single phase from three phase network voltage drop

I'm feeding 208/120v single phase from a 208Y/120v three phase source - typical condo arrangement. Even though its a single phase circuit,
shouldn't I calculate the voltage drop as a three phase (208Y/120) then multiple by .58 (1.73/3) for the single phase 120V drop and by 1.15 for the 208V drop? I'm getting conflicting information. thanks Dave
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----------------------- You won't get 208/120V single phase. The 208V is line to line between phases and the 120V is line to neutral- which can be handled as single phase. This voltage will not be in phase with the line to line voltage. If you have loads from line to line and additional loads from line to neutral (on each phase) then you should be looking at unbalanced 3 phase. A lot depends on your feeder and its independence or lack thereof from other loads. Simply fooling around with factors doesn't make any sense unless you know the basis for the factors.
It is time to actually lay out what you are trying to do and get competent advice locally- it may cost.
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Don Kelly @shawcross.ca
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Don, I'll get two legs and a neutral from the three phase circuit, that's a single phase feeder. It's done all the time. If I need to connect a 208V single phase load to a 208Y/120V three panel board, I simply connect it with a two pole breaker. Similarly, in a residential environment, the utility supplies 208Y120V three phase but each condo gets 208/120V single phase. Dave
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As Don said, it is an unballanced 3 phase feeder. The hots are 120 degreees out of phase.
If it was single phase the voltage would be 240/120 and the hots would be 180 degrees out of phase.
You are only using it for single phase loads, but a hot-hot-neutral connection is not single phase.
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Condos and apartments in Phoenix are connected every day like this. A service at 208 3 phase is set, 400, 600, or 800 amps, then the meter sockets are connected single phase to the 3 phase supply. Each apartment has its own utility meter and bill. Utilities do not seem to worry about the imbalance.
Do your calculations at the feeder voltage. At least that is what I would do.
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