...The circuit ampacity is the lesser of the two calculations (termination vs in the conduit).
...That's what the first paragraph of 110.14(C) tells you...
Not sure how you get that from 110.14(C)....
It sounds to me like you're attempting to reference 310.15(A)(2) Selection of Ampacity when talking about different ampacities on a conductor and being required to utilize the lesser one, which I agree is relevant once reaching 310.15, but is not what 110.14(C) is directing us to do.
110.14(C) Temperature Limitations. The temperature rating associated with the ampacity of a conductor shall be selected and coordinated so as not to exceed the lowest temperature rating of any connected termination, conductor, or device....
My understanding, based on the text and Mike's instructional videos, is that the first half 110.14(C) is saying... if we have 60*C terminations, we have to select our conductors to be from the 60*C column (e.g. types TW, UF).
60*C termination = 60*C wire.... and what they are attempting to do, is to prevent the conductor from having an ampacity that would result in it heating up to a point where it could damage the termination, equipment. Theoretically, that if we place 20A on a #12 TW 60*C wire, it will heat up to 60*C and possibly damage the termination that is rated for 60*C.
However, I would wager that there realistically exists a buffer zone beyond what is allowed by 310.15(B)(16) (e.g. 20A on a #12 TW will not necessarily cause the conductor to reach 60*C exactly and damage equipment... but the code has set this limit and we have to abide by it for safety purposes even if said buffer zone exists, which I believe it must, otherwise, anytime anyone utilized the max allowable ampacity in 310.15(B)(16), the wire would be damaged.
Interpretation #2: This entire section is NOT saying anything about conductor selection, but is simply directing us to look at the temperature column of 310.15(B)(16) associated with the lowest temperature rating in the proverbial chain... with a lot of unnecessary verbal fluff.
110.14(C) …Conductors with temperature ratings higher than specified for terminations shall be permitted to be used for ampacity adjustment, correction, or both."
The latter part of 110.14(C) is giving us permission to utilize conductors with higher temperature ratings, such as 90*C wire, but it explicitly says for the [
sole (implied)
] purpose of "ampacity adjustment, correction, or both." The code is not necessarily, explicitly giving us permission to actually install a 90*C wire on a 60*C termination, not yet.
In context with the previous part of this section, the implication here is that we're going to install a 60*C wire on a 60*C termination, but that we can utilize the max allowable ampacity from the 90*C column for the [
sole (implied)] purpose of "ampacity adjustment, correction, or both."
However, and I should be clear here that I understand the potential problem with this particular interpretation (but this is how it reads, IMO) because you CANNOT, I repeat CANNOT, just go derating from 40A on a #10 TW because you will likely end up with an ampacity that is greater than what the wire itself is rated for.
As with how the rest of this code functions, we have to combine several pieces together, there might be conflicting portions (in which case the most restrictive rule is king), and the later parts of this section as well as 310.15 are part of that equation.
Interpretation #2: The later part of this section is simply telling us that we can utilize a temperature column higher than the lowest rated component for the purposes of adjustment and correction.
Either interpretation implies that we can utilize a temperature column higher than that of the lowest in the proverbial chain for adjustment and correction.
110.14(C)(1) The determination of termination provisions of equipment shall be based on 110.14(C)(1)(a) or 110.14(C)(1)(b).
Unless the equipment is listed and marked otherwise, conductor ampacities used in determining equipment terminations provisions shall be based on Table 310.15(B)(16) as appropriately modified by 310.15(B)(7).
The first sentence here is a little tricky, IMO because of the word "provision" as it could mean either (1) "a thing supplied," which IMO is referring the conductor itself... or it could mean (2) "an amount," in which case it is simply setting a limit for how the terminations of equipment are to be utilized.
However, the second sentence, if taken in context with the first, makes it clear that we are talking about an ampacity limit which is being set and we are to utilize either 110.14(C)(1)(a) or 110.14(C)(1)(b) in combination 310.15(B)(16) to do so.
The part that trips me up is the "as appropriately modified by 310.15(B)(7) part because that's referring to services and feeders. The only explanation I can think of is that this section applies to
all terminations, but when dealing with services and feeders, the rules of 310.15(B)(7) also apply.
Skipping 110.14(C)(1)(a) because it's just setting specific parameters for when it is to be applied, but this is the one I'm looking at here.
110.14(C)(1)(a)(2) Conductors with higher temperature ratings, provided the ampacity of such conductors is determined based on the 60*C ampacity of the conductor size used.
This is where we get permission to install a 90*C wire on a 60*C termination, but we still have to set the ampacity of that wire based on the 60*C column (i.e. we can install #10 THHN 90*C on a 60*C termination, but the ampacity is set at 30A from the 60*C column because the termination is rated at 60*C).
Basically, via 110.14(C), we're setting an ampacity limit in order to control the temperature at the point of termination in order to prevent the terminations from getting damaged by a wire whose ampacity results in a temperature that would exceed what the termination is rated for... by either using 60*C wire on a 60*C termination or by utilizing 90*C wire on a 60*C termination, but in either case, limiting its' ampacity to the 60*C column.
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In summation, we leave 110.14(C) with an ampacity limit.
So, in my example, we have 60*C terminations, but are utilizing #10 THHN, a 90*C wire, and setting the ampacity equal to that of the 60*C column, or 30A.
110.14(C) …Conductors with temperature ratings higher than specified for terminations shall be permitted to be used for ampacity adjustment, correction, or both."
310.15(B) ...The temperature correction and adjustment factors shall be permitted to be applied to the ampacity for the temperature rating of the conductor...
Per the second sentence of 110.14(C) and the first half of the first sentence of the second paragraph of 310.15(B), we can derate this wire from the 90*C column.
However, the second half of the first sentence of the second paragraph of 310.15(B) gives a condition to follow for that derating procedure.
310.15(B) ...if the corrected and adjusted ampacity does not exceed the ampacity for the temperature rating of the termination
in accordance with the provisions of 110.14(C).
So we've left 110.14(C) with an ampacity limit of 30A since the terminations are rated at 60*C, but we are now derating from the 90*C column since that is the temperature rating of the #10 THHN... but that final value has to be BELOW the initial value set by 110.14(C).
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The reason for this condition is that, if we derate from the 90*C column on 60*C terminations and end up with a final ampacity that would exceed what the termination is rated for, we will damage the termination, equipment.
Now, granted, per 240.4(D) Small Conductors, we are still required to put this wire on a 30A breaker, which determines the final overall rating of the branch circuit, but we are not allowed to just skip 110.14(C), 310.15(B) in determining our ampacity for the conductor and just say #10 = 30A, as most people tend to do in my experience, because there exists the possibility that, post derating, this conductor will have an allowable ampacity below that which is set by the OCPD... and could result in a wire that has an allowable ampacity of 25A, but someone puts a 28A total load on it, and the breaker does not trip because it is below 30A.
Really, my interpretation all comes down to how 110.14(C) and 310.15(B) work together... and it is conditional upon a limited situation in which we have 60*C terminals and attempting to utilize a 90*C wire while applying corrections and adjustments to the 90*C column.
Apologies for the length of this, but I had to parse out the actual text and give my thoughts so as to explain the OP. I will await your input and do my best to try and understand where, if anywhere, I have gone wrong.
