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Ambient Temp

2006 5
hello all, long time lurker, first time post.

So in prep for my hot date with the PSI examiner , I decided to give the old NEC a good page by page read through.

When I get to chapter 3 of the saga, I come across something that has always stumped me.

Can someone please explain the difference between 310.15(B)(2) (a) & (b) in layman's terms. All I'm getting is that "a" is for cable tray and "b" is in raceway or cable, but there seems to be more to it than that.