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#51649 - 05/06/05 09:02 AM grounding  
menegt  Offline
Member
Joined: Jan 2002
Posts: 34
sacramento, ca
Situation:
-No 480V 3 Phase Available in location
-only 120/208 coming from 12K XFMR
-Need 480V 3 Phase for new motors being installed

I have a 208V 3 Phase (Wye)/ 480V (Delta) XFMR which I am feeding in reverse to achieve 480V. Electrician I'm working with says that there is no phase to ground on the 480V side (now secondary). Shouldn't he have a ground reference if he is hooked up to the center tap of the primary (208 V) side of the XFMR?

My question is how do achieve a ground (phase to ground) on the 480 side in my situation.

The 120/208 side is properly bonded/referenced (center tapp bonded to ground)


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#51650 - 05/06/05 09:21 AM Re: grounding  
Gregtaylor  Offline
Member
Joined: Mar 2005
Posts: 212
Boise, Idaho, USA
The first disconnect on the secondary side of a transformer is a seperately derived service, and the code says a service must have a grounded conductor. To achieve this in the situation you described you will have to ground one of the secondary phases. This is called corner grounding and it seems counter-intuitive the first time you do it. The conductor that is grounded must now be grey or white (although I usually put a band of the original phase color on the end as well) and then LABEL EVERYTHING because it's confusing for the next guy.


#51651 - 05/06/05 05:26 PM Re: grounding  
resqcapt19  Offline
Member
Joined: Nov 2000
Posts: 2,148
IL
Greg,
Quote
and the code says a service must have a grounded conductor.

Please cite a code reference for that your statement. I do not think that it is correct.
Don


Don(resqcapt19)

#51652 - 05/06/05 05:27 PM Re: grounding  
resqcapt19  Offline
Member
Joined: Nov 2000
Posts: 2,148
IL
menegt,
There should be no connection to the XO terminal on this transformer hook-up.
Don


Don(resqcapt19)

#51653 - 05/06/05 06:00 PM Re: grounding  
winnie  Offline
Member
Joined: Sep 2003
Posts: 649
boston, ma
There is no requirement that the secondary of this transformer be grounded. However there are very specific requirements for _ungrounded_ systems (250.21) and ungrounded systems are a bad idea IMHO, prone to causing additional failures once one occurs.

If you specifically want the benefits of ungrounded systems, then IMHO you should use a high impedance grounded system.

If you don't specifically want the benefits of an ungrounded system, then you should ground the secondary of this transformer.

Corner grounding is one option. Another option is to use a 'zig-zag' transformer to generate a neutral and ground that.

-Jon


#51654 - 05/06/05 08:47 PM Re: grounding  
Gregtaylor  Offline
Member
Joined: Mar 2005
Posts: 212
Boise, Idaho, USA
OK, my off the top of my head reply got a little ahead of the code, technically. 200.3 says that a premise wiring system "shall not be electrically connected to a supply system unless the latter (the supply system i.e. the secondary side of a transformer) contains,for any ungrounded conductor of the interior system, a corresponding conductor that is grounded." This applies to the situation in question,IMO.
Also, since menegt says he already has an appropriate transformer, all the other suggestions about how he could accomplish the same thing are off the point. He needs 480v 3ph. He doesn't NEED a neutral as the term is being used here. He just needs a grounded conductor, and corner grounding is the simplest way to do it.


#51655 - 05/06/05 11:49 PM Re: grounding  
resqcapt19  Offline
Member
Joined: Nov 2000
Posts: 2,148
IL
Greg,
The section that you cited does not require a grounded conductor for this system. It only says that if the premises wiring system has a grounded conductor that it must be connected to a grounded conductor on the supply system. It does not require that the permises wiring system have a grounded conductor. That issue is covered by 250.20 and there is no requirement that a 3 wire 480 volt system have a grounded conductor. That is a design issue, not a code issue. The 3 wire 480 volt system is permitted to be a grounded system, but not required to be a grounded system.
Don


Don(resqcapt19)


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