ampacity of conductors

[eager2learn]

my question is -
what chart do i use when trying to find allowable ampacity for conductors using table 310.16 - 60 degrees, 75 degrees, or 90 degrees - i heard some where before that no matter even if the wire is type thhn you would still use chart 60 degrees - is this true and if so why -

 

[tom baker]

Re: ampacity of conductors

You can use the 90 C rating for ampacity before derating (fill and temp) but 99% of the time the 75 deg c rating is used, base on the terminal temp., for THHN type conductor. The wire type determines the temperature, for example RHW is 75 deg C
See 110.14
Sometimes the 60 deg C rating is required to be used, see 240.4(D) and pay attention to the exceptions E thru G
However the ampacity of 12 NMB is 30 amps but the overall ampacity is based on the jacket at 60 deg C or 20 amps, see 334.80.
If you are asked what the ampacity of 12 THHN is the answer is?

[ June 15, 2005, 02:28 PM: Message edited by: tom baker ]

 

[guesseral]

Re: ampacity of conductors

Not entirely! read 110.14 C, and for NM cable read 334.80!

 

[George Stolz]

Re: ampacity of conductors

I was discussing this the other day with a buddy getting ready for his test. My grasp on this is a little slippery at times, but here's how I laid it out for him.

My question was (off the top of my head), You have 6 12-2 NM cables running through a 30" conduit used for protection. What is the ampacity of each cable?

1. You have an NM cable in your hand. It says it's rated for 90?.
2. You try to install the cable. In order to do it properly, you must adhere to Article 334, which includes 334.80, which knocks down the available ampacity to 60? rating.
3. Once compliant with the most specific article, it must comply with other areas of the code, from most specific to least specific.
4. We come to 310.15, which states we must derate our installation.
5. In the end, we come to 110.14 which is very general.

So the hierarchy on the installation/question is:
Article 334
Article 310
Article 300
Article 110

Is this a correct method, or misleading/confusing?