THNN Vs. THWN Wire - Electrical - Page 2 - DIY Chatroom Home Improvement Forum

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Old 11-13-2009, 10:12 PM   #16
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I realize this is sort of late in the thread, but here's what the wire designation letters mean.

T=Thermoplastic insulation, 60C

H=15C higher. HH=30C higher.

W=Wet locations. Ok to submerge in water. The inside of a conduit installed in a wet location is considered a wet location.

N=Nylon outer covering.

THHN=90C insulation, dry locations only.

THWN=75C insulation, OK for wet locations.

Be careful with the 90C temperature rating though, the vast majority of terminations are rated for a maximum of 75C, and this is the ampacity column you must use in table 310.16 The 90C rating is useful when de-rating due to multiple conductors in the same conduit.

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