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#67691 - 07/12/06 09:47 AM 120/208 and 120/240 sinlge phase
gobblerhuntr Offline
Member

Registered: 05/26/05
Posts: 23
Loc: TN
Can somebody post the diagram for a three phase Y 120/208 transformer bank hook-up and a single phase 120/240 off the same bank. I understand you can use 120/240 single phase pots and not cut the middle pot but increase its size to handle the 120/240 single pase loads.

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#67692 - 07/12/06 10:32 AM Re: 120/208 and 120/240 sinlge phase
winnie Offline
Member

Registered: 09/15/03
Posts: 652
Loc: boston, ma
You may need to describe better what you are asking about.

120/240V is _single phase_ service, obtained from a transformer with a single primary coil and a single center tapped secondary coil.

This single phase transformer can be combined in a bank with 1 or 2 other transformers to form 120/240V 'high leg' _delta_ three phase service. The single phase transformer can be larger than the added 'high leg' transformer in order to supply the three phase load.

This sort of delta service is a totally different beastie from 120/208V _wye_ service.

In theory one could use a bank of three transformers, 2 120V transformers and 1 120/240V transformer, to form a bastard hybrid of 120/208 and 120/240. This is probably better described as a 120/208V three phase wye bank, further banked with a 120/240V single phase transformer, where the neutral of the two banks is tied together, and one of the 120V legs of the wye bank is 'merged' to one of the 120V legs of the single phase bank. I've never heard of such a thing actually used anywhere.

Take a look at these links for standard connections: http://electrical-contractor.net/ubb/Forum15/HTML/000054.html http://electrical-contractor.net/ubb/Forum15/HTML/000053.html
-Jon

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#67693 - 07/12/06 11:44 AM Re: 120/208 and 120/240 sinlge phase
wa2ise Offline
Member

Registered: 11/29/02
Posts: 771
Loc: Oradell NJ USA
 Quote:
diagram for a three phase Y 120/208 transformer bank hook-up and a single phase 120/240 off the same bank.


That would look like the old style peace symbol back from the days of the Vietnam War. Center tied to ground. I don't think any POCO does it though. You could have 120V with both sides hot by connecting say the inverse A phase (here I call the vertical line from the top to the bottom of the peace symbol phase A, bottom of this line "inverse A") to phase B or C. Why anyone would want to do that is another question, even if code allowed it. If a customer had some 240V loads he might want such service from phase A.

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#67694 - 07/12/06 12:01 PM Re: 120/208 and 120/240 sinlge phase
gobblerhuntr Offline
Member

Registered: 05/26/05
Posts: 23
Loc: TN
I have never seen it in use but a bud told me that his company has done this before for a customer that had an existing 120/240 single phase service and added 120/208 three phase equipment. This would be one bank with three single phase pots.

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#67695 - 07/12/06 05:55 PM Re: 120/208 and 120/240 sinlge phase
resqcapt19 Offline
Member

Registered: 11/10/00
Posts: 2209
Loc: IL
 Quote:
bud told me that his company has done this before for a customer that had an existing 120/240 single phase service and added 120/208 three phase equipment.

I don't see how...you would have to ground the bank at two different points and that tends to let the factory installed smoke out.
Don
_________________________
Don(resqcapt19)

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#67696 - 07/12/06 07:03 PM Re: 120/208 and 120/240 sinlge phase
SolarPowered Offline
Member

Registered: 07/05/04
Posts: 615
Loc: Palo Alto, CA, USA
 Quote:
I don't see how...you would have to ground the bank at two different points and that tends to let the factory installed smoke out.


The existing 120/240 transformer remains connected to neutral at the center. The two new transformers are 120, and each connects to the neutral at one end. You end up with a perfect "peace sign," with the 120/240 as the vertical section, and the two new 120s as the angled legs.

[This message has been edited by SolarPowered (edited 07-12-2006).]

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