help with 310-16

[twistedcivic90]

ive been having this argument with an older electrician at work. Question is what column do you size thhn wire to (75c or 90c) when terminating on a standard QO breaker. i thought most breakers are rated at 75c so thats what you should go by....he says since thhn is rated for 90c ,that you go by that....please help me out here

 

[kevinware]

Re: help with 310-16

twistedcivic90,

I am going out on a limb here and say you are right I use the 75 degree column too, but I use the 90 degree column rating when I am derating. See 110.14(C) 2005
Just an apprentices's opinion.........

 

[charlie b]

Re: help with 310-16

No conductor is permitted to carry more current than its rating, or than the rating of the terminations to which it is connected. There aren't any terminations rated for 90C. So you cannot load a THHN beyond the ampacity listed in the 75C table. However, if you have to derate for high ambient temperature or for more than three current-carrying conductors in a raceway, you start with the 90C ampacity, apply the derating factors, then make sure you don't use a final answer that is higher than the 75C value.

Reference 110.14(C).

 

[peter d]

Re: help with 310-16

Remember the "weakest link" principle. If you are terminating one end on a 75C breaker, and the other end on a 60C wiring device, then the entire conductor is limited to the 60C column.

But if you have are feeding a panel board on the other end with 75C terminals, then you are permitted to use the 75C column.

 

[james wuebker]

Re: help with 310-16

Just remember that the breakers are 75c but are your other terminations 75c. If they're not you must start at the 60c table for sizing if it's a 100 amps or less. If they're all 75c terminations you can start with the 75c table for sizing.
Jim

 

[infinity]

Re: help with 310-16

Also I would add that if the THHN were inside of type NM cable than it would be limited to the 60 degree column.

 

[charlie]

Re: help with 310-16

I believe the THHN wire in NM cable is THH without the N and it is not identified except by color. Also, you are absolutely correct about its 60?C application.

 

[engy]

Re: help with 310-16

The Essex cut sheet I have clearly states THHN, Southwire beats around the bush a bit, but does mention Nylon cover...