Grounding of a SDS (transformer), but similar situation.

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sw_ross

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I have a 'real world' scenario where a contracting company needs power for their work trailers at an Army Corps of Engineering spillway project. There is a 480v 60amp, 4-wire receptacle available for them to use in the spillway structure. The company does this type of work on lots of dam sites, so is moderately prepared with material to do this. They have a 30kva, 480v step down xfmr (208/120), and #6 4-wire drop cords.

They want us to set up the drop cords, hook up the transformer, and power up their work trailers. The xfmr and trailers will be located 100-200 feet away from the structure and receptacle. On the primary side of the xfmr will be a fused disconnect, on the secondary side of the xfmr we will put a panel that has three 60 amp breakers that feed three drop cords to the three trailers.

I know that the neutral has to be bonded on the secondary side, either at the transformer or panel, and supposedly the xfmr needs to be connected to the structure GES. With the distance that the xfmr will be away from the structure, I'm thinking of just using 2 ground rods at the panel, creating a service - just as if you had a 4-wire feeder to a separate structure.

This is all a temporary setup for work being done on the spillway.

I would appreciate hearing feedback regarding this grounding scenario,
Thanks,
Sky
 
sw_ross said:
I have a 'real world' scenario where a contracting company needs power for their work trailers at an Army Corps of Engineering spillway project. There is a 480v 60amp, 4-wire receptacle available for them to use in the spillway structure. The company does this type of work on lots of dam sites, so is moderately prepared with material to do this. They have a 30kva, 480v step down xfmr (208/120), and #6 4-wire drop cords.

They want us to set up the drop cords, hook up the transformer, and power up their work trailers. The xfmr and trailers will be located 100-200 feet away from the structure and receptacle. On the primary side of the xfmr will be a fused disconnect, on the secondary side of the xfmr we will put a panel that has three 60 amp breakers that feed three drop cords to the three trailers.

I know that the neutral has to be bonded on the secondary side, either at the transformer or panel, and supposedly the xfmr needs to be connected to the structure GES. With the distance that the xfmr will be away from the structure, I'm thinking of just using 2 ground rods at the panel, creating a service - just as if you had a 4-wire feeder to a separate structure.

This is all a temporary setup for work being done on the spillway.

I would appreciate hearing feedback regarding this grounding scenario,
Thanks,
Sky
Click to expand...

Sky, If you are supplying a properly sized EGC to the primary transformer disconnect, and onto the transformer, all is well. If you are including an equipment bonding jumper from the transformer secondary to the first OCPD in the panel and an EGC to each of the three trailers, all is good. A grounding electrode(s) at the transformer/panel and at each trailer is required (or a common electrode(s)) if in the same area.

I see a few problems with overcurrent protection on the primary and secondary side of the transformer.You didn't mention if the secondary panel has an individual OCPD along with the three 60 feeder breakers for the trailers and what size it is. What size primary fuses are being used?

I don't see rubber cord as a proper wiring method being allowed and if excepted, # 6 awg would only be rated for 45 amps per table 400.5(A)1.
 
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