Is everything I know wrong?

[ggunn]

As long as I have been designing solar (12 years) I have been going by John Wiles' conductor sizing algorithm for 90 degree conductors, which is to say, I multiply the inverter output current by 1.25 and choose a 75 degree ampacity conductor size that exceeds it, then I choose a 90 degree conductor size whose conditions of use derated ampacity exceeds the inverter output current, and then I use the larger of the two wire sizes. Then I choose an OCPD which is the next size up from the conditions of use 90 degree ampacity of the chosen conductor.

I now have a commercial PV inspector who is invoking 110.14(C)(1)(b), claiming that it directs that the 75 degree ampacity of the conductors has to be equal to or greater than the rating of the OCPD. 110.14(C)(1)(b) is written with the same clarity and precision that we have come to expect from the NEC (ahem). Is he correct, and if so, can someone explain it to me in words of two or fewer syllables?

 

[Rock86]

I know this isn't a direct answer... and its from 2014... but take a look at

 

[ggunn]

Sorry; I should have been more specific. I am talking about AC conductors. The Mike Holt video above is for DC circuits.

 

[wwhitney]

110.14(C)(1)(b) doesn't say anything about OCPD sizing. It just limits the ampacity at the terminations to the tabular 75C value, unless otherwise marked.

For OCPD sizing, 240.4(B) still lets you use the next size larger breaker, if not over 800A.

Cheers, Wayne

 

[don_resqcapt19]

If the equipment is marked with 75°C terminations, the ampacity of the conductor must be equal to or greater than the 75°C column ampacity. 215.2(A) sort of permits you to do it they way you are doing it, but it references 1140.14(C) so the adjusted and corrected ampacity of your conductor must be equal to or greater than the 75°C ampacity.

 

[wwhitney]

If the equipment is marked with 75°C terminations, the ampacity of the conductor must be equal to or greater than the 75°C column ampacity.

I'm not sure what you are trying to say here. Taken literally, it would mean, say that a #1 Cu conductor with 90C insulation passing through an area of 50C ambient temperature (0.89 factor, 129A ampacity) couldn't be terminated in equipment marked for 75C terminations, because its ampacity is less than the 75C column ampacity (130A). But I don't think that's what you mean.

Cheers, Wayne

 

[augie47]

I'm not sure what you are trying to say here. Taken literally, it would mean, say that a #1 Cu conductor with 90C insulation passing through an area of 50C ambient temperature (0.89 factor, 129A ampacity) couldn't be terminated in equipment marked for 75C terminations, because its ampacity is less than the 75C column ampacity (130A). But I don't think that's what you mean.

Cheers, Wayne

(side note: You 0.89 multiplier is from 310.15(B)(2)(b); I believe the correct Table would be 310.15(B)(2)(a) for a correction factor of 0.82.. doesn't change the "theory" just the actual numbers)

 

[wwhitney]

(side note: You 0.89 multiplier is from 310.15(B)(2)(b); I believe the correct Table would be 310.15(B)(2)(a) for a correction factor of 0.82.. doesn't change the "theory" just the actual numbers)

Thanks for the correction. Let's change the example to #2 Cu with 90C insulation and 45C ambient (0.87 factor). Now the ampacity is 113A (130A * 0.87), less than the 75C column ampacity of 115A.

Cheers, Wayne

 

[augie47]

Thanks for the correction. Let's change the example to #2 Cu with 90C insulation and 45C ambient (0.87 factor). Now the ampacity is 113A (130A * 0.87), less than the 75C column ampacity of 115A.

Cheers, Wayne

Then, as I understand it, you would have to install it so that the load would not exceed 113 amps.
Based on Art 240, your overcurrent device could still be 125 amps. The `130 amp rating would be irrelevant except for derating.

 

[ggunn]

All I can say is duh. Sometimes (too many times) I try to look for a convoluted solution when the answer is very simple. As Wwhitney (and my other engineer) said, in a word, 240.4.

 

[bwat]

I'll see if I can reword Don's statement.

From this:

If the equipment is marked with 75°C terminations, the ampacity of the conductor must be equal to or greater than the 75°C column ampacity.

to this:

If the equipment is marked with 75°C terminations, the 75°C column ampacity of the conductor must be equal to or greater than what is required.

Would that work?

This is a concept that I certainly understand, but is a little hard to put into words. One of those "you don't really know something until you try to teach it".

 

[wwhitney]

Then I choose an OCPD which is the next size up from the conditions of use 90 degree ampacity of the chosen conductor.

This part is not quite right, you need to start at the lesser of the 90C conditions of use ampacity and the 75C tabular ampacity, then you can go to the next size up OCPD.

Simple example: #3 Cu has a 75C ampacity of 100A and a 90C ampacity of 115A; standard breaker sizes are 100A, 110A, and 125A. So for an 80A inverter current with no ampacity adjustment or correction, your algorithm picks #3 Cu. But then the next size up breaker from the 90C ampacity would be 125A. You can't protect the #3 at 125A; it needs to be protected at 100A.

Cheers, Wayne

 

[jaggedben]

Is it me or is 110.14(C)(1)(b)(2) just impossible to make sense of?

It says you may use 'conductors with higher temperature ratings [than 75C], provided the ampacity of such conductors does not exceed the 75C ampacity of the conductor size used..."

Shouldn't it say the direct opposite? 'Exceeds' instead of 'does not exceed'? Is this an error?

 

[wwhitney]

I think when it says "75C ampacity" it implicitly means "75C tabular ampacity". So it's saying you are limited to the 75C tabular ampacity, unless the equipment is "listed and identified" for a higher termination temperature.

Cheers, Wayne

 

[jaggedben]

I think when it says "75C ampacity" it implicitly means "75C tabular ampacity". So it's saying you are limited to the 75C tabular ampacity, unless the equipment is "listed and identified" for a higher termination temperature.

Cheers, Wayne

'75C ampacity' is not confusing to me. The part that's confusing is 'provided... the ampacity ... does not exceed' the 75C amapcity. It literally seems to prohibit using 90C conductors (unless the equipment is listed for them), because of course their (90C) ampacity will always exceed the 75C ampacity. It just seems like an improper edit that has ended up saying the opposite of the intent and common practice.

 

[wwhitney]

I don't think that the bare word "ampacity" is referring to a tabular ampacity, rather the sentence is trying to say you have to cap the ampacity at the tabular 75C ampacity. I.e. because of the equipment termination temperature limitation of 75C, the ampacity of the a 90C conductor connected to 75C equipment will be the lesser of the 75C tabular ampacity and the adjusted for conditions of use ampacity based on the 90C tabular ampacity.

I.e. I read "provided" as "with the provision that" rather than as "if". Because that's the only way to get a reasonable meaning out of that convoluted text.

Cheers, Wayne

 

[jaggedben]

Yeah, I'm gonna continue to contend that it doesn't say what it means to say, unless someone can give me a more substantive explanation.

 

[wwhitney]

So if it instead of "does not exceed" it read "is limited to" would that take care of your concerns? Because that's my understanding of what it means to say.

Cheers, Wayne

 

[jaggedben]

It seems it should just the use the same language as the previous section. i.e. 'provided the ampacity of such conductors is determined based on the 75C ampacity of the conductor sized used' or 'the equipment is listed and identified for use with such conductors.'

 

[don_resqcapt19]

It seems it should just the use the same language as the previous section. i.e. 'provided the ampacity of such conductors is determined based on the 75C ampacity of the conductor sized used' or 'the equipment is listed and identified for use with such conductors.'

It can't say that, as saying that would not permit you to use the 90°C ampacities for ampacity adjustment and correction.

The intent is that where the adjusted and corrected ampacity of the 90°C conductor exceeds that of a 75°C conductor, you cant use the 90°C conductor at an ampacity that exceeds the table ampacity as found in the 75°C column.

Where the adjusted and corrected ampacity of the 90°C conductor is less than that found in the 75°C column, that becomes the maximum ampacity of the conductor and may require the use of a smaller OCPD, or a larger conductor.