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#64752 - 04/14/06 06:14 AM grounding a step down transformer review  
lite bulb  Offline
Member
Joined: Feb 2006
Posts: 78
wilm,de.
I have a 800 amp 277/480 volt mcc panel running a small plant,in the same room with the mcc panel i am adding a 30 kva transformer 480-208/120 for a subpanel,out of the mcc panel,i tapped the main nippled out of the mcc with a galv.nipple bond bushing on each end to a 40 amp disconnect, i ran 4 #8 3ph and a ground. in 1" sealtight with bond bushings on each end, i ran 8'to the transformer 3ph and the ground that is connected to the transformer frame. out of the transformer to the sub panel with 10' of pvc with 4 #2 AND 1 #6 ground, bonding done in the subpanel, #6 grounding electrode to plant steel frame.


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#64753 - 04/15/06 07:57 AM Re: grounding a step down transformer review  
Steve McKinney  Offline
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Joined: Apr 2004
Posts: 79
Marion, Illinois
Lite,

Did you tie in to your Xo connection on the secondary. All steel needs to be bonded as well.

Steve


#64754 - 04/15/06 08:44 AM Re: grounding a step down transformer review  
lite bulb  Offline
Member
Joined: Feb 2006
Posts: 78
wilm,de.
just the neutral to xo there is a strap from xo to the transformer frame,bonding is done in the panel. could i have just run the 3ph and the neutral and not the ground from the trans. to the panel just like a incoming service, or was it right to run the ground, now i look at it there are 2 paths of ground coming from the 120/208 panel (1) the neutral,(2)the ground.


#64755 - 04/15/06 11:05 AM Re: grounding a step down transformer review  
resqcapt19  Offline
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Joined: Nov 2000
Posts: 2,148
IL
You can't make a bond in both the transformer and the panel on a SDS. You only do it at one location. No matter where you make the bond, there has to be a path from the transformer case to XO. If you bond at the transformer, the path is direct from the case to XO. If you bond at the panel, the path from the case is via the equipment bonding jumper to the panel. In either case an equipment bonding jumper is required between the transformer and the panel.
Don


Don(resqcapt19)

#64756 - 04/16/06 09:07 AM Re: grounding a step down transformer review  
lite bulb  Offline
Member
Joined: Feb 2006
Posts: 78
wilm,de.
so the bonding strap that came with the transformer xo to the case has to be removed when bonding in the panel.


#64757 - 04/16/06 04:04 PM Re: grounding a step down transformer review  
resqcapt19  Offline
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Joined: Nov 2000
Posts: 2,148
IL
Quote
so the bonding strap that came with the transformer xo to the case has to be removed when bonding in the panel.

Yes. Look at 250.30(A)(1).
Quote
(1) System Bonding Jumper An unspliced system bonding jumper in compliance with 250.28(A) through (D) that is sized based on the derived phase conductors shall be used to connect the equipment grounding conductors of the separately derived system to the grounded conductor. This connection shall be made at any single point on the separately derived system from the source to the first system disconnecting means or overcurrent device, or it shall be made at the source of a separately derived system that has no disconnecting means or overcurrent devices.

Don


Don(resqcapt19)


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