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Thread: Sizing of the conductors for a subpanel

  1. #1

    Default Sizing of the conductors for a subpanel

    Can I use a nuetral for a subpanel that is one size smaller than the hot conductors?

    I am in a sitaution where I am completing a job I did not start. The existing pvc conduit run from the main panel to the location of a subpanel is 1-1/2 inch schedule 40. I need to get the maximum amperage feeding this subpanel from the existing 200A main panel.

    I plan on using 2x #1 conductors for the hots, 1x #2 for the nuetral and a #4 for the ground, all copper. And feeding the hots through a 125A 2p circuit breaker.

    Does this sound right???? I don't think I can easily get anything larger through the 1-1/2 inch conduit. Any thoughts?

  2. #2
    Join Date
    Jul 2005
    Location
    London, Ontario
    Posts
    263

    Default

    Well, yes you can use a smaller neutral conductor, but you are then limited to the current that the neutral conductor can carry (vs the higher current that the larger current can carry).

  3. #3
    Join Date
    Dec 2003
    Location
    Kent, WA
    Posts
    8,427

    Default

    What type of wire are you using? If its copper with at least one H, then #1 can do 130 amps and #2 can do 115 amps. If this is aluminum, you're limited to 100A breaker and 90A neutral load (or perhaps 110A/100A if 310.15(B)(6) can be applied to subpanel feeders in your area). If it helps, you can run a #6 copper grounding wire instead of a #4.

    Since you have used a 125A breaker, if the calculated load is 115A or less then you are fine on copper wire (you can round up the breaker to the next standard size which is 125A). If the load calculates to 125A, then you need to determine if the unbalanced load is 115A or less. All it would take would be a 240V only load of 25A or so to accomplish (and perhaps less depending on the % factor applied to the load).

    A range or dryer allows a 70% factor to be applied to the neutral loads. A 240V water heater, pump, or HVAC system would provide a lot of relief to that neutral load.
    Mark
    Kent, WA

  4. #4

    Default

    I did it today and actually used three copper #1's (THHN) for the hots and neutral plus a #4 for the ground. They pulled easily through the 1-1/2" PVC.

    It's a weird job in that I came in at the middle of and don't have accurate plans. There are some circuits wired for 240v. One is a dryer and I believe there are plans for a small AC system.

    Thanks for the advice though.

  5. #5
    Join Date
    Jul 2005
    Location
    London, Ontario
    Posts
    263

    Default

    Wasn't thinking about 240V loads in my previous post.

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