310-16 & derating + 60? & 70? columns

[peter]

How to you connect the derating factors of 310-16 with the 60? and 75? columns?
For instance,using good ole THHN to carry 55 amps at 100? with 6 conductors, I know enough to try #8 and to multiply 55 X .91 X .80 = 40.04 amps. So I try #6 wire and get 75 X .91 X .80 = 54.6 amps which is good enough for me.
But if I am using 60? Centigrade devices, then the first column says I have to use #6 gauge wire.
So now I am confused. :roll: How does this work?

 

[bob]

Re: 310-16 & derating + 60? & 70? columns

If the equipment terminals are rated @ 60/75 degrees you can use the the 75 degree col.
To derate #6 thhn you may use the 90 degree rating
which is 75 amps. The derating factor for 4 - 6
conductors is 0.80.
75 amps x 0.80 = 60 amps.
If the equipment termials is rated 60 degrees
you still use the 90 degree rating but you must use the 60 degree col. #6 rated at 75 amps 90 degrees. 75 x .8 = 60. Wire capacity in the 60
degree col = 55 amps. You may have to use #4 thhn.
The wire is rated at 75 amps. The problem is the
rating of the terminals.

[ March 27, 2003, 11:53 PM: Message edited by: bob ]

 

[iwire]

Re: 310-16 & derating + 60? & 70? columns

This info was originally posted by "charlie b" in another post and I think it is a clear way of explaining how to derate.

originally posted by charlie b
STEP 1: Look up the conductor ampacity in the 90C column of 310.16.

STEP 2: Apply the correction factor for temperature from the bottom of 310.16.

STEP 3: Apply the adjustment factor for more than 3 current-carrying conductors per 310.15(B)(2)(a).

STEP 4: Look up the conductor ampacity in the 75C column of 310.16 (or the 60C column, if that is the limit of your terminations).

STEP 5: Use the lower of your results from STEP 3 or STEP 4.

 

[peter]

Re: 310-16 & derating + 60? & 70? columns

So the secret is to use the lower of the two?
Is there a Code reference for this handy?
It seems the temperature correction factors don't make much difference - except in extreme circumstances - and could be eliminated.
~Peter

 

[brad troxel]

Re: 310-16 & derating + 60? & 70? columns

look under 110-14C
Temperature correction.

 

[jschultz]

Re: 310-16 & derating + 60? & 70? columns

you can only use the 90C column if the type of conductors you are using are rated for 90C. for example thwn is rated for 75C, not 90C. So if you use thhn/thwn in a wet location and need to derate, you have to start in the 75C column.