as a single phase service and a three phase service and call it two services.