Question about derating of conductors. I’m in inland CA – summertime temps here can stay over 100F for a week at a time. In an attic, that means temps of 130F or more. Does anybody derate their electrical systems for temps? I understand it’s common practice (and required) to derate for bundling raceways together and for current-carrying conductors in conduit.
But according to 2002 NEC Table 310.16 and assuming ambient temps of 130F, my 12ga NM rated for 20amps (by 240.4D) is now down to 15A, and my 14ga is down to 11.4A; and this before any bundling deratings. I asked an EC buddy: he said he doesn’t bother derating for temp, and my instructor hemmed and hawed about rather it’s really necessary, because the temps are only for short durations and in a residence these circuits almost never see continuous duty (3hrs+), and if they do, they’re basically doing light duty like lighting.
Can any sparkies give their input/feelings? Thanks.
Replies
Look at table 310.16
Then look at the section on NM wiring.
NM cable is rated at 105 c. For that it has an ampacity of 30 amps.
While you are restricted to using the 60 c column for NM you can use the 75 ot 105 degree column for the starting value for calculation derating.
If the run of the wire is the only thing that gets hot then you can use the 105 column. If there is a device that it connects to in the heated area or near the bundled wires then you need to check the ratings of the terminals on that device. In most case it will limit you to the 75 column.
#14, 12, & 10 usually don't need to be derated less than the "nominal" value (15, 20, 30) in residential appliactions unless there is large bundle of cables in a group.
Bill,
Thanks for your reply. You've answered lots of my elect. Q's and have not failed me yet. For further clarification, when specifically is one restricted to using the 60C column?
And if all cable, devices, and breakers are rated for 90C for example, can one use that column to determine ampacities etc.? Thanks Bill.
"And if all cable, devices, and breakers are rated for 90C for example, can one use that column to determine ampacities etc.?"Yes, but I don't think that 90C devices are too common.However, you need to look at all of the details. The rating on the terminals only come into effect where those terminals will be exposed to the conditions that cause the derating.For example a cable run from the panel in the garage up through a hot attic and down to a room in the house. The part of the cable in the attic is exposed to the heat. But that heat is far from the terminals on the devices, so their rating does not come into play.And also, by CODE for NM you can only use the higher temp as a starting point when calcuating the derating. For that cable you are limited to the SMALLER amount give in the 60C column or the derated amount. I have no idea why tht is in the code.For THNN in conduit you can use the 90C column without restriction.
Thanks Bill. For every electrical question that's answered, one more appears, but I don't pay you enough as it is. Thanks again.
You should not confuse ampacity and maximum overcurrent protection. Go to section 334.80 in your 2002 code and you'll find that NM wire is rated at 90*C. Use the 90*C column in 310.16 and you'll find that 14 AWG is rated at 25 amps and 12 AWG is 30 amps. At a temp. of 140*F the ampacity would be derated to 17.75 and 21.3 amps respectively. If you start bundeling your wires you would derate from these values. I think if this was a problem you would know about it.
Thanks tim,
It seems I was confusing overcurrent protection per 240.4D, "overcurrent protection shall not exceed 15A for 14AWG, 20A for 12AWG, and 30A for 10AWG..." and ampacity. Thanks, that clears up a mystery for me.
Just to make sure your not mixing degrees C and degrees F. 130F is 54C