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What would be the proper wire size?

Question asked by matthew@haskris.com on Apr 5, 2018
Latest reply on Apr 12, 2018 by matthew@haskris.com

I have a feeder circuit - power coming into a terminal block rated for 175A then wired to a 50A contactor to a compressor with an RLA of 28.8A.  The terminal block states 75C wire and the compressor states use minimum 75C wire for ampacity determination.  We are contemplating either 8AWG or 10 AWG MTW - THHW wire for this circuit.  The circuit would be considered dry location.  Am I limited to the 75C column in the wire ampacity table in NFPA 79 due to the statements of using 75C on the terminal block and compressor even though the dry location MTW - THHW wire is rated 90C?  8AWG:  75C wire = 50A, 90C wire = 55A and 10AWG:  75C wire = 35A, 90C wire = 40A.  Also, I have not included sizing the wire at 1.25 times as required in clause 12.5.3.  I am referencing NFPA 79 - 2012.  

 

If limited to the 75C column, I figure the total load here is 36A {{ 28.8 * 1.25 }}.  My thought then would be to use 8AWG.  However, I've been challenged that the 10AWG at 90C would be fine.  Any thoughts would be appreciated in helping me understand this a bit better.  Thanks.

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