Using a 3-phase transformer to feed a 1-phase panel?

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sw_ross

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I have a situation where some work trailers are being place near an Army Corp. dam spillway for some work a company is doing on the spillway. There is a 480v 60amp 4-wire receptacle available to feed power to the trailers. The company thats doing the work is familiar with this situation and does this type of work in a variety of locations.

They have provided a 30kva 3-phase transformer and a 480v disconnect, and want us to hook up the trailers. They've also provided SO drop cords to feed the transformer from the receptacle (4-conductor, #6) as well as feed each of the work trailers from the transformer.

They want us to hook up the system to power up the trailers. We want to add in a 3R panel on the load side of the transformer, using that spot to re-establish the service (bonding the neutral, and adding two ground rods), and then having 3 60amp breakers that feed the drop cords to each trailer.
Each trailer has a sub-panel with a 60 amp main.

My question is whether there are any major ramifications of feeding a single phase panel from this 3-phase panel (other than having 208v L1-L2)?
Otherwise any other comments are appreciated.
Thanks,
Sky
 
Is there 240 volt loads in the trailers? If so they will only be getting 208 volts, may not be a problem but needs considered. If they are all 120 volt loads there will probably be no problem at all.
 
sw_ross said:
I have a situation where some work trailers are being place near an Army Corp. dam spillway for some work a company is doing on the spillway. There is a 480v 60amp 4-wire receptacle available to feed power to the trailers. The company thats doing the work is familiar with this situation and does this type of work in a variety of locations.

They have provided a 30kva 3-phase transformer and a 480v disconnect, and want us to hook up the trailers. They've also provided SO drop cords to feed the transformer from the receptacle (4-conductor, #6) as well as feed each of the work trailers from the transformer.

They want us to hook up the system to power up the trailers. We want to add in a 3R panel on the load side of the transformer, using that spot to re-establish the service (bonding the neutral, and adding two ground rods), and then having 3 60amp breakers that feed the drop cords to each trailer.
Each trailer has a sub-panel with a 60 amp main.

My question is whether there are any major ramifications of feeding a single phase panel from this 3-phase panel (other than having 208v L1-L2)?
Otherwise any other comments are appreciated.
Thanks,
Sky
Click to expand...

I would use a 3 phase panel and connect each trailer in a way that balances the load.

One trailer A-B, the next A-C, the next B-C etc.
 
I agree, a 3-phase panel would be best, but these guys are anxious to get their work trailers powered up and we already have a single phase 3R panel set up on plywood for temporary power at jobsites. Maybe we'll start with the panel we have until we can get a 3-phase panel.

Of the three trailers, power is mostly for lights and outlets. There's also heat in each one. I looked at the nameplate on the externally mounted heater on two of the trailers, and it showed a voltage requirement of from 196volts - 256volts operating range, so I think 208volts will be okay.

My other area of question is the grounding situation. Simplifying the scenario, we basically have about a 100-200 foot drop cord (4-wire, #6) that will feed 480volts out to a fused disconnect we'll mount by the transformer. That cord plugs into a 480v, 60a outlet located in the spillway facility and is protected by a breaker somewhere in the facility. The transformer will be located out, away from the facility probably as far as the cord will reach. Then on the secondary side of the transformer we'll have our temporary panel and out of that panel we'll have 3 60amp breakers that feed drop cords to each of the 3 trailers.

I know that transformers are supposed to be bonded to the building steel (i.e. connected to the structures grounding system). In this case have an equipment grounding conductor coming out to the disconnect/transformer, but we're a ways away from the structure to be able to bond to the building. I know on the secondary side (either in the transformer or in the panel) we have to bond the neutral to the ground and I'm thinking we should plan on driving two ground rods, as if it were a feeder out to a separate detached structure, thereby re-establishing a service at the temporary panel.

Can anybody critique my thinking about how I want to ground this, and slap me in the face if I am over-complicating this?

Thanks,
Sky
 
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