Conductor derating

Steelhead

Hey guys,

When you guys have to derate conductors do you derate for ambient temp and the amount of conductors in a raceway or is it one or the other? I was under the understanding that you derate for both, but I've heard both scenarios. What is the proper way?

Thanks in advance for your responses.

jbfan

Both, however I have never derated for temp except on test.

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JPraski

Both, but check the rating of the wire, you only derate when it goes over that. If you get 90 degrees celius rated wire, you probably will never derate for ambient.

nap

Huh? You derate simply due to conductor count regardless of ambient temps.

never mind. Your wording threw me.

Steelhead

I thought the 90 degree rating of the wire had to do with the amount of heat the conductor insulation can safely handle. According to 310.16 the ambient temp for that table is 86 degrees F

Steelhead

Haven't you had to run a circuit somewhere where the ambient temp was over 86 degree F, thus having to possibly derate for temp and amount of conductors in a raceway if there were more than three current carrying conductors?

nap

yes, the 90º (C) (194ºF) is as you say.

the 86º (F) is simply a base temp to calculate the current rating of the wire at that will not cause heating that would cause the wire to heat to a temp exceeding the capacity of the insulation. If you look at the correction table for temp, you will notice that as the ambient temp goes up, the rating is decreased. This is so when you take the ambient temp plus the heat created by the current at the max rating, it will not surpass the temp rating of the insulation.

Steelhead

And that is what I thought too. I guess I'm also wondering how often do most electricians have situations where they have to derate for both.

oleguy74

table 310.15(b)(2)(a)for more than three wires.other tables 310.16 to 310.20 take care of temp correction factors.

JPraski

My bad. you derate for number of conductors in a raceway. You derate for temperature, as shown in the correction factors in the same table you were looking at. You derate for being in sunlight on a roof.

(I meant, derate if it is over 86 ambient. Sorry very unclear when re-read.)

So yes, you do both deratings, although as mentioned I've never bothered with the ambient one- but I live 'up north', 90 degrees is unusual. Problem being, of course, that there is no definition of ambient. If it hits 90 degrees 2 days a year, is that what you need to derate for? I've been told by the inspectors around here not to worry about it.

Check 240.4 (D) though, if you're using #12 or #14 you're probably all right anyhow.

"The temperature rating of a conductor [see Table 310.13(A) and Table 310.13(C)] is the maximum temperature, at any location along its length, that the conductor can withstand over a prolonged time period without serious degradation."

oleguy74

ambient is the surrounding air temp wher ever your at.inside or out.

Steelhead

Thanks guys. I'm no longer confused.

Saturday Cowboy

Good electricians try to design their wiring to avoid where posible. Most residential locations with thinking it can be completely avoided.

JPraski

Most residential locations don't use pipe much. When they do it's frequently underground, or not a complete run (like for going underground and then direct buried cable). I bet they use the derate a lot in Chicago, where their code requires pipe in residences, but I've never done work there.

I do industrial a lot, and we don't really even use the derate there. I've done it, but it's only in obviously hot locations (over paint ovens that are always 200+ deg F, that sort of thing).

Does anyone actually know when you're supposed to use the derate for temp? There is no definition for ambient, so there's no telling exactly how long something has to be hot before it's considered a factor. I use the 'Duh Rule'. As in, 'Duh. You better do something about that.' But I don't work down south, where it does get very hot for a significant portion of the year. You can't just use the temp at the time of construction, everyone would calculate loads in winter to make it cheaper. Someone mentioned 'inside and outside' temp somewhere, that's wrong because you have to rely on cooling and heating systems to maintain the inside temp, and it doesn't count as ambient.

brric

IMO because of factors such as load diversity derating in a residential setting is ridiculous.