Re: OT - Pulling Single Phase from 3 Phase Service

Posted by John T on August 06, 2007 at 19:30:23 from (66.244.83.147):

In Reply to: OT - Pulling Single Phase from 3 Phase Service posted by Jim in DE on August 06, 2007 at 18:49:03:

Jim, Am I correct in assuming the 3 phase service you have is 480 Y 277 V 3 Phase 4 Wire Y grounded??? If thats so, its 277 volts from any leg to NEUTRAL and 480 VAC from line to line. That being said, if you need 240 VAC single phase, yes indeed you can theoreticaly (provided you can get a 277/240 transformer) use any of the three 277 legs (to NEUTRAL) and transform it down from 277 to 240 VAC single phase to run a 240 VAC single phase motor.

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 reallyyyyyyyyyy 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

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