EXam Prep Book. Ambient temperature correction and terminal rating.

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pkelectrical

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I have emailed Mike holt and I am told that I am doing the problem wrong. I believe I am right.

The question asks:

What THHN conductor size is needed for 40A non-continuous load? The ambient temperature is 90F and the terminal size is rated for 60 degrees.

Looking at table 310.15B16 and 60 degree column we find that #8 is rated for 40A. we then use the ambient temperature correction Table and use the insulation rating of the conductors which is 90 degrees and 90 degree F. the correction is .91. We then apply the correction to the 40A we found from table 310.15B16.
So 40A x .91= 36.4A is for #8 THHN at 90F and 60 degree terminals. Mike says that this is not correct, and that #8 Awg is sufficient.

Any thoughts?
 
1) You use the 60?C column value for the terminal temperature limitation.

2) You use the 90?C column value for adjustment and correction.

Lesser of 1) or 2) result is circuit ampacity.
 
pkelectrical said:
I have emailed Mike holt and I am told that I am doing the problem wrong. I believe I am right.

The question asks:

What THHN conductor size is needed for 40A non-continuous load? The ambient temperature is 90F and the terminal size is rated for 60 degrees.

Looking at table 310.15B16 and 60 degree column we find that #8 is rated for 40A. we then use the ambient temperature correction Table and use the insulation rating of the conductors which is 90 degrees and 90 degree F. the correction is .91. We then apply the correction to the 40A we found from table 310.15B16.
So 40A x .91= 36.4A is for #8 THHN at 90F and 60 degree terminals. Mike says that this is not correct, and that #8 Awg is sufficient.

Any thoughts?
Click to expand...

As Smart $ said.
#8 THHN is a 90 degree conductor, so that is 55 X .91 = 50.05 adjusted ampacity. #8 THHN meets the requirement.
You have 60 degree terminations so your conductor has to have a minimum ampacity of 40 @ 60 degree. # 8 THHN meets that.
# 8 THHN is the correct answer.
Note that if you used #8 TW it would not meet the requirements.
 
Here are my thoughts!

A # 8 AWG THHN copper conductor ampacity is 55 amps under the 90 degree c column in table 310.15b16. We can use this ampacity for adjustment for number of current carrying conductors and ambient temperature corrections. If there is not more than three current carrying conductors then you would only need to make a correction for the ambient temperature. Using table 31015.b2a you find 90 degree f in right column next look under the temperature rating of the conductor. In this case it is 90 c. The multiplier is .96. .96 * 55 = 52.8 amps.
The terminals are only rated 60 degree c. Therefore it can't be protected at anything higher than the 60 degree c column. 40 amps.

Typically THHN has a dual rating as thwn also. If the installation is in a wet location than you would have to derate from the 75 degree column.
 
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