Using ROMEX® to feed subpanel, do I have to derate from Table 310.15(B)(6)

Discussion in 'Electrical Forum discussion & Blog' started by Mr_Magoo, Sep 23, 2008.

  1. Mr_Magoo

    Mr_Magoo New Member

    Messages:
    16
    I want to put a subpanel in the garage. According to Table 310.15(B)(6), I would need #4 Copper for a 100amp subpanel. Is this table for individual conductors in conduit or can you use ROMEX® to feed the subpanel at 100amp? The reason I ask is I read that when using ROMEX® you had to derate the THHN in ROMEX® to the 60*C rating.

    Thanks,
    Mike
    Last edited by a moderator: Dec 7, 2011
  2. Mr_Magoo

    Mr_Magoo New Member

    Messages:
    16
    Further research has indicated that I do need to derate when using NM.
  3. jwelectric

    jwelectric Electrical Contractor/Instructor

    Messages:
    2,532
    Location:
    North Carolina
    Do just a bit more research and you will see that Table 310.16 must be used also
  4. Speedy Petey

    Speedy Petey Licensed Electrical Contractor

    Messages:
    992
    Location:
    NY State, USA
    You are not "derating" anything. NM cable simply must use the 60 deg C column.

    IF you did need to derate the cable you would do so from the actual insulation temperature column which is 90 deg C.
  5. Chris75

    Chris75 Electrician

    Messages:
    608
    Location:
    Litchfield, CT

    As already mentioned, you cannot use table 310.15(B)(6).
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