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#178366 - 05/28/08 09:04 PM Useless neutral 277/480
Redsy Offline
Member
Registered: 03/28/01
Posts: 2056
Loc: Bucks County PA
I need to install a step-up transformer from a 240 panel to get 480 volts for 3-phase equipment. No 277 required. However, in order to avoid the debate about grounded v. ungrounded 480 v. 3 phase 3 wire delta, I'm thinking about specifying a 277/480Y secondary and grounding the neutral, but not using it.

Comments, please?

Thanks,
John
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#178368 - 05/28/08 09:22 PM Re: Useless neutral 277/480 [Re: Redsy]
Tesla Offline
Member
Registered: 06/16/04
Posts: 1273
Loc: Sacramento, CA
Probably not economic.

Stay with a pure delta-delta transform.

Monitor the power for phase loss.

Is this a VFD driven load?
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#178374 - 05/29/08 04:21 AM Re: Useless neutral 277/480 [Re: Tesla]
SteveFehr Offline
Member
Registered: 03/19/05
Posts: 1195
Loc: Chesapeake, VA
If you're worried, why not just use a delta-wye transformer, ground the secondary neutral, and just not run a 4th wire? If you don't have to worry about phase shift, it's really the way to go.

Though the coils in a delta secondary aren't grounded, the core is, and magnetic core conductance and reactance will hold a delta secondary near equal potential, especially if the phases are balanced. Think of it like a wye-wye with a high impedance ground on the neutral.
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#178382 - 05/29/08 07:44 AM Re: Useless neutral 277/480 [Re: SteveFehr]
gfretwell Offline


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Registered: 07/20/04
Posts: 9036
Loc: Estero,Fl,usa
Corner ground it?
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Greg Fretwell
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#178406 - 05/29/08 04:11 PM Re: Useless neutral 277/480 [Re: Redsy]
Redsy Offline
Member
Registered: 03/28/01
Posts: 2056
Loc: Bucks County PA
Thanks guys!

I hesitate to corner ground the delta because 480 to ground concerns me. However, I'm not sure I want to float the delta either, hence my question.

However, a delta wye step-up xfmr seems to be special order and costs a premium. I will let the customer decide.

The other question...

The xfmr rep. says that although their step-down xfmr can be connected backwards, the inrush will be 10X primary current. A true step-up xfmr inrush would be 4-5X primary current, but a true step-up xfmr would cost more.

I am concerned about tripping primary OCPD if I connect backwards.
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#178409 - 05/29/08 05:24 PM Re: Useless neutral 277/480 [Re: Redsy]
Tesla Offline
Member
Registered: 06/16/04
Posts: 1273
Loc: Sacramento, CA
The power calculations on three phase transformers are identical to three phase motors. ( Truly identical )

So just treat the transformer as if it were a motor load and protect it accordingly from the high side.

You'll still need protection on the secondary ( higher ) side because transformer faults still have to be guarded against. Use the Separately Derived System rules.
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Tesla
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#178468 - 05/31/08 05:32 PM Re: Useless neutral 277/480 [Re: Tesla]
HotLine1 Offline


Member
Registered: 04/03/02
Posts: 6775
Loc: Brick, NJ USA
Redsy:
FWIW, I never had an issue with a back-feed on 75 to 150 kva 480-120/208 delta/wye xfrs. All were for 480 machine loads; balance was not an issue.
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John
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#178510 - 06/02/08 04:10 PM Re: Useless neutral 277/480 [Re: Redsy]
Redsy Offline
Member
Registered: 03/28/01
Posts: 2056
Loc: Bucks County PA
Thanks everyone.

Thanks John,

Do you prefer to corner ground, or float the 480 volt side?


Edited by Redsy (06/02/08 04:12 PM)
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#178517 - 06/03/08 04:37 AM Re: Useless neutral 277/480 [Re: Redsy]
pauluk Offline
Member
Registered: 08/11/01
Posts: 7520
Loc: Norfolk, England
If you don't want to ground a delta secondary solidly but are concerned about just letting stray capacitances balance out the voltage to ground on the phases, how about using a bank of three resistors to provide a ground reference?
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#178526 - 06/03/08 12:45 PM Re: Useless neutral 277/480 [Re: pauluk]
HotLine1 Offline


Member
Registered: 04/03/02
Posts: 6775
Loc: Brick, NJ USA
Redsy:
I float the 480 side. 3 phase, 3 wire.



Edited by HotLine1 (06/03/08 12:46 PM)
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John
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