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Thread: Please check my derating calculations.

  1. #1
    DIY Junior Member rick15752's Avatar
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    May 2010
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    Default Please check my derating calculations.

    I've been looking for some help on derating and hope someone here can help me.

    Home was built in 1985 and, apparently, derating wasn't closely observed then. But, I've run some new circuits to my breaker panel and I want to make sure I follow the NEC 2008 334.80. Here is my question / situation:

    Top plate at breaker panel, holes for wiring appear to be 1 1/4" in 2x4 framing and are foam sealed. I found two of these holes, containing some 14-2 wire, that allowed me to easily fit more circuits (NM-B) to the panel. I can't drill anymore holes in this stud bay because of other wiring to the panel. I'll reseal with foam.

    What I ended up with:
    Hole #1: three 12-2 plus one 14-2 (each has their own breaker, three 20 amp and one 15 amp)

    Hole #2: two 12-2 plus one 14-2 (each has their own breaker, two 20 amp and one 15 amp)

    Derating calcs:

    Hole #1: 8 ccc
    NEC 310.15(B)(2)(a) 7-9ccc = 70 %
    NEC Table 310.16 Column 90 12ga = 30 amps, 14ga = 25 amps
    30 x .7 = 21.00 amps 20 amp breakers for the three 12 ga wires into the panel
    25 x .7 = 17.50 amps 15 amp breaker for the one 14 ga wire into the panel

    Hole #2: 6 ccc
    NEC 310.15(B)(2)(a) 4-6ccc = 80 %
    NEC Table 310.16 Column 90 12ga = 30 amps, 14ga = 25 amps
    30 x .8 = 24 amps (20 amp breaker for the 12 ga wire)
    25 x .8 = 20 amp (15 amp breaker for the 14 ga wire)

    Are these calculations correct? Should they be calculated separately as done here?
    Last edited by rick15752; 02-09-2011 at 11:42 AM.

  2. #2

    Default

    1. You do not have to derate.
    2. 12-2 wire requires a 20A max breaker. 14-2 wire requires a 15A max breaker. Table 310.16 has a little asterisk next to AWG size 14,12, and 10 that leads you to article 240.4(D). You may want to read that.

    -rick

  3. #3
    Electrical Contractor/Instructor jwelectric's Avatar
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    Default

    Quote Originally Posted by rick15752 View Post
    Are these calculations correct? Should they be calculated separately as done here?
    looks good .

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