Do I need to use 60 degree C ampacity rating for feeders to 60 amp subpanel?

Question

I calculated my wire size for feeders to a 60 amp subpanel, for a pump house, based on 75C column of ampacity chart. Terminals out of main and into subpanel are both rated 75C. But then I saw something about circuits under 100 amps need to use 60C column. It is my understanding that I only need to use the 60C column if the connections aren't rated. Is that correct?

Answer 1

First, there's no such thing as a 60 amp subpanel. The subpanel sizes on offer are 30A, 70A, 100A and up. There is also no such thing as 60A wire - take a gander at Table 310.15(B)(16) and see if you can find it. I can't. So 60A is a fiction. That can be freeing.

But then I saw something about circuits under 100 amps need to use 60C column

Yes, it's in NEC 110.something. However if you read more closely, it grants an exception when the wire, cabinets and terminals all have a higher temperature rating.

I have never heard of a subpanel with a 60C temperature rating.

So yes, for a panel-panel run, where you don't run it through any enclosures with only a 60C rating (huh?), as long as your wire isn't NM, UF or TW you should be all set to use 75C thermal.

However I do not recommend using 60A-ish wire for that job. I recommend 2-2-2-4 aluminum (90A). It's far cheaper than any copper option, and is at par with 4-4-4-6 aluminum (65A) due to popularity. And it's at a large enough size that everyone agrees it's safe, and really, aluminum large feeder has always proven itself safe. Because professionals torque large connections to spec, and they are are aluminum rated (made of aluminum actually).