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Joined: Feb 2006
Posts: 78
I'm installing a service to a detached building, main building is 480/277 the detached bulding will be feed from the main building and then step down to 120/208 do I have to carry a neutral and ground to the detached building.

Joined: Jan 2005
Posts: 5,370
Likes: 1
Cat Servant
Where's the 480/208 transformer? Are you keeping the neutral for the 277 isolated from the neutral for the 120 isolated from each other in the transformer?

No matter what you do, you need a ground wire between the buildings, and a grounding electrode at the detached building as well.

IMO, you should keep the 277 neutral apart from the 120 neutral at the transformer. Indeed, I see no reason to bring the 'gray' neutral in with the transformer feed at all. This makes the transformer a 'separately derived service'.

[red] ECN regulars double-check me on this: [/red]
With a separately derived service, the first breaker after the transformer becomes the point where you separate your grounds from your neutral, just as with your service from the PoCo. You'll want to use bonding bushings on your connectionc between the transformer and that breaker.

Joined: Jul 2004
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If you bring the feeder to the second building as a 480 delta and set the transformer in the second building, I agree with Reno. This becomes a SDS and your neutral starts there. That is probably the best way to do it anyway, saving wire and reducing I2R losses. Establish your ground electrode system just like a service entrance.

Greg Fretwell
Joined: Feb 2006
Posts: 78
thanks for your reply guys

Joined: Feb 2003
Posts: 939
I do the same way as Reno and Greg expain just run delta to detached building then treat them as SDS however pay attetion to the delta connection I know some area may required a CGD { Corner Grounded Delta } that can get tricky if not heeded with connections.

The CGD is only used if you have CGD in main service otherwise just run full delta as those guys mention.


Pas de problme,il marche n'est-ce pas?"(No problem, it works doesn't it?)

Joined: Jun 2004
Posts: 1,273
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.


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 10:37 PM.

Joined: Jan 2005
Posts: 5,370
Likes: 1
Cat Servant
Nonsense, Tesla. I know I've put in transformers that had 480 (three phase) going in, and 208/120 coming out. Indeed, I can't see any way to create 240/120 in a "Wye" configuration at all.

True, I did ASSUME the OP was talking about a 3-phase transformer - which, technically, is three transformers in one box.

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