Installing a subpanel on a separate building that has existing, separate service?

[Ohno Raccoon]

Hi everyone,

Kind of a long post, but the more background info available will help for discussion.

We have a new construction building in Washington DC, which follows NEC 2014. Due to the structure of our new building (for the sake of conversation, Building A), the structural reviewer with the city is forcing the owner to address increased snow load on the neighboring buildings (Building B directly adjacent, physically touching our building; Building C separated by a driveway). The owner has made an agreement with both property owners to provide snow melt systems onto each of their respective roofs. The owner has directed us to provide a design for snow melt systems and to have them powered from their Building A's service.

Each snow melt system will need a subpanel installed on the roof of Building B and C to serve the array of heat mats to be installed. These branch circuits need to have ground fault protection.

This proposed installation sounds wrong to me for multiple reasons. I've proposed alternate designs, however the owner is insistaet that this is how it needs to be done. So before we move onto an alternate design, I need to justify why we cannot provide what they are proposing.

First concern - I don't like the idea of bringing feeders to a separate building, per NEC 225.30. However, I think it can be argued that NEC 225.30(D) applies. I even brought this concern up with the city's electrical reviewer to see what they think before submitting for permit. And they didn't even hesitate to think anything was wrong with providing equipment on a separate building.

Second concern - Having two separately derived systems at Building B and C. How would this be installed safely, with regards to grounding? I'm concerned if not done correctly the ground fault protection may not function or there would be a shock hazard present.

Appreciate any insight you can offer. Thanks.

 

[winnie]

Not even close to my area of expertise, so take the below with a huge grain of salt:

IMHO the concern is to make sure that power is removed from a building in the event of an emergency, so I'd ask the fire department what they would want to see.

It seems to me that you could do what the building owner desires and provide the equivalent safety of separate services by having some sort of shunt trip or contactor arrangement so that if power to building B gets turned off power to the snow melt system is removed.

With respect to grounding of the SDS, you will need to tie the grounding systems of the two buildings together, so would need to consider how this has an effect on things such as service objectionable currents or lightning protection system grounding. But if the buildings are actually touching then it is likely things are already connected.

Each SDS would need to be grounded to (local) building steel, water piping, GEC, etc. just as normal, and each SDS will have an EGC run with the primary.

-Jon