ambiant temp corrections

kwired

Electron manager
Other then in a hot attic, NM cable is not run that often in places where ambient temp is much of an issue is probably the main reason this topic seldom comes up. Ambient temp adjustments do have provisions where one can actually increase conductor ampacity if the ambient is low enough, but this topic also doesn't come up too often.
 

Sahib

Senior Member
Ambient temp adjustments do have provisions where one can actually increase conductor ampacity if the ambient is low enough, but this topic also doesn't come up too often.
However, in parts of places where ambient temperature is higher, the ampacity of the run of the same cable has to be reduced there!
 

kwired

Electron manager
However, in parts of places where ambient temperature is higher, the ampacity of the run of the same cable has to be reduced there!
I merely mentioned that low ambient temp is one place where you actually can increase the conductor ampacity though it is something seldom talked about, high ambient's are quite often a topic of discussion at places like this site - including this thread.:huh:
 

luckylerado

Senior Member
Applying T310.15(B)(2)(a) and without the application of any other adjustments or corrections, a #12 NM cable ran in an attic would need to be exposed to 159?F before it would need to be protected with a 15 amp OCP device. A #14 NM would need to be exposed to 168?F before a 15 amp OCP device would be insufficient.

Those conditions, even above insulation in the hottest attics, are going to be exceedingly rare and will NEVER occur over the majority of North America.
 

Smart $

Esteemed Member
... A #14 NM would need to be exposed to 168?F before a 15 amp OCP device would be insufficient. ...
Actually, the temperature limit for #14 on a 15A breaker is 185 degrees (limit of correction table for 90 degree C wire)... but the load cannot exceed the adjusted ampacity. The reason for this is 15A is the smallest standard breaker rating.
 

luckylerado

Senior Member
Actually, the temperature limit for #14 on a 15A breaker is 185 degrees (limit of correction table for 90 degree C wire)... but the load cannot exceed the adjusted ampacity. The reason for this is 15A is the smallest standard breaker rating.
For all practical purposes you are correct and I would agree however playing devils advocate for a minute:

Per 240.6 10Amp is a standard Fuse rating.

240.4(B) ..."The next higher standard overcurrent device rating..."

(2)The ampacity of the conductors does not correspond with
the standard ampere rating of a fuse or a circuit breaker
without overload trip adjustments above its rating (but that
shall be permitted to have other trip or rating adjustments).



Also, now that I re-read 240.4(B)(1). Is this to say that the provision of 240.6 DOES NOT apply to 120V branch circuits feeding other than home runs to dedicated single recepts?

I may need to reconsider my stance on this one.

240.4(B) Overcurrent Devices Rated 800 Amperes or Less. The
next higher standard overcurrent device rating (above the ampacity
of the conductors being protected) shall be permitted to be
used, provided all of the following conditions are met:

(1) The conductors being protected are not part of a branch
circuit supplying more than one receptacle for cord-and plug-connected
portable loads.

(2) The ampacity of the conductors does not correspond with
the standard ampere rating of a fuse or a circuit breaker
without overload trip adjustments above its rating (but that
shall be permitted to have other trip or rating adjustments).

(3) The next higher standard rating selected does not exceed
800 amperes.

 
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Smart $

Esteemed Member
For all practical purposes...
It's a "gray area" depending on interpretation. For instance, you are taking the phrase "fuse or a circuit breaker..." as meaning one has to use a fuse for a corrected ampacity lower than 10.5A. The alternative interpretation is that when using a circuit breaker, the next higher standard rating is 15A. AHJ interpretation will rule.
 

luckylerado

Senior Member
Yes, I may be splitting hairs a bit with the fuse argument however do you take 240.4(B)(1) to mean that for 120V receptacle branch circuits serving anything but dedicated equipment that the adjusted and corrected ampacity may not be less than the OCP setting.

Essentially 240.4(B) does not apply to general purpose and convenience outlets.

This puts the temperature range from earlier in this discussion a little bit more in the realm of possibility but under insulation I still feel like these temps are going to be few and far between. I would bet there is a formula for finding a design temp for attics.

Max temp limits for receptacle circuits per my new understanding of 240.4(B)
140?F for #12
122?F for #14
 

Smart $

Esteemed Member
Yes, I may be splitting hairs...
I agree with your assessment of 240.4(B)(1).

Also, under insulation should be cooler... but at the same time the insulation will hinder self-generated heat from dissipating. That leaves us where????

And I get 149?F for #14... 15A@60?C/25A@90?C is 0.60... 150-158?F correction is 0.58, so next lower-temp range 141-149?F@0.65.
 
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