ECN Forum Forums General Topics of the Trade General Discussion Area 350 mcm for 300 amps?

Lots of posts on all aspects of this. I will try to summarize;

Using table 310.16:

350Kcmil Cu is rated for 260A if 60C insulation and terminations, 310A is 75C insulation and terminations, and 350A if 90C insulation and terminations.

For 300Kcmil, the values are 240A, 285A, and 320A.

(Assuming the ambient temperatures, etc...)

Most type THHN wire is dual rated as THWN. You can use whichever rating applies to the circumstances; eg. if the wire is in a wet location, then you cannot use the wire as THHN, but can use it as THWN. THWN wire is only rated at 75C insulation. If the wire is dual rated to THWN-2 then you can use it at 90C.

If you want to be able to use the 90C ampacity, then all of the terminations will also need to be rated for 90C.

If your wire is rated for 75C in wet locations, or if your terminations are rated for 75C, then you will need to use the 75C ampacity for the wire. This means 350 Kcmil to supply 300A.

However if the calculated load is <285A, then you could use 300 Kcmil wire protected with a 300A breaker.

Ryan, did I get the above correct?

-Jon