Heat kit wire whip sizing.

Discussion in 'Electrical Forum discussion & Blog' started by mrbeing123, Feb 18, 2014.

  1. mrbeing123

    mrbeing123 New Member

    Messages:
    19
    Location:
    Louisiana USA
    Could someone please tell me if I am correct, or please correct me if I'm wrong.
    -3.5 ton Heat Pump Package unit
    -1 10kw heat strip
    -100 amp appliance disconnect next to unit

    The unit needs two feeds off the disconnect.
    -1 22.2A(MCA: 26.2/MOP: 40) for compressor and air handler portion

    -1 10kw(MCA: 52/MOP: 60) for the heat kit

    The compressor and handler needs #10 with MOP 40
    However, for the heat strip, since it is for the most part a constant load,
    the wire size for the whip should account for the 80% ampacity(210.19(A)(1)).
    The 10kw element calculates to ~41.7A. And which 41.7 * 1.25 = 52(same as MCA)

    I just wanted to verify what I believe, the heat kits whip should be a 8/2 whip of 90c Cu correct?
    also, the EGC should be #8 as well right?

    Would this normally be done in one conduit whip? Using Liquidtight, the LFNC-B fill table shows 3/4 sufficient for up to 6 #8 conductors.

    Thanks for any help provided
  2. Speedy Petey

    Speedy Petey Licensed Electrical Contractor

    Messages:
    988
    Location:
    NY State, USA
    The only thing you have wrong is you CANNOT use 8/2NM cable. NM cable must use the 60 deg C column, so it is god for only 40A.
    #8cu conductors in conduit can use the 75 deg C column so it is good for a 50A circuit. Either way, the MCA is 52A so #6cu is the minimum size conductor you can use.

    You can use individual conductors in a sealtite whip. The ground must be sized for the largest circuit which is 60A, so one #10cu ground would be minimum. You DO NOT need separate grounds for multiple circuits on a raceway.
  3. mrbeing123

    mrbeing123 New Member

    Messages:
    19
    Location:
    Louisiana USA

    Please excuse the way I wrote that. By 8/2 I was meaning 2 individual thhn wires in the conduit and not nm cable. ie. the whip as a whole would contain 2 conductors. I have a question. Just out of curiosity, wouldn't it be allowed to use the 90c column if the terminations on the disconnect were rated for 90c? Mine is not though. Does anyone even offer 90c rated terminals on there disconnects?
  4. mrbeing123

    mrbeing123 New Member

    Messages:
    19
    Location:
    Louisiana USA
    I'm only asking because I would like to understand.
  5. Speedy Petey

    Speedy Petey Licensed Electrical Contractor

    Messages:
    988
    Location:
    NY State, USA
    No. It is all but guaranteed that one cannot use the 90 deg C column for this reason. Typically 75 deg C is the best you will ever do when figuring circuit ampacity.
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