OT - Pulling Single Phase from 3 Phase Service

Thanks
Jim

Gerald J.

NOTE your question is NOT correct in claiming you use two legs of your service to get 277 volts, if the service is what I think it is, as noted above, its 277 volts from any leg to NEUTRAL and 480 line to line. The other alternative is to use a 480 to 240 volt single phase transformer and feed its input line to line (any 2 phases) for the 480 and use the 240 volt as the single phase output.

If your service is a 480 volt three phase three wire open floating Delta, THERE IS NO 277 VOLTS which is why when you mention 277 volts I took it to be a three phase four wire Y system which is 277 any phase to neutral but 480 line to line.

I prefer where possible to balance the phases as best possible but you can still get by using a single leg of 277 to Neutral (or 480 line to line) to transform down to 240.

You reallyyyyy yyyy need to insure if the service is a 480 Y 277 volt three phase 4 wire or a 480 volt three phase 3 wire open floating delta (no 277 volts) ????? ????? ?

John T long retired electrical engineer n a lil rusty SO NO WARRANTY LOL

John T. - yes the service is 480 Y 277 V 3 Phase 4 Wire Y grounded, as I have 277V from each leg to ground and 480V between legs. I'm thinking I want to go with the 2 wire 480V single phase to 240V transformer as theorically this should help balance the load. Would the 240V output on this type of transformer have a single 240V output or 2 - 120V? If a single 240V how would I wire to accomodate a 3 wire 120V,120V,nuetral type outlet? Thanks.

-Jim

In order to arrive at two legs of 120 i.e. a 120/240 volt single phase THREE wire service, the transformer you use would have to have TO BE CENTER TAPPED. If it were 480 volt single phase in and 120/240 volt single phase three wire out, you would take the center tap, connect it to a made or other grounding electrode (it becomes a "seperately derived service" like when you place a dry transformer in a building) and then feed a main breaker equipped service entrance type panelboard where the common Neutral and Ground Busses would be bonded together.

I guess all you need is to find a dry transformer of the appropriate KVA you require thats 480 VAC single phase in with a 120/240 VAC (center tapped) single phase three wire output,,,,, ,,,tie the center tap Neutral to a grounding electrode (i.e. ground it),,,,, ,,,,,feed a service entrance panelboard with those three wires (L1 L2 Neutral),,,,, ,Bond the Neutral and Ground buss bars inside the panel and bond the panels metallic frame to the ground buss.

John T Again a lil rusty n long retired EE

A line to line load will still unbalance the 480 currents, and so the motor terminal voltages and thus the motor currents which exagurate the unbalance and if too unbalanced, fry a winding of the motor.

Gerald J.

NOTE your question is NOT correct in claiming you use two legs of your service to get 277 volts, if the service is what I think it is, as noted above, its 277 volts from any leg to NEUTRAL and 480 line to line. The other alternative is to use a 480 to 240 volt single phase transformer and feed its input line to line (any 2 phases) for the 480 and use the 240 volt as the single phase output.

If your service is a 480 volt three phase three wire open floating Delta, THERE IS NO 277 VOLTS which is why when you mention 277 volts I took it to be a three phase four wire Y system which is 277 any phase to neutral but 480 line to line.

I prefer where possible to balance the phases as best possible but you can still get by using a single leg of 277 to Neutral (or 480 line to line) to transform down to 240.

You reallyyyyy yyyy need to insure if the service is a 480 Y 277 volt three phase 4 wire or a 480 volt three phase 3 wire open floating delta (no 277 volts) ????? ????? ?

John T long retired electrical engineer n a lil rusty SO NO WARRANTY LOL

good luck