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New kitchen project -- a few quetsions

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  • #16
    Re: New kitchen project -- a few quetsions

    Originally posted by bluecon View Post
    You feed the 3 wire from a double pole breaker and the opposite 120v phases will take care of that. You won't run 40a in the neutral.
    Yeah now that you say that, I know where I got the thought from, because it actually did happen.

    I was recently sent out to a restraunt with a 3 phase service and someone had pulled a multi wire branch circuit in to the kitchen for the prep counter recepticles and had three phases sharing a nuetral. That one the nuetral did burn up, but it had a load of like 38 on a #12.

    You are correct, with only two phases sharing the nuetral, the maximum unbalance could only be 20 amps. I stand corrected. I have been doing way to much 3 phase only lately. I need to go back to my roots and do some houses again.

    Jeff

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    • #17
      Re: New kitchen project -- a few quetsions

      Mind you, that when you do use a 2 circuit 3 wire feed, you WON'T be able to use GFCI breakers in the panel or else they will conflict, but you can use separate GFI outlets to protect the individual circuits once they have been split off. Now If I am not mistaken with 3 phase power, you CAN run all 3 phases off of one neutral and the nuetral will not be overloaded. In a 3 phases system if all 3 pahses are drawing current the load on the nuetral will be the unbalanced load of all 3 phases, in which case the nuetral load will never be higher than any one maximum current. So lets say you have a 3 phase system, you have a 20 amp breaker on each 3 phase hot leg and only one nuetral, if all 3 phases are drawing 20 amps the current on the nuetral is 0. If 2 phases are 5 amps and one is drawing 20 amps the nuetral is carrying only 10 amps. As proof to this, if you look at a 3 phase panel box the nuetral is never bigger than any of the line voltage wires because the current on the nuetral will never be more than the differnce in current in any of the 3 phases at any given time. Check that out

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      • #18
        Re: New kitchen project -- a few quetsions

        I want ot correct my above statement by saying that though it is true that you can run all 3 phases on one nuetral and not overload the nuetral on a 3 phase system to nuetral loads, it is not as I said above by simply adding and subtracting the loads. Though it is true that if all 3 pahses were carrying 20 amps the current in the nuetral is 0, if all 3 phases are drawing current the resultant current in the nuetral is not simple addition subtraction as I mentioned before, but rather it is vector anaylasis using trig that calculates the current in the neutral, but the fact still remains that the current in the nuetral will not be higher than the highest current carrying conductor, in which case the nuetral will NOT become overloaded. Sorry for the mistake, but I have clarified it here. My apologies.

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