Please advise. Is there some media that would explain the definitive answer to using NEC 2017 to use tables 310.15 in correlation with 210.19 (A) and (B), ambient temperature and correction factors and 310.15 (B)(3)(a).
Please advise. Is there some media that would explain the definitive answer to using NEC 2017 to use tables 310.15 in correlation with 210.19 (A) and (B), ambient temperature and correction factors and 310.15 (B)(3)(a).
Your branch circuit conductors have to be sized large enough to handle the connected load in 210.19. The Tables in Article 310 show the ampacity for each type of conductor material and insulation type, and the adjustment factors for the number of current carrying conductors and temperature.
If you had a 34 Amp load, 4 current carrying conductors in a raceway at 135 F, and installed THHN copper conductors;
then the following applies.
#8 THHN is good for 55 Amps with 90C insulation, it has to be derated by 80% per 310.15(B)(3)(a) = 44 Amps
310.15(B)(2)(b) requires another deration of 84% = 37 Amps.
This means you can install #8 copper conductors on a 40A OCPD for the 34 Amp load because 37A is not a standard size and you are permitted to select the next standard size (240.4(B)). In addition since the ampacity is below 100A, you are restricted to the 60C column, and #8 is restricted to 40A. (110.14(C)(1))
Thank you that cleared up some of my questions. My next question is for 110.14(c)(a)(2) and 110.14(c)(b)(2). They are worded differently but does it basically mean I can use a 90 degree c conducter as long as the ampacitys used are from there respected columns? Example copper conductor rated 90 degree c 115 amps can be used for 75 degree c termination as long as I use 2awg from 75 degree column? Thank you in advance for knowledge and willingness to share.
The ampacity always has to limited to the weakest link in the circuit, which is typically the terminal/lug. A #3 copper THHN is a 90C rated conductor with an ampacity of 115A at 90C. If the conductor is terminated on a lug rated at 75C, the conductor ampacity will be restricted to the 75C rating, 100A. If the lug was rated at 60C, the ampacity drops to 85A, however the conductor would be permitted to be protected by a 90A OCPD because 85A is not a standard size.
The NEC allows the 90C ampacity to be used for any derating calculations provided that the final adjusted ampacity does not exceed the temperature rating of the terminal.
Using the same #3 copper THHN, what would the permitted ampacity be on a 120/208 feeder(4 currentcarrying conductors) installed in EMT to a panel with 75C rated lugs.
115A X 80% (Table 310.15(B)(3)(a)) = 92 Amps. Since 92A is not a standard size, a 100A OPCD is permitted and the lugs are rated for exactly 100A.
Hello Brian and everybody. I have a question regarding this.
NEC 310.15 (B)(3)(C) states the correction factors on temperature for exposed conduit on a roof.
However the Handbook example 310.4 states the following:
You have your full load of 175 A, you divide this current by the temperature correction factor (0.82) and you get the final ampacity you need of 213 A.
Why do I go first to the 90ºC column to check if we comply with the 213 A and then go to the 75ºC column to check if my original current of 175 A complies with the 205 A of my selected conductor @75ºC.
Shouldn't I use for both checks the 213 A?
I hope you can please answer this question because it's a little urgent for me.
Thank you in advance to everybody for the attention.
Regards.
chiefns830 Hi Nick, I've seen you know pretty much about NEC. Can you also help me with this please?
Thank you in advance.
Regards.
What type of cable, or conductor? 90degree column is your starting point when derating.
Depending upon the factors that you mention in your original post, you may need to do more than one derating  a triplederating, perhaps.
Let's say a conducter that falls in both 75 and 90 degree c table and with no adjustments or corrections. Im just over zealous.
I'm not sure what you mean. It would help if you would identify a conductor or cable, and then we can go from there.
Thhw
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Re: Correct use of NEC 2017 to establish 310.15?
reply from Nick Sasso in Electrical  View the full discussion
I'm not sure what you mean. It would help if you would identify a conductor or cable, and then we can go from there.
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The adjustment factors come into play when there are 4 or more current carrying conductors in a raceway or cable or when the ambient temperature requires an adjustment.
THHW is 75C rated insulation, so the starting point will be the values in the 75C column. Even if the lugs had a higher temperature rating, the ampacity cannot exceed the 75C value.
I'm not really sure what you are asking. It sounds like you are talking about four different topics, all lumped together. If you are asking whether 310.15(B)(16) takes all that into consideration, then the answer is no. 310.15(B)(16) is your starting point. Then you can apply [whatever you have going on] to the values given in 310.15(B)(16).