Error in Annex D Example D7?

[wwhitney]

Example D7 concerns the use of 310.12, the 83% "factor" for sizing residential services/main feeders. It start by noting that for a 175A rated service, if no ampacity correction or adjustment is required, only 145A of ampacity is required, so at least 75C rated #1/0 Cu or #3/0 Al would suffice.

It then considers the case of XHHW-2 conductors (which have 90C rated insulation) installed in a 38C ambient environment, which gives 0.91 ampacity correction factor. That means the required tabular ampacity is 175*83%/0.91 = 160A. But in translating that into conductor size, the example still uses the 75C column and gets #2/0 Cu or #4/0 Al.

As far as I can see, this is a mistake? Since the conductor insulation is rated 90C, we can use the 90C column, so the answer remains #1/0 Cu or #3/0 Al. That is, under the conditions given, the usable ampacity of #1/0 Cu will be the lesser of the 150A (the 75C ampacity, for terminations) and 170A * 0.91A = (the 90C ampacity, corrected for temperature). So #1/0 Cu still has a 150A ampacity, and suffices for the 175A residential service.

Am I missing something here? Or should I submit a PI?

Cheers, Wayne

 

[JoeStillman]

You are on to something bigger. I think we all imagine that we understand the language of 310.15(A) (second paragraph) to mean that we use the temperature rated ampacity to calculate a corrected and adjusted ampacity, and we can apply the wire at that adjusted ampacity or less. But let's look closer...

... The temperature correction and adjustment factors shall be
permitted to be applied to the ampacity for the temperature
rating of the conductor, 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).

The actual words say "does not exceed" but we treat it like it really means "is equal to or exceeds."

If you use the literal interpretation of the paragraph above, then Example D7 is correct.

 

[wwhitney]

The actual words say "does not exceed" but we treat it like it really means "is equal to or exceeds."

So there is an interpretation that goes like that, for a 90C conductor on a 75C termination. You start with the unadjusted/uncorrected 75C ampacity; then if you need to do adjustment and/or correction, you try applying those factor to the 90C ampacity; if the value is no more than the unadjusted/uncorrected 75C ampacity, that's your ampacity; otherwise you have to go back and apply those factors to the 75C ampacity. And I agree that a literal reading of the words used supports that meaning.

However, the resulting procedure has the property that as the adjustment/correction factors get worse (lower ampacity, you get an increase in the final ampacity. [As you pass from the regime where the adjusted/corrected 90C ampacity is more than the unadjusted/corrected 75C ampacity to the regime where it is not.] That's completely nonsensical and tells us that interpretation is wrong.

So the only rational meaning of the sentence is that you take the minimum of the two values.

Cheers, Wayne