ser cable

[James L]

How does one know what temperature to actually use as ambient in a given installation?

Even directly measuring doesn't mean you checked when the conditions were the hottest.

Ambient simply means in the immediate vicinity.
Immediately surrounding

 

[Rick 0920]

I see what you're saying, but those ampacities aren't static. They're all based on a certain ambient temperature - 30⁰c (range of 77-86⁰F is specified, too).

So in other words, if you have an ambient temperature of 80⁰F, you have to keep the load under 20 amps on a #12 NM to keep the conductor temp under 140⁰

But if you have an ambient temp under 50⁰F then a load of 25.8 amp will still keep that same conductor under 140⁰F

I see your point. So are you saying that if you are using NM-B cable in an ambient of 48 F, you could terminate the #12 conductor on a 25 amp breaker? Your calculation says that the conductor will still handle it and it would not exceed 140 F. I believe when you read Art. 334.80 Table 310.16, Art. 240.4 (D)(5), Table 240.4 (G) (special conditions for smaller conductors), you still can't protect #12 cu. at more than 20 amperes. The ampacity can always be greater, we just can't utilize the full ampacity of said conductor.

 

[LarryFine]

Ambient simply means in the immediate vicinity.
Immediately surrounding

I get that, but how would you know how hot it can get in a given attic space, for example?

There is more to ambient temperature than geography. Black roof vs white roof, sun vs shade, etc.

 

[Rick 0920]

I get that, but how would you know how hot it can get in a given attic space, for example?

There is more to ambient temperature than geography. Black roof vs white roof, sun vs shade, etc.

Good point. Sometimes we just do the best we can do! NM-B is a step above the old Romex with TW insulation.

 

[James L]

I see your point. So are you saying that if you are using NM-B cable in an ambient of 48 F, you could terminate the #12 conductor on a 25 amp breaker? Your calculation says that the conductor will still handle it and it would not exceed 140 F. I believe when you read Art. 334.80 Table 310.16, Art. 240.4 (D)(5), Table 240.4 (G) (special conditions for smaller conductors), you still can't protect #12 cu. at more than 20 amperes. The ampacity can always be greater, we just can't utilize the full ampacity of said conductor.

For sure, general use circuits have a breaker limit.

But heat pumps, motors, etc aren't that way.

 

[James L]

I get that, but how would you know how hot it can get in a given attic space, for example?

There is more to ambient temperature than geography. Black roof vs white roof, sun vs shade, etc.

I'm thinking more in terms of conditioned space with an ambient temperature much lower than what the chart shows, which is a range of 79-86⁰F

 

[Rick 0920]

For sure, general use circuits have a breaker limit.

But heat pumps, motors, etc aren't that way.

Those special conditions are covered in Table 240.4 (G) See post #22

 

[Rick 0920]

I'm thinking more in terms of conditioned space with an ambient temperature much lower than what the chart shows, which is a range of 79-86⁰F

We are still talking about NM cable and its allowable ampacity. 334.80 spells it out in black and white. The final derated ampacity (or in this case, an increased ampacity), shall not exceed that of a 60 C. conductor. Table 310.16 - #12 @ 60 C. has an allowable ampacity of 20 amps. So unless Table 240.4 (G) applies, it's a code violation to use an OCPD of larger than 20 amperes for #12 cu.