voltage difference on the three legs
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  1. #1
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    Default voltage difference on the three legs

    when I check the voltage on my three phase I have two legs with 115 volts and one with 230 volts, this is coming out of breaker box installed in the building, How do I determine which leg on the lathe,milling machine etc goes to each leg or is there something needed to knock down the the 230 leg to 115

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    Default Measure voltage leg to leg on 3ph

    the "high leg" should not be used for computers or for control circuits.

    Of lights for that matter. just use it for 3ph motors, and you will be fine.

    Cheers

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    Default

    You seem to have "wild leg" 3 phase.....aka "farm" 3 phase..... wired so you can use 2 of the three for lights etc, just like house wiring.

    Basically the same as RPC output..... Use the "high leg " for motors only.

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    On the 3ph the right wire is the wild leg. The two left wires can be used as previously described. On the motors it makes no difference which wire goes where. I have 3ph at my house and run the house off the two 120 lines, while my wild leg is only 200v. That probably cleared the mud up. lol tommie

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    Default

    As i understand it, the service is standard three phase with the ground on the center tap of one of the transformer windings. One that has 115 volts on each end of that winding will theoretically have 199.18 volts to the ground. There is no reason that the high leg should be any less stable or unreliable than the others. It is coming from the same power lines as the other legs and I would expect that the power company takes it from different phases from one service to another to balance the load because there would be a higher draw on the one supplying single phase. If you read from leg to leg, all three should read the same. It sounds like Tommied should be happy because his appears to be well balanced.

    It is all the same power. All it does is save transformers and wire.

    http://en.wikipedia.org/wiki/High-leg_delta

    Their example shows 240 V across the windings rather than 230, so the high leg to ground is correctly given as 208. Nothing the least bit arcane or mysterious about it once you get your viewpoint straight.

    Younpete, check your readings again. 230 V is too high.

    Bill

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    NEC forbids use of the wild leg for single-phase loads....... I forget the section, but..........

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    Quote Originally Posted by JST View Post
    NEC forbids use of the wild leg for single-phase loads....... I forget the section, but..........
    Does the transformer have a heavier winding on the base leg? It would make sense if you prohibited single phase use of the other two legs because the load on the base would always equal to or greater than the other two.

    Bill

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    Default

    the "high leg or wild leg" was the answer I needed, this goes to the right or l3 terminal on the machines and by doing this the voltage variance is taken care of

    thanks for the replies

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    Hi Youn I have quite a bit of experience with the "wild leg" 3 phase power. Maybe in the big cities it might be called "farm" but where I came from it was Normal to have "wild leg" 3 phase. You are measuring the 3 phase to neutral (ground) Measure the voltage between phases. I have seen the "wild leg" as low as 190vac and as hight as 250vac measured to ground especially on open delta systems! Yet line to line was 240vac + or - a few volts. Connection to your machine should not be a problem but you must check see if any one line was used to ground. Most machinery have a transformer connected to the input and all single phase power taken from it. However modifications might have been made such as someone connected a light to the system and used one leg to neutral. Frank


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