50 amp thhn?

[sparkync]

According to the wire table, #8 thhn in the 90 degree column is good for 55 amps. I understand that we only use 90 degree column for calculation purposes, but the 75 degree column doesn't list thhn , only thwn etc. which list at 50 amps.
Some sources say it will take #6 thhn for 50 amps, and other sources say #8. Which is right?
Thanks

 

[david luchini]

According to the wire table, #8 thhn in the 90 degree column is good for 55 amps. I understand that we only use 90 degree column for calculation purposes, but the 75 degree column doesn't list thhn , only thwn etc. which list at 50 amps.
Some sources say it will take #6 thhn for 50 amps, and other sources say #8. Which is right?
Thanks

#8 THWN has an allowable ampacity of 50. So #8 THHN would be acceptable for a 50A circuit, assuming no adjustments or corrections needed.

 

[augie47]

Please keep 110.14 into account also. There are, I believe, some devices which will accept a #8 but are 60° rated.

 

[Dennis Alwon]

According to the wire table, #8 thhn in the 90 degree column is good for 55 amps. I understand that we only use 90 degree column for calculation purposes, but the 75 degree column doesn't list thhn , only thwn etc. which list at 50 amps.
Some sources say it will take #6 thhn for 50 amps, and other sources say #8. Which is right?
Thanks

If an insulation is rated 90c then it is also rated 75C so there is no need to list it in the 75C column. Most insulation today is not thhn but thwn-2 or something similar to that. the -2 makes it good at 90C for wet and dry locations.

We are also assuming you are not using nm cable because if you are then you are limited to 60C. See 334.80

 

[winnie]

To summarize:

THHN may be used at 90C or less.

The allowed temperature for a THHN installation is usually set by terminations, which may be 60C or 75C.

If you have 75C terminations and everything the THHN touches is rated for 75C or more, then #8 THHN is good for 50A.

Jon

 

[Dennis Alwon]

To summarize:

THHN may be used at 90C or less.

The allowed temperature for a THHN installation is usually set by terminations, which may be 60C or 75C.

If you have 75C terminations and everything the THHN touches is rated for 75C or more, then #8 THHN is good for 50A.

Jon

Unless it is nm cable...

 

[winnie]

Unless it is nm cable...

Agreed. I'll cover my posterior with the claim that THHN in NM is 'touching stuff, that requires rating at 60C

 

[jap]

I may be odd but any time I hear THHN or THWN when thinking about amperage ratings, I immediately picture individual conductors in my head, not what may be inside a cable such as NM.

Must be my Commercial/Industrial background.

JAP>