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Ambient Temp

2010 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.
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Discussion Starter · #5 ·
Thanks for the response Harry. After reading through again, I realized my question was based on my misunderstanding that index (a)=open air (b)= closed raceway. After reading through again I realize this is not the case.

So what is still confusing me is when to use each one. As Dennis mentioned (b) includes higher temp rated wire, yet 60, 75 and 90c rated wire are listed in both indexes. Not only that but the factors are different for the same ambient temps. What is the purpose of index (a) and why does it contradict (b)?
 
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