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Thread: Understanding ampacity ratings

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    Understanding ampacity ratings

    I was looking at the ampacity ratings chart - NEC Table 310.16:

    There are 3 ratings for each sized wire. Using #2 THHN wire as an example:

    Allowable Ampacity: 95 Amps at 60C / 115 Amps at 75C / 130 Amps at 90C

    I have also seen wet rating and dry rating.

    Is there a bottom line to which rating you use to know the rating of the particular wire? Is it at 60, 75 or 90 degrees? Does it depend on where you live and what the air temperature is?


    Thanks

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    JasonLion's Avatar
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    Re: Understanding ampacity ratings

    There are several answers. The simple answer is that if you have to ask questions like that you probably shouldn't be doing your own wiring. The "don't count on this because the answer is a lot more complex than this" answer is that it depends on which ever is lowest of the temperature rating of the wires insulation and the temperature rating of the insulation of the things it will be connected to at either end, but with several exceptions and special cases. The real answer is that you should ask questions like that one on an electrical forum, rather than a swimming pool forum.
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    Re: Understanding ampacity ratings

    JasonLion, thank you for your response. I think you confused me more, lol. Thanks for the advice about not doing my own wiring.

    If anyone cares, this seems to explain it pretty well. It is a little confusing. https://www.youtube.com/watch?v=WL81zOzNagM

    This is helpful as well http://ecmweb.com/code-basics/conductor-size-matters

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    Re: Understanding ampacity ratings

    Phew that was heavy reading, I'll have a go at most things but if you need to know that, for me its sparky territory...
    During my build we put in a new 63A supply to the pool area from the house main board. Sparky temporarily rigged up the 32A supply for my Spa but the cabinet had nowhere to go as the pool shed wasn't built until last, it was bolted to a plank concreted into the dirt. I rather helpfully moved this box(turned it off first) for the chippies when they came to build the shed. I came to turn it back on, spa didn't work, I opened up the power board, the movement of the cabinet had worked the main incomer loose from its switch, first thing I saw was this dirty great Red wire with its copper out looking at me....Not much concerns me, but one thing nipped up tight seeing that...
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    Propbndr's Avatar
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    Re: Understanding ampacity ratings

    Quote Originally Posted by jgarden View Post
    I was looking at the ampacity ratings chart - NEC Table 310.16:

    There are 3 ratings for each sized wire. Using #2 THHN wire as an example:

    Allowable Ampacity: 95 Amps at 60C / 115 Amps at 75C / 130 Amps at 90C
    I think you are misreading the table. Look carefully at the column headings. THHN is in the center column not the others. Using your example #2 THHN is good for 115 amps @ 75C. The other two columns are for other insulation types.

    But, +1 what JasonLion said "if you have to ask questions like that you probably shouldn't be doing your own wiring."
    Steve
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    Mod Squad Bama Rambler's Avatar
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    Re: Understanding ampacity ratings

    And just to add to the confusion it's Table 310.15(B)(16) now.

    Propbender has it correct. Different insulation formulations have different thermal characteristics and therefore are in different columns.

    Most people don't have to worry about the temperature rating of conductor insulation. There are a few instances where higher temp rated wiring is required and in those cases it'll be listed on the equipment/appliance.

    And just to add more confusion, #2 copper w/THHN is listed for 90C now and is good for 130 amps.
    Dave J. TFP Moderator
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    Re: Understanding ampacity ratings

    When manufacturers thermal test devices, they attach thermocouples to every joint in a wire throughout the entire device (locations are given by UL or other test certifiers). They then run high current through the device, typically full load if not higher. The current causes the device to heat up and the thermocouples record that temperature rise. When the device becomes heat soaked (the rise quits and temperatures remain stable) the highest reading is used to choose the insulation value for that entire device.

    If one tiny spot way down in the corner gets up to 90 degrees C, then the entire device must have 90 degree C rated wire. Thats why there are different temperature ratings for the wire insulation. Thats also why the enclosuer cabinets are the size they are, to allow some of the heat to escape. Never jam an electrical box full!

    During my testing days, I've run as much as 5000 amps through things. That gets HOT!
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