Temperature Correction and Terminations

[wwhitney]

I understand that the termination limitations in 110.14(C) are commonly understood to based on the table ampacity at the termination temperature rating without correction for ambient temperature. Two questions on that:

1) Is there any physics or listing reason that it is unnecessary to correct for temperature at the terminations? E.g. (to make up a possible explanation), that the UL listing for a piece of equipment would require it to be labeled with the allowable operating temperatures, and that the listing standard would confirm operation at the upper end of the allowable temperature range, where the conductors at the terminals are sized just by the table without temperature correction. Then the temperature effects at the terminals would be baked in already, with no need to do any further correction. Are any listing standards like that?

2) Code wise, what is the reading of 110.14(C) that supports this common understanding? (2017) 110.14(C)(1) says (second sentence): "Unless the equipment is listed and marked otherwise, conductor ampacities used in determining equipment termination provisions shall be based on Table 310.15(B)(16) as appropriately modified by 310.15(B)(7)." But Table 310.15(B)(16) is titled " . . . Based on Ambient Temperature of 30C" and has a footnote that says "*Refer to 310.15(B)(2) for the ampacity correction factors where the ambient temperature is other than 30°C (86°F)." So doesn't any reference to Table 310.15(B)(16) automatically include temperature correction per Table 310.15(B)(2) per the footnote?

Thank you,
Wayne

 

[wwhitney]

To follow up, question #2 was discussed some in this other thread of mine: https://xenforo.mikeholt.com/thread...mperature-correction-at-terminations.2568653/

Annex Example D3(a) is also informative as to the intention. And these have been long standing questions, as this old thread is one example of: https://forums.mikeholt.com/threads/equipment-termnation-temperature-limitations.127214/ I particularly like Smart$'s comments in post #7.

So my updated comments on the two questions in the OP

1) Circuit breakers are rated for 40C ambient, so it makes some sense that there is no need to correct for temperature at circuit breaker terminations for temperatures between 30C and 40C. I have not looked into listing standards for other equipment. On the fundamental physics, obviously if the ambient temperature at the termination is higher, the actual operating temperature at the termination will be higher.

2) The consensus on the intention is that you don't do temperature correction at the terminations. The code language is somewhat contradictory--the primary counterargument to the reference chain in the OP is that the second sentence of 110.14(C) grants permission to use the insulation temperature as the starting point any time that temperature correction is required. So you never do both temperature correction and impose the termination temperature at the same time. 110.14(C)(1)'s reference to Table 310.16 should therefore be amended to indicate that you ignore the Notes on the table.

Cheers, Wayne

 

[wwhitney]

A simple example of the physics issue with ignoring temperature correction at terminations:

A 120' branch circuit consisting of a breaker and 100' of conductor/conduit at 30C ambient, followed by 20' of conductor/conduit and the load at 45C ambient. 100A non-continuous load, 75C terminations at both ends, a 100A breaker, and #3 Cu conductors, 3 CCCs.

If the #3 Cu conductors have 75C insulation, then there's a violation: where the ambient temperature is 45C, the ampacity of the #3 Cu in the conduit is 0.82 * 100 = 82A due to temperature correction. Less than the 100A load. That means if/when the load exceeds 82A, the conductors in that region may operate above 75C, we have no guarantee they will not.

If the #3 Cu conductors have 90C insulation, there's no NEC violation: where the ambient temperature is 45C, the ampacity of the #3 Cu in the conduit is 0.87 * 115A = 100A. And the termination check ignores the 45C ambient.

However, we know from the previous example that the conductors may operate above 75C, and that would apply to the load enclosure as well. Very odd that we are permitted to operate the 75C rated terminations at a temperature that may exceed 75C.

Cheers, Wayne