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Thread: 18 amps going back on the neutral

  1. #11
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    Jul 2003
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    New Jersey
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    Quote Originally Posted by Rick F View Post
    it could possibly be 3 phase.. but i don't understand how that would altar the fact that nothing is being fed 3phase, it's all 120 or 240v nothing is is seeing all three phases...I didn't look that much into the panel..
    Here's one thing that the NEC says regarding the neutral current when using only two phases in the MWBC from a 3Ø system.

    (5) Neutral Conductor.
    (b) In a 3-wire circuit consisting of two phase conductors and the neutral conductor of a 4-wire, 3-phase, wye-
    connected system, a common conductor carries approximately the same current as the line-to-neutral load currents
    of the other conductors and shall be counted when applying the provisions of 310.15(B)(3)(a).
    Rob

    Moderator

    All responses based on the 2014 NEC unless otherwise noted

  2. #12
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    Quote Originally Posted by LarryFine View Post
    I have another possibility: The supply is 208Y/120 3ph, not 120/240 1ph.
    Not just a possibility, very likely - and was the first thing that came to my mind when reading OP.

    Quote Originally Posted by Rick F View Post
    but can you explain to neutral current? seems like a big waste of money
    Quote Originally Posted by ActionDave View Post
    The neutral current does not cancel out on two legs of a three phase system the way it does on a single phase system, It carries all the neutral current from all the 120V loads.
    In addition to what Dave said - you need all three legs of a wye system with equal current on all of them to get a balance that has a net of zero current on the neutral. 120 vs 180 degree phase angle is the reason. See if this starts to help make sense without getting into the vector math-

    If you have 120/240 system and put two identical loads on opposing lines connected to neutral - each draws same current, the neutral will read read zero. Disconnect the neutral from the supply and nothing changes for the most part, the loads will still operate in series with 240 volts supplied and half the volts dropped across each load.

    Now put same two loads on a 208/120 system. Each still draws same current as before because each load still is connected to 120 volts, but 120 plus 120 is not 208 - neutral is going to have to make up for that difference. Now disconnect the supply neutral and you only have 104 volts across each load when they are in series, and current is also going to drop accordingly no neutral to pick up the difference.

    When load on A is equal to load on B some of A current is returning to neutral, some is returning to B, and at same time some of B current is returning to A as well as some to N. The end result when A and B are equal loads is neutral current is approximately same level also. Add C phase with same current level and neutral will be zero, but is splitting 3 ways instead of two ways like it does with 120/240 single phase supply.
    I live for today, I'm just a day behind.

  3. #13
    Join Date
    May 2005
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    Another way to explain it easily: Start with a fully-balanced 3ph load of 20a, 20a, and 20a; zero neutral current. Now, reduce one phase's load by 1a; neutral current is now 1a. For every amp that one phase's load reduces, the neutral current rises the same amount. Zero load on that phase results in 20a on the neutral.
    Code references based on 2005 NEC
    Larry B. Fine
    Master Electrician
    Electrical Contractor
    Richmond, VA

  4. #14
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    Apr 2005
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    Quote Originally Posted by Rick F View Post
    We have lunch trucks on a street where I work,
    Quote Originally Posted by Ingenieur View Post
    sounds normal
    L1 17 A
    L2 4 A
    N 18 A

    L1 is loaded much more than 2
    the N sounds a bit high but not alarming

    If L1 = 20, L2 = 0 then N = 20
    as you said, balance the pnl
    Not that easy to balance a panel for this equipment.
    It may be more balance as different compressor motors kick in. One 10 A motor on one leg can change every thing. When it's running thing are more balance but when it shut off after reaching set temp.

    I'll bet that if they check the unbalanced load at different times they will come up with different currents readings on the neutral.
    The 95% of people that you can't trust give the other 5% a bad name.

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