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Thread: How to read ampacity chart 310.16 - 310.19

  1. #1
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    How to read ampacity chart 310.16 - 310.19

    Can anyone tell me how to properly determine wire sizes using the ampacity tables in 310.16
    For example: table 310.16 says that it's based on ambient temp of (86f deg) but then there are three different temp ratings on the chart two of which have the same insulation. Take a #10
    THHW for example, it's in the (167F) and the (194F) column how do I know which one to apply all my correction factors to if the temp I need is (75f).

  2. #2
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    Look at table 300.13 in the 2008 and earlier and see that thhw is rated 75C in wet location but 90C in dry location. So it depends on the install.

  3. #3
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    310.16 {2008} contains two different temperature characteristics. One has to do with the ambient temperature surrounding the conductors the other has to do with the operating temperature limit of the conductor based on it's insulation type. For example a #12 THHN conductor has an ampacity of 30 amps in the 90° C column with an ambient (surrounding air) temperature of 30° C or 86 ° F. THHN is a 90° C conductor based on it's insulation (top of the table). That means that it can safely carry 30 amps of current without damaging it's insulation in an ambient temperature of 30° C. Now run that same conductor in a higher ambient temperature and then the heat from the air will impact the conductor ampacity. That's when you would use the correction factors at the bottom of 310.16. There are other aspects of this table that would also apply, one being the temperature ratings of terminations used with the conductors, and two the number of condcutors in a raceway or cable, etc.
    Rob

    Chief Moderator

    All responses based on the 2011 NEC unless otherwise noted

  4. #4
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    thanks for the response

    Thanks for the response to my question, after I posted this I went on to read several different forums asking similar questions. This is what I came up with, when dealing with THHN and several others that are listed in both the 75c and the 90c columbs we are actually talking about the max temp of the insulation, the 90c columb can only be used when you have to de-rate for other variables (more than three and or temp). After de-rating all the factors, if the ampacity comes out to be more than that listed in columb 75c then you may use the ampacity listed in 75c but never more, if it comes out to be lower than that listed in 75c, you must start over with your caculations taking your ampacity from 75c before de-rating it. Can anyone verify that this is the proper way to use the 90c columb, or do I have more studying to do?

  5. #5
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    Thats a pretty good scenario for most applications.
    There are limited situations where you can terminate at the 90° column and a few where you must terminate at the 60° column.
    Be sure to read 110.14
    At my age, I'm accustomed to restaurants asking me to pay in advance, but now my bank has started sending me their calendar one month at a time.

  6. #6
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    Take a look at table 310.104(a) in the '11 code book. When you come across insulation types listed in mulitple columns, such as thhw or xhhw, it depends on whether its a dry or wet location. Like Dennis said, if its a wet location, use the 75 degree column. Dry locations use 90 degrees. However, 110.14(C)(1) says to use the 60 degree. Really depends on the situation. Not sure about your calculation method. Haven't heard that one myself. Best of luck

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