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OK. 75 C refers to the temperature rating of the wire insulation. Wire ratings start at 60 C (centigrade) and go up in increments of 15 C. In simple terms, the ampacity of a wire of a given size increases as the temperature rating of the insulation increases. For instance, 6 AWG UF wire has an ampacity of 55 amps in the code tables. 6 AWG THHN has an ampacity of 75 amps. The copper wire size is the same, but the electrons can be driven faster on the higher rated wire.
Terms such as THHN, THWN-2, THHW, UF, NM-B and so forth refer to the actual insulation material of the wire. For instance, THHN has heat resistant thermoplastic insulation. THWN has moisture and heat resistant thermoplastic insulation.
The actual wire gauge should be selected based on a number of factors such as required circuit ampacity, length of run, number of wires in the conduit, conduit fill percentage, highest expected ambient temperature, etc. After taking all this into account, a wire size is selected. When in doubt, always go to a larger size.
: John, ok, but....what does "75 C" mean? 75 Centigrade? Also,
: what does
: THHN, THWN-2 mean? What gauge SHOULD the wire be?
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