ambiant temp corrections

does the ambiant temp correction factor apply to romex wire in attics?

a recent study book (not a mike holt book) states a 25a load ac unit needs a #6 wire @ 60c with ambiant temp in attic of 122f.

in all my years in residential wiring working for large companys, we always would run it in 10/2.

i asked a friend master electrician about this and he thought i was crazy to suggest a #6 for a 25a ac!

a few months back, a new master electrician working as a city inspector asked me about the ambiant temp of the 14 and 12 guage wires in the attic of a house and i looked at him completly clueless.
 

luckylerado

Senior Member
does the ambiant temp correction factor apply to romex wire in attics?
Yes but you would not use the 60 degree ampacity in the calculation. Not for Romex at least

At 122 degrees F a #10 THHN is derated to 33 amps

40 amps (90 degree column) times 82% (2014 Table 310.15B2a)
 

Smart $

Esteemed Member
Yes but you would not use the 60 degree ampacity in the calculation. Not for Romex at least

At 122 degrees F a #10 THHN is derated to 33 amps

40 amps (90 degree column) times 82% (2014 Table 310.15B2a)
30 amps.

Derated ampacity shall not exceed the 60?C value [334.80]
 

luckylerado

Senior Member
30 amps.

Derated ampacity shall not exceed the 60?C value [334.80]
334.80
"...90?C (194?F) rating shall be permitted to be used for ampacity
adjustment and correction calculations...."


My example was to show #10 Romex is adequate to carry 25A through a 122 deg F attic. Nothing more.
 

kwired

Electron manager
"...provided the final derated ampacity does not exceed that of a 60?C (140?F) rated conductor."
Adding that "final derated ampacity" makes a big difference - otherwise you first had me thinking what good is there to be able to start making the adjustments from 90C values in the first place.



johnsmith09800
ambiant temp corrections
does the ambiant temp correction factor apply to romex wire in attics?

a recent study book (not a mike holt book) states a 25a load ac unit needs a #6 wire @ 60c with ambiant temp in attic of 122f.

in all my years in residential wiring working for large companys, we always would run it in 10/2.

i asked a friend master electrician about this and he thought i was crazy to suggest a #6 for a 25a ac!

a few months back, a new master electrician working as a city inspector asked me about the ambiant temp of the 14 and 12 guage wires in the attic of a house and i looked at him completly clueless.​
Are you running the cable above the insulation where it will reach 122F or below insulation where it likely isn't much warmer then the temp below the ceiling? It only needs adjusted for the temp the conductors are actually subjected to.
 
We run the wires in the attic on rough in before the insulation is blown but here in Texas in the summer the outside temp could easily reach 100 - 110 f, i never taken a thermometor up in an attic with me but i would say guessing maybe 160 in the attics, it gets soo hot you cant work up there more than 10- 15 minutes b4 you need a break or you could overheat. I have been wiring houses for over 20 years and none of the large companies have ever said anything about temp adjustments for attics, now im studying and learning new stuff and wonder if I have been doing it wrong all these years even though i was just following directions.
 

kwired

Electron manager
We run the wires in the attic on rough in before the insulation is blown but here in Texas in the summer the outside temp could easily reach 100 - 110 f, i never taken a thermometor up in an attic with me but i would say guessing maybe 160 in the attics, it gets soo hot you cant work up there more than 10- 15 minutes b4 you need a break or you could overheat. I have been wiring houses for over 20 years and none of the large companies have ever said anything about temp adjustments for attics, now im studying and learning new stuff and wonder if I have been doing it wrong all these years even though i was just following directions.
You kind of get away with it in most dwellings because the circuits often are not loaded all that much to begin with.

If it is 160 deg in the attic you are not going in there at all. I don't know how long it takes to burn you but they recommend setting water heater temps at 125 deg because some will get burned at higher temps - but does take some exposure time, but 160 is not going to take more then a few seconds to burn most people I would think.

161 degree temp for 15 seconds is enough for HTST pasteurization processes.
 

kwired

Electron manager
Saunas are typically in the 160-200 deg F range. Whether that temperature would burn would depend on the humidity.
I guess I can understand that, dry air would have less mass in contact per surface area unit with your skin then a liquid media would. Splash a 160 degree cup of hot beverage on you and you could get burned in an instant, splash a room full of 160 degree air all over you and it will likely take much longer before you do burn, some sensitive areas will burn faster then other areas though.
 

MasterTheNEC

Senior Member
Nothing in Section 310.15(B)(2)(a) exempts Type NM-B Cable from the Ambient Adjustments. While many don't take them into consideration and while many fires are not reported.....the NEC says what it says....isn't there a charlie rule on that somewhere:angel:

In fact, Section 334.80 directs you back to Section 310.15 and says the ampacity of the cable SHALL BE determined in accordance with 310.15. The additional demands placed by the rest of Section 334.80 do not change any of that, in fact it adds to the requirements.
 

kwired

Electron manager
Nothing in Section 310.15(B)(2)(a) exempts Type NM-B Cable from the Ambient Adjustments. While many don't take them into consideration and while many fires are not reported.....the NEC says what it says....isn't there a charlie rule on that somewhere:angel:

In fact, Section 334.80 directs you back to Section 310.15 and says the ampacity of the cable SHALL BE determined in accordance with 310.15. The additional demands placed by the rest of Section 334.80 do not change any of that, in fact it adds to the requirements.
If you install in a manner so as not to have to adjust for "bundling", a 14AWG NM-B can be run in a attic with max ambient of 149 degF and a 12AWG in max ambient of 140 degF, and have them actually part of respective 15 and 20 amp circuits. A cable serving a fixed load only needs adjustments made according to the connected load, but general purpose recptacle outlet circuits need to assume possible full 15 or 20 amp loads may be present.
 

MasterTheNEC

Senior Member
If you install in a manner so as not to have to adjust for "bundling", a 14AWG NM-B can be run in a attic with max ambient of 149 degF and a 12AWG in max ambient of 140 degF, and have them actually part of respective 15 and 20 amp circuits. A cable serving a fixed load only needs adjustments made according to the connected load, but general purpose recptacle outlet circuits need to assume possible full 15 or 20 amp loads may be present.
Fact..if you install Type NM-B Cable in an elevated ambient temperature...you are going to "correct" versus "adjust" accordingly for all conditions found in Section 310.15. Create all the scenarios you would like but that is a fact. If you apply the "Correction Factors" correctly...then you are good to go....apply the "correction factors" incorrectly......don't cry foul to the manufacturers who guide you otherwise.

You are still required to comply with Section 310.15.....unless expressed otherwise.
 

ActionDave

Moderator
Staff member
Nothing in Section 310.15(B)(2)(a) exempts Type NM-B Cable from the Ambient Adjustments. While many don't take them into consideration and while many fires are not reported.....the NEC says what it says.....
Clever turn of a phrase here. I would like to see any evidence of fires caused by NM sitting in a hot attic. There is miles of NM up and down my block, lots of it not even NM-B, in insulated attics. Some of it above the insulation, some of it in it, some of it both. No smoke comes out of any of them in July or August.
 

MasterTheNEC

Senior Member
Clever turn of a phrase here. I would like to see any evidence of fires caused by NM sitting in a hot attic. There is miles of NM up and down my block, lots of it not even NM-B, in insulated attics. Some of it above the insulation, some of it in it, some of it both. No smoke comes out of any of them in July or August.
So you are re-writing code based on evidence.....the code is clear...is it not. It does not matter what you consider to be quite frank.
 

MasterTheNEC

Senior Member
So you are re-writing code based on evidence.....the code is clear...is it not. It does not matter what you consider to be quite frank.

I should add....I know they are not spontaneously combusting in attics everywhere.....but what it does make you think about it the ability for that conductor to carry a specific amount of amperes in a given condition. Good news is we (and the others) make products that stand up to a lot of stress.....But given the way the NEC is written you can't ignore the statement at the beginning of Section 334.80 which references you back to Section 310.15 and the various items that can affect the ampacity of a conductor.

In a real world...will a slightly undersized GEC or even a EGC be a real problem....maybe...maybe not....all we know is that CODE says do it.......and we do it.
 
I was really hoping someone would have a legal answer why we dont adjust for temp but it looks like we have just simply been ignoring that code, how long have temp corrections been in the book? How many people here actually adjust for the temp in residential, have you ever been red tagged for not doing it. I understand that most time the circuit does not pull anywhere near the max capacity of the circuit so it wouldn't matter. As far as the example I gave about ac's, where i live, we have a min circuit ampacity and a max marked on the unit, we size the wire to the max ampacity, so if it 20a min, 30a max and we run 10/2 and it only pulls 20a, it should still be fine if the temp adjust is applied, I just very surprised i never heard of this in 28 years until i started to study for my test. Thanks for all the replies, very helpful ppl here :)
 

edward

Senior Member
IMO, NM should be calculated for ambient. However, I have yet to see one do it and an inspector ask for it. I believe the reason it does not get done, is because in residential the circuits do not get loaded to their maximum capacity for any long period of time.

The only load that stays ON for some time is the pool motor (at least that comes to mind). Of course different parts of the country and different clients may have different loads and different needs.
 
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