Ambient Temperature Correction Factors art. 310.15 B(2)

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A new problem has appeared in the 2011 code: The formula for ampacity corrected for a different ambient temperature than in table 310.15B(16) uses a value Tc called "temperature rating of conductor".

However, this is misleading. People are plugging in 90 degrees there for THHN or THWN-2. This is wrong. The value Tc is not the conductor temperature RATING, but rather is the DESIRED conductor operating temperature.

This problem arises when using THHN connected to 75 degree terminals. Since you cannot allow the temperature of the THHN to exceed 75 degrees, even though the wire would be happy (but the teminal would not); you have to use 75 degrees for Tc and use the ampcity for 75 degrees from table 310.15-B(16) even though THHN is not listed in the 75 degree column.
So those who insist on blindly following the Code like the devil reads the Bible are going to plug in 90 degrees for THHN come what may.

The problem is actually more general and related to the headings of the colmuns in table 310-15B(16). The temperature colmns should not be given the heading "Temperature rating of conductor" but rather the heading should be "Desired conductor operating temperature". The values 60, 75 and 90 degrees indicate how hot the conductor will get when passing the indicated number of amps, whatever the "rating" of the wire.

I recommend changing the definition of Tc in 310.15B(2) to

"Tc = Desired conductor operating temperature"

and likewise changing the heading of table 310.15B(16) to the same words
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charlie b

Staff member
I don?t understand your concept of ?desired temperature.? Let?s run through an example.

You start with a tabulated ampacity that was based on an ambient temperature of 30C. For your application, you are told that the actual ambient temperature will be 42C. You plan to use 90C rated wire, #6 THHN. There are two ways to find the ampacity.

Method 1:
? The 90C rating of #6 THHN is 75 amps.
? Table 310.15(B)(2)(a) gives a correction factor of 0.87.
? Multiply 75 times 0.87, and you get 65.3 amps
? We take note that 65.3 is higher than the 75C rating of #6 THHN (i.e., 65 amps).
? Conclusion: the ampacity is 65.

Method 2:
? The 90C rating of #6 THHN is 75 amps.
? Subtract 90 ? 42, and you get 48.
? Subtract 90 ? 30, and you get 60.
? Divide 48 / 60, and you get 0.8.
? Take the square root of 0.8, and you get 0.89.
? Multiply 75 amps times 0.89, and you get 67.
? We take note that 67 is higher than the 75C rating of #6 THHN (i.e., 65 amps).
? Conclusion: 65 is still the ampacity.

I suspect that there will be very few real life examples in which the use of the formula changes the ampacity in our favor.

jim dungar

Staff member
60?C, 75?C, and 90?C are ratings at which the insulation of the conductor is damaged or its life is adversely shortened. These ratings have nothing directly to do with how hot the conductor actually gets when a certain amount of current is flowing through it to the termination. In fact the UL Listing for a breaker or fused switch rated for use with '75?C conductors' requires the terminations to not exceed about 55?C.


Senior Member


To further mention, insulation ratings in Table 310.13A...T310.104 (2011) "Type Letter" title column should be revised to read 'Conductor Insulation Type Letter.' Interesting that sheathing ratings are not directly associated in the construction specifications for NM, SE-R, SE-U...even though UF cable is noted. Having additional references like these would defray some confusion of rating adjustments for cables not found in the 310 charts.
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