Nec 310.15 B2 temperature adder 60 deg f

[Jpflex]

NEC 310.15B2

Says to add a 60 degrees F to the ambient temperature when calculating and correcting conductor's ampacity, when it is in a cable or raceway with its bottom less than 3/4 from a roof or exposed to direct sunlight.


My 2023 handbook provides an example

A feeder runs less than 3/4 inches above a commercial building roof. This feeder supplies a calculated load of 175 A with an ambient temperature of 95 degrees ferenheight. None of the loads are continuos and the neutral is not considered current carrying (bundling effect)

QUESTION:

What is the size of THWN-2 ALUMINUM FEEDER rated at 90 degrees C is needed for this installation?


ANSWER:

New ambient temperature
95 degrees F + 60 degrees F = 155 deg f

Table 310.15B11

THWN-2 insulation rated 90 degrees c. Therefore, a 0.58 reduction to the conductor ampacity at 90 degrees c is required

175 ampere / 0.58 = 302 ampere aluminum conductor is to be selected from table 310.16

A 400 kcmil ALU conductor at 305 amperes at 90 degrees c is chosen


Then the book says according to 110.14 C, we must select a wire for 75 degree c when terminating onto equipment rated for 75 deg c

However, instead of including the 60 deg F adder to ambient temp as with the wire subject to 90 degree c limit, they do NOT include this adder to the 75 degree limit.

The book says the 400 kcmil aluminum THWN-2 can be used here for termination because at 75 deg F, the ampacity is 270 i amperes and is enough for the 175 i ampere load.

What does not make sense is that if NEC assumes that the condition of use by subjecting wire in direct sunlight or less than 3/4 above a roof causes elevated conductor temperature for a given ampere limit, then the ampacity is either lowered or a larger size wire is selected

Why then is nec not adding 60 degree adder to the wires ampacity at 75 degrees F and increasing the conductor size under the same condituons of direct sunlight ar distance from a roof?

If direct sunlight or distance above a roof less than 3/4 inch causes wire temperature to increase above its 90 degree C insulation temp limit at the 310.16 tables ampacity shown, it will surely do the same for the tables ampacity limit for the conductor at 75 degrees C

 

[Jpflex]

based on table 310.15B11, this new ambient temperature off 155 degrees F falls in between 66 to 70 degrees celcius which is acceptable for the wire and may answer part of this in just this one example but would it be possible for ambient temp plus the adder to exceed the termination temp?

 

[CoolWill]

You use the 90 degree column for your derating to start with. This gives us an out because we can start with a higher ampacity than we need and still end up close to where we want to be anyway.

Also, it's probably cheaper in most cases to raise the conductors a little higher than to pull huge conductors in larger conduit.

 

[wwhitney]

Why then is nec not adding 60 degree adder to the wires ampacity at 75 degrees F and increasing the conductor size under the same condituons of direct sunlight ar distance from a roof?

For better or worse, the NEC seems to leave sizing of wires at terminations to the equipment manufacturer. That is, the NEC does not require temperature correction for termination-related wire sizing. Temperature correction only applies to wire sizing in the run of the conductor, away from the terminations.

So if the equipment manufacturer says their equipment can be installed in full sunlight on the roof, and at the applicable ambient temperatures, and the terminations are rated 75C, that means that the minimum wire size as far as the equipment manufacturer is concerned may be chosen from the 75C column of Table 310.16. That is what the manufacturer expects, and the equipment is designed such that nothing will overheat if you provide at least that size wire. If they need the larger wire size that a 60C termination rating would imply in order to make their thermal design work in those conditions, they will specify that the terminations are 60C rated, even if all of their components are actually good for 75C or 90C or more.

Now if you have to upsize the wire further because of conditions like in the OP and the NEC requirements, that's fine, and the equipment obviously won't be negatively thermally impacted by the larger wire size (as long as the equipment termination provisions accept that larger wire size).

Cheers, Wayne

P.S. A related question, and perhaps less clear, is suppose you manage to find a wire nut (or other splicing connector) that is only 75C rated, and you want to install a junction box on the roof using that connector. Is it still OK in the OP to use 400 kcmil Al, as the connector counts as an equipment termination? (I think that's my understanding of the NEC rules). Or is it not OK, as the connector could in fact get up to 90C under those conditions (I think that's my understanding of the physics, at least that there's no guarantee it won't exceed 75C, if we take the 60F adder as rational).

 

[don_resqcapt19]

You use the 90 degree column for your derating to start with. This gives us an out because we can start with a higher ampacity than we need and still end up close to where we want to be anyway.

Also, it's probably cheaper in most cases to raise the conductors a little higher than to pull huge conductors in larger conduit.

Or use XHHW conductors.