Clarity on ampacity tables

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JoeNorm

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I would like to have a better understanding of the "degree" columns and ampacities of various wire types.

My basic understanding is that I can use the 90deg column to start derating calculations. Other than that I am pretty much just blindly using the 75deg(unless of course the wire is listed in the 60deg column) column, assuming that all the equipment I am using has at least this rating.

We use a lot of THHWN, which is a 90deg wire, is there ever a time I can actually use the full 90deg ampacity? Is a lot of equipment rated to 90deg?

Do you'll use rules of thumb here or are you checking ratings of every piece of equipment you have and going from there?

What is a reasonable way to make this table useful, without assuming too much and doing too much guesswork?
 
Take a look at 110.14(C)(1). Over #1 AWG the equipment is required to have 75° C terminals. Almost all equipment is rated for a maximum of 75° C so that would be the limiting factor when determining the ampacity of the conductor. If there is a 60° terminal involved then you would use the 60° ampacity of the conductor.
 
Do you have everyday examples of equipment with 60deg terminals?
For 30-35 years now most equipment has had 75C terminals.

Equipment older than that is where you are most likely to find 60C terminals for the most part.

NEC has maintained the language in 110.14 through all this time even though nearly all new things have 75C terminations.
 
Amp rating?

If 30 amp or less it is somewhat non issue because of 240.4(D) and you would run same size conductor whether you were using 60 or 75C in most instances.

Generac 6344 50-Amp 125/250V Raintight Aluminum Power Inlet Box
 
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