Sizing Conductors

CNC

In one of my classes i seemed to have read that when sizing conductors, you size it to the lowest degree rating in the circuit you connecting too.

Example if using 90* c thhn, and your breakers or bussing says 60* or 75*, can you NOT use the 90* ampacity of the chart to size your wire for a sub panel? or do you use the 75 degree chart for the ampacity?

Magnettica

Insulation is the main concern. The only time you can use the 90º column is for derating or if the equipment you are working with is rated for 90º. Most of the time I use the 75º column when working with THHN/ THWN.

CNC

AND most panels aren't rated for 90*, so i cant use the 90 degree column? or can i order a panel that is rated for it? wait, i suspect my exsiting isnt a 90* panel, so it doesn't matter.....

Celtic

Have you read 110.14(C)(1)(b)(2)?

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nEighter

It is more of a safety measurement correct? Rated for 90deg C, but only supposed to be used at 60degC or 75deg C. So if I understand it right, it is not really that you SHOULD use it up to that, but that it is rated (tested) up to 90deg C.

Magnettica

The bottom line is that if the equipment is not rated for 90º than you cannot use the ampacity listed in the 90º column. So if you had a panel buss rated for 90º but the lugs were rated for only 75º well then you would have to use the 75º column. 110.14 explains that. Thanks Celtic

Yes, think HEAT. When you have more than three current carrying conductors (CCC) in one raceway than those conductors need to be derated to account for the added heat in the wire, err, conduit.

What kind of wire is it? #12 THHN.

Good, use the 90º column in TABLE 310.16 for derating. The 90º column says that #12 THHN is good for 30 amps. However, let's say we have 10 CCC, all the same size, #12 THHN. What is the ampacity permitted for each conductor? First we have to use TABLE 310.15 (B)(2)(A) and we find that 10 CCC's need to be derated by 50%.

30 amps X 50% = 15 amps

So now those 10 CCC's are good for all of 15 amps. This is a prime example of why you see #10 thhn used in commercial applications, more ampacity.

hope that helps.

CNC

awesome, that brings some light to the subject, thanks guys.