Derating conductors

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bphgravity

Senior Member
Location
Florida
Is there such a rule that says "double derating" is not required. The argument is that if you are required to derate the ampacity of a conductor per 310.15(B)(2)(a) and 310.16 correction factors, then you don't have to derate for temperature limitations on terminals per 110.14(C). I believe that you must do all that apply. Once you have made all corrections for ambient temperature and amount of current carrying conductors, you still can only select the wire size based on the column of the lowest terminal rating on the circuit, regardless of the insulation rating of the used conductor.
 

charlie b

Moderator
Staff member
Location
Lockport, IL
Occupation
Retired Electrical Engineer
Re: Derating conductors

I’m not sure I understand what you mean by “double derating,” but here’s the process:
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.

It seems to me that this process agrees with what you have said. Am I right about that?
Charlie B.
 

dereckbc

Moderator
Staff member
Location
Plano, TX
Re: Derating conductors

Charlie, excelent analogy. That is exactly what I teach to new engineers comming on board. I wished the NEC spelled it out that way.
 

iwire

Moderator
Staff member
Location
Massachusetts
Re: Derating conductors

It would be nice if they put that in as a FPN, but I guess then it would be an instruction manual not a code book.

[ March 13, 2003, 11:55 AM: Message edited by: iwire ]
 

andrew grant

Member
Location
California
Re: Derating conductors

2002-110.14(C) appears to require that the ampacity adjustments be made prior to looking in Table 310.16 for terminal provisions, rather than it being a separate test. See example in Handbook at 210.20.

However, example in 220.10 shows the old way.

This confuses me - help
 

charlie b

Moderator
Staff member
Location
Lockport, IL
Occupation
Retired Electrical Engineer
Re: Derating conductors

If there are reasons to de-rate (i.e., for temperature or for number of conductors), then that will impact the ampacity of any conductor that you propose to use. What 110.14(C) does for us is that it allows us to start with the 90C column of Table 310.16 (assuming that your conductor is rated for 90C), instead of de-rating the number given in the 75C column. However, if your answer exceeds the number shown in the 75C column, you don?t get to use the higher number.
 
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