Two Buildings - Multiple Services - Both Directions

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drktmplr12

Senior Member
Location
South Florida
Occupation
Electrical Engineer
Edition: 2014 NEC

I am having trouble coming to terms with the application of 225.32, Exception 1 and 225.36 requirement that equipment be service entrance rated. I have a unique application:

The plant has two buildings, each with a 480V service from two different transformers. Each 480V service has a main circuit breaker. Each building also has a generator with a main output breaker, connected to an ATS. The ATS is connect The Owner would like to interconnect the two buildings and power the process from either service or generator.

I do not believe I need a neutral between the two buildings, but need to confirm this. Assume all distribution is 3P4W. I've notated what is desired on the one-line in red.

It's clear what to do when the feed only goes one direction and there is a neutral carried between the two buildings. My confusion is (1) there is no neutral to be carried between and (2) that the feed is intended to go either direction and both sides would require SE disconnect per 225.36. How exactly should the grounds be connected between the two (apparently) required SE disconnects for each branch circuit?

Currently the thought is to have an equipment ground connect the ground bus in the MCC and SWBD. Am I missing something?

The one-line shows a switchboard with a new breaker, but I'm thinking that may have to be a SE breaker in a separate enclosure.


Elementary One-Line
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225.36 Refers to 250.32(B)
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drktmplr12

Senior Member
Location
South Florida
Occupation
Electrical Engineer
I re-read the requirements and am thinking that since there will be no neutral between the buildings, the equipment need not been SE rated, since Exception 1 of 250.32(B) is not be applied. Just run a equipment ground sized per 250.122 between the two.
 

JFletcher

Senior Member
Location
Williamsburg, VA
How is this design considered safe for the power company?

I don't know... With two separate everything, there is no problem. Interconnecting the buildings as proposed would at the very least require the automatic transfer switches to be paralleled, as well as synchronization gear to run both gensets at one time, or some sort of electrical lockout to prevent the second generator from starting.
 

LarryFine

Master Electrician Electric Contractor Richmond VA
Location
Henrico County, VA
Occupation
Electrical Contractor
Fletch, he also wants the services to be able to supply both from either, not just the gennies. To merely do that, the interconnection should be on the genny side of each ATS, with the sync issues addressed, and I would expect a neutral to be used.

I also agree that there is no safe or compliant way to make such a double-ended supply from two discreet, distinct services*. Perhaps two interconnected, mutually-excluding ATS's (after the genny ATS's) could allow either to feed both, one at a time.

*​ Especially when located at two separate structures.
 
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