690.8(B)(2)(c) from NEC2011...where did it go?

[Carultch]

As the title implies, what happened to 690.8(B)(2)(c) from NEC2011?

The text reads:
"The conductor selected, after applications of conditions of use, shall be protected by the overcurrent protective device, where required."

And particularly, does this rule still apply, even though not explicitly written in 690.8 for the PV-specific wire sizing algorithm? It seems like a general enough rule, that this might be the case. But it would help if it were still part of 690.8, to simplify making sure that your checklist is complete for all the wire sizing requirements.

The absence of this rule in 690.8 makes it difficult to rebut challengers of your design, when this particular rule would require an increased size above 1.56*Isc for terminations and 1.25*Isc/total derate for conductor ampacity.

So the question remains. Is this a rule that was intentionally eliminated to no longer require it? Or is it eliminated because it is implied by the first four chapters?

 

[pv_n00b]

The information in 2011 690.8(B) (2) on sizing conductors was in other parts of the code already and was just reprinted here so that it was all in one place and people did not have to dig it out of other code sections. In 2014 the section was significantly rewritten by the CMP to combine a number of proposals together and simplify the section.

690.8(B)(2)(a) became 690.8(B)(1)
690.8(B)(2)(b) became 690.8(B)(2)
690.8(B)(2)(c) was moved to 690.9(A) as simply a reference to use 240 for selection of protection.

2011 690.8(B)(2)(c);

The conductor selected, after application of conditions of use, shall be protected by the overcurrent protective device, where required.

2014 690.9(A) in part;

PV source circuit, PV output circuit, inverter output circuit, and storage battery circuit conductors and equipment shall be protected in accordance with the requirements of Article 240. Protection devices for PV source circuits and PV output circuits shall be in accordance with the requirements of 690.9(B) through (E).

Now 2011 690.8(B)(1)(b) did get deleted but is still in force because it was just reprinted from another section of the code, 110, anyway. 2011 690.8(B)(1)(c) got deleted with no reason given that I can find. My thought would be that the CMP decided this was a manufacturer's requirement best addressed by the manufacturer and not the code.

 

[ggunn]

As the title implies, what happened to 690.8(B)(2)(c) from NEC2011?

The text reads:
"The conductor selected, after applications of conditions of use, shall be protected by the overcurrent protective device, where required."

And particularly, does this rule still apply, even though not explicitly written in 690.8 for the PV-specific wire sizing algorithm? It seems like a general enough rule, that this might be the case. But it would help if it were still part of 690.8, to simplify making sure that your checklist is complete for all the wire sizing requirements.

The absence of this rule in 690.8 makes it difficult to rebut challengers of your design, when this particular rule would require an increased size above 1.56*Isc for terminations and 1.25*Isc/total derate for conductor ampacity.

So the question remains. Is this a rule that was intentionally eliminated to no longer require it? Or is it eliminated because it is implied by the first four chapters?

If you size your conductors correctly with the proper derates and assign the proper OCPD ratings then this will not be an issue; at least it hasn't for me in designing hundreds of systems. The derated for conditions of use 90 degree ampacity has always turned out to be greater than the rating of the next size down OCPD.

 

[Carultch]

If you size your conductors correctly with the proper derates and assign the proper OCPD ratings then this will not be an issue; at least it hasn't for me in designing hundreds of systems. The derated for conditions of use 90 degree ampacity has always turned out to be greater than the rating of the next size down OCPD.

Not true. 1.25*Isc/total derate is less than 1.56*Isc/total derate. So systematically, when you ignore the "rounding errors", the OCPD is more likely to govern wire size than 1.25*Isc/total derate.

Unless the second 1.25 margin is between two standard OCPD sizes, this rule isn't automatically satisfied.

Another question. Suppose you have an inverter that could either have a 400A fuse or a 500A fuse, but not a 450A fuse, despite 450A being a standard size. Then suppose that your wire ampacity after conditions of use is between 400A and 450A. Could you use the 500A fuse, without having to bump up the wire to 451A worth of wire?

 

[ggunn]

Not true. 1.25*Isc/total derate is less than 1.56*Isc/total derate. So systematically, when you ignore the "rounding errors", the OCPD is more likely to govern wire size than 1.25*Isc/total derate.

Unless the second 1.25 margin is between two standard OCPD sizes, this rule isn't automatically satisfied.

Another question. Suppose you have an inverter that could either have a 400A fuse or a 500A fuse, but not a 450A fuse, despite 450A being a standard size. Then suppose that your wire ampacity after conditions of use is between 400A and 450A. Could you use the 500A fuse, without having to bump up the wire to 451A worth of wire?

Not in any case I can remember, but we default to #10 for PV source circuits and a minimum of #8 for inverter outputs. I cannot remember a single case where the OCP determined conductor size for me.

In any case, only the 90 degree ampacity derated for conditions of use must be greater than the next size down OCPD, not the 75 degree ampacity derated for continuous use.

Why would an inverter be able to use a 500A or a 400A fuse but not a 450A one, and why would you use a 500A fuse where a 400A fuse would suffice? Are you talking about input or output? But yes, of course, in any case the 90 degree ampacity of the conductors derated for conditions of use must be greater than the next size down OCPD whether it can be used (for whatever reason) or not.

Maybe it's my methodology. For DC, 1.25 X Isc is Imax and I treat it the same as any other Imax. I use 1.25 Imax (AC or DC) to select a conductor from the 75 degree column and the OCPD. For my chosen condutor size I derate the 75 degree ampacity for continuous use and the 90 degree ampacity for conditions of use, and both derated ampacities must be greater than or equal to Imax. With that method in hundreds of designs I have yet to encounter a case where the conductors are not protected by the OCPD. I'm not saying I know for certain that it can't happen, only that it hasn't.

 

[Carultch]

Not in any case I can remember, but we default to #10 for PV source circuits and a minimum of #8 for inverter outputs. I cannot remember a single case where the OCP determined conductor size for me.

In any case, only the 90 degree ampacity derated for conditions of use must be greater than the next size down OCPD, not the 75 degree ampacity derated for continuous use.

Why would an inverter be able to use a 500A or a 400A fuse but not a 450A one, and why would you use a 500A fuse where a 400A fuse would suffice? Are you talking about input or output? But yes, of course, in any case the 90 degree ampacity of the conductors derated for conditions of use must be greater than the next size down OCPD whether it can be used (for whatever reason) or not.

Maybe it's my methodology. For DC, 1.25 X Isc is Imax and I treat it the same as any other Imax. I use 1.25 Imax (AC or DC) to select a conductor from the 75 degree column and the OCPD. For my chosen condutor size I derate the 75 degree ampacity for continuous use and the 90 degree ampacity for conditions of use, and both derated ampacities must be greater than or equal to Imax. With that method in hundreds of designs I have yet to encounter a case where the conductors are not protected by the OCPD. I'm not saying I know for certain that it can't happen, only that it hasn't.

The example in question has an Isc of 277A. 1.25*Isc = 346A. 1.56*Isc = 432A. I'm using 2 parallel sets in the same conduit. Any time a derate factor is anything more than a basic ambient temperature derate, the OCPD where required, is likely to govern sizing.

I don't understand why the inverter omits standard fuse sizes. It even has exotic fuse sizes like 355A, despite having the standard neighboring size of 350A.

 

[pv_n00b]

Another question. Suppose you have an inverter that could either have a 400A fuse or a 500A fuse, but not a 450A fuse, despite 450A being a standard size. Then suppose that your wire ampacity after conditions of use is between 400A and 450A. Could you use the 500A fuse, without having to bump up the wire to 451A worth of wire?

No, the NEC allows going up to the next standard size, not up to the next manufacturer allowed size.

 

[ggunn]

The example in question has an Isc of 277A. 1.25*Isc = 346A. 1.56*Isc = 432A. I'm using 2 parallel sets in the same conduit. Any time a derate factor is anything more than a basic ambient temperature derate, the OCPD where required, is likely to govern sizing.

I don't understand why the inverter omits standard fuse sizes. It even has exotic fuse sizes like 355A, despite having the standard neighboring size of 350A.

Wow, I don't know what happened. I must not have hit submit before I shut my computer down.

Anyway, I wrote a somewhat lengthy response where I ran your numbers and found that you could tolerate a 90 degree COU derate of 0.77 (if memory serves) before a 450A OCPD could not protect two sets of 4/0 copper. So yes, I can see where that could happen. And yes, if you have to go to 500A, the 4/0 is right out.

My original response was much better. Pity.