You've missed the big issue: does the customer intend to utilize 480Y277 circuits in the dependent building.
Having actually run circuits identical to your proposal -- we ended up pulling a neutral for a 480Y277 Panel -- which was tapped to create a SDS 208Y120 system.
Reno: there is no 480:208 transform.
H1-H2 = 480V
X1-Xo = 120V
The turns ratio is 4:1...
208V arrises as a consequence of adding two legs of 120V power -- adjusting for their phase angle difference.
The 208Y120 system lags the 480Y120 by 30 degrees -- at all times.
The additional conductor expense ( the High Board ) is balanced by using 277V lighting -- especially if site runs are a consequence of the new building.
A bonding run, properly sized, is required -- of course.
A Ufer for the new building is required -- by my AHJ -- ground rods don't cut it out my way.
We stay away from Corner Grounded Deltas. If the primary Service is 480Y277 -- which our Poco insists upon -- then Grounding a 277 leg would induce some horrific ground currents.
I've actually seen that happen -- by accident. A dedicated fire pump Service was corner grounded. It drew 90Amps on that leg -- at 277Volts.
Yiikes.
Fortunately, I'd bonded everything in sight -- so no one died. ( Particularly, me. I became a 'bird on a wire.' )
Your Low Board is going to have to have a properly sized C/B -- else you'll have to tack on a Safety Switch right at the XFMR -- and bonding bushings are required on all of the dependent Low Board raceways -- until you hit the OCPD.
It all sounds like a lot of fun.
Last edited by Tesla; 09/04/11 11:37 PM.
