110.14(C)(1) Requires Ambient Temperature Correction at Terminations?

[wwhitney]

I was perusing (2020) 110.14(C), and I note that the second sentence of 110.14(C)(1) says "Unless the equipment is Listed and marked otherwise, conductor ampacities used in determining equipment termination provisions shall be based on Table 310.16 as appropriately modified by 310.12."

Table 310.16 Note 1 says "Section 310.15(B) shall be referenced for ampacity correction factors where the ambient temperature is other than 30°C (86°F)." Presumably any reference to Table 310.16 includes the Notes to the table.

So that means that ampacity correction for ambient temperature applies at terminations? This is contrary to my prior understanding, am I missing something?

I.e. if you have an outdoor 100A OCPD with 75C terminations, and the ambient temperature high is 104F, then a 0.88 correction factor applies, and the 75C termination ampacity of #3 Cu is only 88A. Meaning you'd need to use #2 Cu to get a termination ampacity of 101A, even with 90C conductor insulation.

Cheers, Wayne

 

[jim dungar]

If your 100A OCPD is located in an ambient temperature greater than 40C it will also need to be derated.

 

[wwhitney]

If your 100A OCPD is located in an ambient temperature greater than 40C it will also need to be derated.

Good point, although conveniently in my example, 104F = 40C, so a 100A breaker would not need derating.

Should I take your lack of comment to the contrary to mean you agree that for 110.14(C) one does apply temperature correction at terminations?

Reading this all again, now I'm confused by exactly how 110.14(C) interacts with, say, 215.2(A)(1). Is 110.14(C) saying "here's an additional ampacity check you must do for terminations?" So with the two checks in 215.2(A)(1), we now have 3 checks for a feeder.

Or is 110.14(C) just modifying the 2 checks called for in 215.2(A)(1)? Effectively saying "whenever you do an ampacity check required elsewhere in the NEC, you must consider terminations. If the ampacity check called for is before correction/adjustment, then use the lower of the termination temperature rating and the conductor temperature rating. If the ampacity check is after correction/adjustment, then you may use the conductor temperature rating without regard to termination temperature rating."

Thanks, Wayne

 

[jim dungar]

Should I take your lack of comment to the contrary to mean you agree....

No, you should not.

 

[wwhitney]

No, you should not.

OK, so what's wrong with the simple analysis in the OP of the referential chain 110.14(C)(1) -> Table 310.16 -> Note 1 -> 310.15(B)? As far as the text of the NEC goes.

I mean my understanding from past discussions here was that the intention and industry practice was not to do ampacity correction at terminations. However, that doesn't appear to be what the NEC says. So if the intention is not to do ampacity correction at terminations, or if there is debate as what the text actually means, I would propose a PI for 2026.

Cheers, Wayne

 

[wwhitney]

To follow up, it is worth noting that in Annex D Example D3(a), when sizing the ungrounded feeder conductors, two ampacity checks are made:

(1) For terminations, using the termination temperature, a 125% factor for continuous loads, and no ampacity correction or adjustment. Presumably reflects 215.2(A)(1)(a).
(2) For the raceways, using the insulation temperature, no 125% factor for continuous loads, and ampacity correction and adjustment. Presumably reflects 215.2(A)(1)(b).

So the lack of ampacity correction in check (1) above correlates with my prior understanding that ampacity correction is not done at terminations. [Which still raises the question of why, physics-wise.]

Yet the reference to Table 310.16 in 110.14(C) may reasonably (typically, I would say) be read to encompass Note 1. So if the Annex D Example D3(a) is correct and the intention is not to do ampacity correction at terminations, I would say that 110.14(C)(1) would need to be amended such as:

" . . . shall be based on Table 310.16, without regard to the Notes, as appropriately modified by 310.12."

Cheers, Wayne

 

[wwhitney]

If your 100A OCPD is located in an ambient temperature greater than 40C it will also need to be derated.

Perhaps I missed your point yesterday: are you saying that since a typical circuit breaker has an ambient rating of 40C, there's obviously no need to do temperature correction at the circuit breaker termination for temperatures between 30C and 40C? That makes sense.

Looking into this more, I found Square D's Data Bulletin on rerating their breakers. It says that for QO and QOB breakers, a 100A breaker will have a rating of 100A at 40C, 91A at 50C, and 81A at 60C (doing my best to read the graphs).

Now suppose that breaker has a 75C insulated #3 Cu conductor terminated on it, which has an uncorrected ampacity of 100A. The correction factors for 40C, 50C, and 60C are 0.88, 0.75, and 0.58, respectively.

So at 40C, the breaker is rated for 100A, or 80A continuous, while the conductor is rated for 88A continuous, and 88 > 80. At 50C, the breaker is rated for 91A, or 73A continuous, while the conductor is rated for 75A continuous, and 75 > 73.

But at 60C, the breaker is rated for 81A, or 65A continuous, while the conductor is rated for 58A continuous. Isn't that a mismatch, meaning that the #3 Cu should be upsized, based solely on termination temperature considerations?

For this example, let's assume ambient in the OCPD enclosure is 60C, while ambient in the conduits leading to it is only 45C, with no more than 3 CCCs. So in the conduit the temperature correction factor is 0.82, the #3 Cu ampacity is 82A, bigger than the 81A rating of the OCPD.

Thanks, Wayne

 

[jim dungar]

If the ambient is 60C is a 60C conductor at maximum capacity even with zero load?

As hard as you try, I think you will find listed OCPD match up with conductors selected using the NEC, other than in very extreme cases.

 

[wwhitney]

If the ambient is 60C is a 60C conductor at maximum capacity even with zero load?

Obviously yes? Is that a trick question? My example has only 75C conductors.

As hard as you try, I think you will find listed OCPD match up with conductors selected using the NEC, other than in very extreme cases.

OK, I agree my example above is an extreme case. So as is often the case, the rules seem to be "good enough."

Thanks for the response.

Cheers, Wayne

 

[jim dungar]

Obviously yes? Is that a trick question? My example has only 75C conductors.


OK, I agree my example above is an extreme case. So as is often the case, the rules seem to be "good enough."

Thanks for the response.

Cheers, Wayne

The rules are more than just good enough.
In your example, clearly the conductors would not be properly protected therefore the installation would not comply with the NEC.

 

[wwhitney]

In your example, clearly the conductors would not be properly protected therefore the installation would not comply with the NEC.

Sorry, I don't follow that. The example (tweaked conduit temperature):

100A QO breaker in an enclosure with 60C ambient, rerated to 81A per Square D.
#3 Cu wire with 75C insulation
Conduit in 55C ambient with <=3 CCCs
63A continuous load

NEC required checks:
Termination check: 125% * 63A = 79A < 100A = #3 Cu 75C uncorrected ampacity
Conduit check: 63A < 100A * 0.67 = #3 Cu corrected ampacity, 75C insulation at 55C ambient
OCPD size check: 125% * 63A = 79A < 81A rerated OCPD

So where's the NEC violation?

If temperature correction applied at terminations, then the ampacity of the 75C #3 Cu at 60C ambient in the enclosure would be 58A, less than the 63A continuous load. Which seems like a corner case the NEC is missing.

Cheers, Wayne

 

[jim dungar]

Sorry, I don't follow that.

So where's the NEC violation?

Reread your posts. You were talking about a breaker in 60C and conduit in 45C.

 

[wwhitney]

Reread your posts. You were talking about a breaker in 60C and conduit in 45C.

Correct, I mentioned I adjusted the conduit temperature. But that makes no difference to the illustrative power of the example. For the original numbers there's only one change:

Conduit check: 63A < 100A * 0.82 = #3 Cu corrected ampacity, 75C insulation at 45C ambient

So there's no NEC violation either way. And I figured a 5C delta between conduit/enclosure was more plausible an example than a 15C delta. Although 60C ambient at the enclosure is obviously an extreme case.

Cheers, Wayne

 

[david luchini]

Sorry, I don't follow that. The example (tweaked conduit temperature):

100A QO breaker in an enclosure with 60C ambient, rerated to 81A per Square D.
#3 Cu wire with 75C insulation
Conduit in 55C ambient with <=3 CCCs
63A continuous load


So where's the NEC violation?

The violation is 240.4. You can't protect a conductor with an ampacity of 63 with an 81A ocpd (as Jim noted in post #10.)

 

[wwhitney]

The violation is 240.4. You can't protect a conductor with an ampacity of 63 with an 81A ocpd (as Jim noted in post #10.)

Thank you very much for your response. Somehow I didn't connect Jim's comment "the conductors would not be properly protected" and 240.4, although now it's obvious.

As it happens, that comment in post #10 was premature, as I only introduced the 240.4 violation in post #11 when I changed the conduit temperature from 45C to 55C. So for the original example in post #7, which I reprise below, what's the NEC violation, if any?

Cheers, Wayne

100A QO breaker in an enclosure with 60C ambient, rerated to 81A per Square D.
#3 Cu wire with 75C insulation
Conduit in 45C ambient with <=3 CCCs
63A continuous load

NEC required checks:
Termination check: 125% * 63A = 79A <= 100A = #3 Cu 75C uncorrected ampacity
Conduit check: 63A <= 82A = 100A * 0.82 = #3 Cu corrected ampacity, 75C insulation at 45C ambient
OCPD min size check: 125% * 63A = 79A <= 81A rerated OCPD
OCPD max size check: 81A rerated OCPD <= 82A = #3 Cu corrected ampacity