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Thread: Wire insulation letters. Thhn

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    Wire insulation letters. Thhn

    Hey there. I can't find where in the code book it tells what the letters on conductor insulation. Like then. Thank you.

    Sent from my Moto G (4) using Tapatalk

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    Look at 310.104
    Without data you’re just another person with an opinion – Edwards Deming

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    Note that thhn auto-corrects to then.

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    You mean what the letters actually mean / stand for ?

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    Quote Originally Posted by chuckgassmannm View Post
    Hey there. I can't find where in the code book it tells what the letters on conductor insulation. Like then. Thank you.

    Sent from my Moto G (4) using Tapatalk
    310.104 gives you some idea but doesn't exactly tell you every detail either.

    A few common things:

    T: thrermoplastic
    H: heat resistant
    W: moisture resistant
    N: Nylon jacket (mostly for gasoline and oil resistance)
    X: cross linked polyethylene
    M: machine tool wire
    R: rubber

    some combination examples:
    TW = thermoplastic, moisture resistant
    THW = thermoplastic, heat resistant (75C), moisture resistant
    THHN = thermoplastic, double heat resistand (90C), moisture resistant
    THWN = thermoplastic, heat resistant (75C), moisture resistant, nylon jacket

    Then you find a -2 sometimes this turns THWN-2 into same thing as THWN but with a 90C rating. For some reason they'd rather do that than call it THHWN.
    I live for today, I'm just a day behind.

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    Thank you. That does answer my question.

    Sent from my Moto G (4) using Tapatalk

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    The first place I can recall finding this being used goes back to the early decades of the 1900s and the NECs published then.

    Rubber insulated conductors were being perfected, using a largely synthetic compound that was mandated to contain at least (something like) 13% real rubber. Three different identifiers were used: R, H and W

    R = The basic 60 degree C, "rubber" insulation with a cotton weave slip that is impregnated with bituminous compounds and further coated with wax.
    H = Rubber insulation "Heat" rated for 75 degree C.
    HH = "High Heat" rubber insulation rated for 90 degree C.
    W = "Wet" moisture rated rubber insulation.

    The "T" came with the post World War II evolution of plastics and stands for "Thermoset Plastic."
    Another Al in Minnesota

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    Quote Originally Posted by al hildenbrand View Post
    The first place I can recall finding this being used goes back to the early decades of the 1900s and the NECs published then.

    Rubber insulated conductors were being perfected, using a largely synthetic compound that was mandated to contain at least (something like) 13% real rubber. Three different identifiers were used: R, H and W

    R = The basic 60 degree C, "rubber" insulation with a cotton weave slip that is impregnated with bituminous compounds and further coated with wax.
    H = Rubber insulation "Heat" rated for 75 degree C.
    HH = "High Heat" rubber insulation rated for 90 degree C.
    W = "Wet" moisture rated rubber insulation.

    The "T" came with the post World War II evolution of plastics and stands for "Thermoset Plastic."
    The wires with "T" are thermoplastic and not thermoset.
    Don, Illinois
    (All code citations are 2017 unless otherwise noted)

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    "T" doesn't always mean thermoplastic. As in MTW (Machine Tool Wire).

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    Quote Originally Posted by 480sparky View Post
    "T" doesn't always mean thermoplastic. As in MTW (Machine Tool Wire).
    Well then W doesn't always mean moisture resistant either
    I live for today, I'm just a day behind.

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