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Discussion Starter · #1 ·
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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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.
Welcome aboard..:thumbup:

Wire in cable tray is in open air, but in pipe the wires will get hotter because they are close together and not on open air.
 

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While we're on the subject could someone please explain to me what 310.15 is all
About? Every table after A confuses me. So table A makes sense. 15a for 14awg, 20a for 12awg and so on. And then table B shows way higher allowable ampacities. Is this due to the difference of insulators?
 

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Table B is for conductors of higher temperature ratings and where the ambient temp is 40C instead of 30C as table A
 

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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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Table B is for conductors of higher temperature ratings and where the ambient temp is 40C instead of 30C as table A
I like Ryan C. Jackson's recommendation. He advises you to draw a big red X through that big table of derate factors that stands out on a full page. So tempting to use that table, instead of the 30C table that applies to the most common types of wire.

I've also wondered why all those exotic wire types (Ni plated, medium voltage, etc) are tested at 40C instead of 30C like all the rest. It would make more sense to me for everything to be tested at a common ambient temperature.
 
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