Neutral wire size smaller than hots for 100 amp subpanel?

Discussion in 'Electrical Forum discussion & Blog' started by Ted M, Feb 10, 2012.

  1. Ted M

    Ted M New Member

    Messages:
    12
    Location:
    Northern New Hampshire
    For convenience I'm installing a 100 amp subpanel within one foot of the main panel, which is 200 amps.

    NO new circuits are being added; only moved from main panel to subpanel.

    Three THHN #3 wires and a #8 EGC wire will be run through 1 1/4 inch PVC.
    (Other conduits for circuits)

    In the subpanel will be 6 20 amp CBs and two double-width 240 volt CBs.

    The 240 volt CBs comprise about 25 percent of the load.
    No neutral current on these two circuits.

    So, can I reduce the neutral wire size to #4 ?

    Thank you.

    Ted
  2. jwelectric

    jwelectric Electrical Contractor/Instructor

    Messages:
    2,534
    Location:
    North Carolina
    The size of the neutral must be the maximum unbalanced load. In order the figure this you must figure the load for the feeders and find the maximum load between the ungrounded conductors and the difference between these and the neutral.

    It is much easier to just size the neutral the same as the ungrounded conductors in your case. The cost savings between the sizes would only be a few cents. To answer your question I would need the numbers of the load and each branch circuit.
  3. ballvalve

    ballvalve General Engineering Contractor

    Messages:
    3,261
    Location:
    northfork, california
    I was told in a early thread when I was using #2 for 100 amps that it was undersized. Why is he using #3? Although he did not say the size of breaker he is feeding through.

    May I consistently use one size smaller wire for the ground wire in a pull to an appliance or a subpanel?
  4. Ted M

    Ted M New Member

    Messages:
    12
    Location:
    Northern New Hampshire
    I'll be using a 100 amp CB.

    In a much earlier thread JW advised me that this is the correct wire size, which is what I had already bought.
    THHN, copper, #3. It's difficult to read the dot-matrix print on the wire, but it's also gasoline and oil rated.

    The reason I wanted to try to downsize the Neutral is because I have a 70 amp neutral ground lug (Square D LK70AN).
    I wanted to avoid finding an LK100AN. I live in the boonies, and no supply house around here sells Square D stuff.

    Ted
  5. nukeman

    nukeman Nuclear Engineer

    Messages:
    711
    Location:
    VA
    There a difference between a 100A fixed load (like that boiler thread) where a 125A breaker is needed and say a 100A subpanel, which I assume will be on a 100A breaker (could be on a smaller one, depending on what the OP needed it for).
  6. jwelectric

    jwelectric Electrical Contractor/Instructor

    Messages:
    2,534
    Location:
    North Carolina
    2 aluminum is good for 90 amps but 3 copper in a raceway is good for 100 amps
  7. jwelectric

    jwelectric Electrical Contractor/Instructor

    Messages:
    2,534
    Location:
    North Carolina
    Do a calculation of the 120 volt circuits that are on one line and if it is less than 70 amps then you will be alright.

    Should you have bought size 1 aluminum SE-R it would have come with a reduced neutral already
  8. ActionDave

    ActionDave Electrician

    Messages:
    346
    Location:
    Colorado
    I would have no problem using #4 for the neutral.
  9. ballvalve

    ballvalve General Engineering Contractor

    Messages:
    3,261
    Location:
    northfork, california
    Again, is it approved to consistently use a ground wire in a conduit pull one size smaller than the others?
  10. jwelectric

    jwelectric Electrical Contractor/Instructor

    Messages:
    2,534
    Location:
    North Carolina
    See Table 250.122 of the NEC for the correct size of EGC
  11. ActionDave

    ActionDave Electrician

    Messages:
    346
    Location:
    Colorado
    Are you asking about grounded/neutral or equipment grounding conductors? Either way, there are many cases where it is code to use a smaller wire than the hots for both, but not in every case.
  12. hj

    hj Moderator & Master Plumber Staff Member

    Messages:
    26,607
    Location:
    Cave Creek, Arizona
    220/240 circuits do not use the neutral, so it is only germaine for the 120 volt circuits. If their loads on either leg is less than 75 amps you can reduce the neutral. If you are unsure or you have unbalanced legs with greater load on one side then your safest option is to use a full size neutral.
  13. ballvalve

    ballvalve General Engineering Contractor

    Messages:
    3,261
    Location:
    northfork, california
    4 wire pulls to 240v appliances, and also a pull from a meter panel to a sub panel with a ufer ground....

    Only interested in case grounding conductors, green wires.
  14. jwelectric

    jwelectric Electrical Contractor/Instructor

    Messages:
    2,534
    Location:
    North Carolina
    A remte pane does not need a "ufer ground" or any other type of electrode unless it is in a remote buiding

    As for the green grounding conductor it will be sized using Table 250.122
  15. ballvalve

    ballvalve General Engineering Contractor

    Messages:
    3,261
    Location:
    northfork, california
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