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10-16-2008, 11:01 PM   #1
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## Temperature correction question

Hi,

I am finishing an attic space and I have used #14 NM for the lighting circuits. The surrounding unfinished attic space gets extremely hot, despite all attempts, since there is inadequate intake ventilation due to a lack of soffits.....

Anyway, I am not totally clear on how to reduce an ampacity for high temperature. Truth is, if I were starting over, I think I would know what to do but given what I have already done, some questions arise that I can't seem to answer from the code....

Let's say, the load on a circuit (noncontinuous):

1) Sums to, let's pretend, 8 amps
2) Is less than the temperature corrected (TC) wire ampacity of, say, 14 amps,

but the wire (15 amps) itself exceeds the TC wire ampacity, is this a problem? I am not sure whether the code is saying that the TC ampacity must exceed the non-TC wire ampacity, or whether it does not matter as long as the load is below the TC ampacity...

I can totally understand that the latter could be a problem if somebody later unknowingly added onto a circuit with #14 thinking that 15 amps is the upper bound, when due to temp correction it is not. If this is the case, is a possible solution to use a smaller circuit breaker (if they even make a 10 amp breaker)?

My situation is that wires in some attic hot spots near the roof will get blazingly hot. If the ambient temps in these areas fall within 132-140 F or less, the calculated TC ampacity is still above 15 (which of course then gets lowered to 15 amps). However, in the 141-158 range (which I can't rule out might be reached), the TC ampacity is about 14.5 amps, still above the load but slightly less than the 15 amps of the conductor....

Will I need to go to a larger wire size?...

Thanks!!!

Mark

10-17-2008, 02:33 AM   #2
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#14 NM cable temperature corrected to a worst case scenario of 141-158F is 14.5 amps. Lighting loads are considered continuous and limited to 80% of circuit ampacity, 12 amps. IMO you're fine.

If you read art. 240.4(B), it allows the next standard size up overcurrent device to be used over the calculated ampacity of the wire. In this case, a 15 amp breaker.

 10-17-2008, 01:06 PM #3 Newbie   Join Date: Oct 2008 Posts: 3 Rewards Points: 10 thanks cow! this is good info- ill check out that code section later today

 10-17-2008, 01:16 PM #4 the Musigician     Join Date: Jul 2008 Location: I'm right here! Posts: 10,404 Rewards Points: 2,000 i'd agree the wire's fine, but you need to get some venting up there! can you put in a ridge vent? gable vents? kinda kill 2 birds, etc. DM
 10-17-2008, 01:39 PM #5 Newbie   Join Date: Oct 2008 Posts: 3 Rewards Points: 10 I put in a ridge vent about one month ago after having previously installed some gable vents for intake. I tell ya, having no soffits is a major pain. ...Truth is, I don't feel too confident that the ridge vent will make much of a difference given the state of my current intake ventilation though. I should be able to add more intake vent but I don't want to do anything until I can assess the situation next summer.... mark
10-17-2008, 02:34 PM   #6
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wow.... maybe a solar vent fan?? free power and it comes on and runs faster as it gets hotter outside? best installed in spring or fall, of course. =o)

DM

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