Conductor and OCPD sizing

[ggunn]

690.8(B)(2): Circuit conductors shall be sized to carry not less than the larger of 690.8(2)(B)(a) or (2)(b).

(2)(a) is the bit about multiplying by 1.25 twice.

(2)(b) is the bit about multiplying by 1.25 once and applying conditions of use.

OK so far.

(2)(c): "The conductor selected, after applications of conditions of use, shall be protected by the over current protective device, when required."

I interpret this to mean that if the derated ampaciity falls below the OCPD size below the one you have determined is the correct size, then you either have to lower the OCPD size to the first one above the derated ampacity, or you have to upsize the wire until its derated ampacity is higher than the OCPD below the one you have chosen, as long as it is less than 800A, in which case the ampacity of the wire must be equal to or greater than 95% of the rating of the OCPD. In other words, you can round up to the next OCPD size as long as the current is less than 800A, like everywhere else in the code.

William Brooks PE and James Dunlop PE do not agree. In their NABCEP Installation Professional Resource Guide, on page 67 in an example problem, they say:

"Step 5: Determine if 15A OCPD can protect the conductor under conditions of use [690.8(B)(2)(c)]: 12 AWG -> ampacity = 30A X 0.8 X 0.58 = 13.92A (fails because 14A fuse will not protect this conductor under conditions of use)."

It seems to me that if the code meant to say that the derated ampacity must be equal to or greater than the rating of the OCPD it would have said that, and that "protected by the OCPD" is defined elsewhere in the code, namely that you can round up to the next size up if the current is less than 800A. It seems to me that the 14A fuse is compliant. BTW, the 15A to 14A switch is theirs, not mine.

What say you? It makes a difference in system design.

 

[ggunn]

...as long as it is less than 800A, in which case the ampacity of the wire must be equal to or greater than 95% of the rating of the OCPD.

Scratch the bit about 95%; I obviously was thinking about something else.

 

[Smart $]

...

I interpret this to mean that if the derated ampaciity falls below the OCPD size below the one you have determined is the correct size, then you either have to lower the OCPD size to the first one above the derated ampacity, or you have to upsize the wire until its derated ampacity is higher than the OCPD below the one you have chosen, as long as it is less than 800A, in which case the ampacity of the wire must be equal to or greater than 95% of the rating of the OCPD. In other words, you can round up to the next OCPD size as long as the current is less than 800A, like everywhere else in the code.

William Brooks PE and James Dunlop PE do not agree. In their NABCEP Installation Professional Resource Guide, on page 67 in an example problem, they say:

"Step 5: Determine if 15A OCPD can protect the conductor under conditions of use [690.8(B)(2)(c)]: 12 AWG -> ampacity = 30A X 0.8 X 0.58 = 13.92A (fails because 14A fuse will not protect this conductor under conditions of use)."

It seems to me that if the code meant to say that the derated ampacity must be equal to or greater than the rating of the OCPD it would have said that, and that "protected by the OCPD" is defined elsewhere in the code, namely that you can round up to the next size up if the current is less than 800A. It seems to me that the 14A fuse is compliant. BTW, the 15A to 14A switch is theirs, not mine.

What say you? It makes a difference in system design.

You cannot downsize the minimum OCPD rating. It must be 125% of currents calculated in 690.8(A)... or greater. If the calcualtion results in a non-standard rating, you can apply 240.4(B) and go to next larger standard size, but that next standard size may not protect the conductor according to other requirements of Article 240. The conductor also has to meet the same 125% rating AND be protected by the OCPD.

I'm thinking where the disagreement between authors stems from is what they are individually considering as standard OCPD ratings. The NEC only recognizes breakers down to 15A. You can go lower with standard fuse ratings at 1, 3, 6, and 10. Not 14... [240.6(A)]. You could also use an adjustable trip breaker, but I've not seen any at the lower end of the scale.

Anyway, IMO, as long as the derated conductor ampacity is 11A or greater, it will be protected by a 15A-rated breaker.

 

[jaggedben]

It seems to me that ... "protected by the OCPD" is defined elsewhere in the code, namely that you can round up to the next size up if the current is less than 800A. It seems to me that the 14A fuse is compliant. BTW, the 15A to 14A switch is theirs, not mine.

...Anyway, IMO, as long as the derated conductor ampacity is 11A or greater, it will be protected by a 15A-rated breaker.

I agree with both of you, and would only add that this is bolstered by 690.8(B)(1)(d), which in turn refers to 240.4(B).

 

[ggunn]

You cannot downsize the minimum OCPD rating. It must be 125% of currents calculated in 690.8(A)... or greater. If the calcualtion results in a non-standard rating, you can apply 240.4(B) and go to next larger standard size, but that next standard size may not protect the conductor according to other requirements of Article 240. The conductor also has to meet the same 125% rating AND be protected by the OCPD.

But of course. I was speaking of the article itself. I did not mean to imply that the OCPD could or should be downsized if it was sized according to the rules you cite, only the ways in which the OCPD rating and the conductor size could be brought into agreement.

 

[ryan_mayfield]

"Step 5: Determine if 15A OCPD can protect the conductor under conditions of use [690.8(B)(2)(c)]: 12 AWG -> ampacity = 30A X 0.8 X 0.58 = 13.92A (fails because 14A fuse will not protect this conductor under conditions of use)."

It seems to me that if the code meant to say that the derated ampacity must be equal to or greater than the rating of the OCPD it would have said that, and that "protected by the OCPD" is defined elsewhere in the code, namely that you can round up to the next size up if the current is less than 800A. It seems to me that the 14A fuse is compliant. BTW, the 15A to 14A switch is theirs, not mine.

What say you? It makes a difference in system design.

If the check is really against the 15A OCPD (which would correspond to a standard module max fuse rating, but is not stated clearly in the example) then I say they are correct. The guide is based on the 2011 NEC and their example is dealing with PV source circuits. In 2011 690.9(C) Photovoltaic Source Circuits, the second paragraph indicates OCPDs shall be in one amp increments starting at one amp and up to and including 15A. Given that, the derated 12AWG would need to be protected by a 14A OCPD. Note, in 2014 this one amp increment language was removed.

Now even if the module in their example has a max fuse rating of 15A, you could place a 14A OCPD in that circuit, given Isc x 1.25 x 1.25 < 14A. And that would make the 12AWG OK.

 

[ggunn]

If the check is really against the 15A OCPD (which would correspond to a standard module max fuse rating, but is not stated clearly in the example) then I say they are correct. The guide is based on the 2011 NEC and their example is dealing with PV source circuits. In 2011 690.9(C) Photovoltaic Source Circuits, the second paragraph indicates OCPDs shall be in one amp increments starting at one amp and up to and including 15A. Given that, the derated 12AWG would need to be protected by a 14A OCPD. Note, in 2014 this one amp increment language was removed.

Now even if the module in their example has a max fuse rating of 15A, you could place a 14A OCPD in that circuit, given Isc x 1.25 x 1.25 < 14A. And that would make the 12AWG OK.

Actually, I am not that worried about individual string conductors;I don't know anyone who uses #12's for string conductors, anyway; #10's are pretty ubiquitous.

What concerns me is PV output circuits, i.e., the DC outputs of combiner boxes. For a big system with DC runs short enough not to have to be upsized for voltage drop, this could make an economically significant difference.

Does anyone know if 690.8(B)(2)(c) is unchanged in the 1024 code?

 

[SolarPro]

Here is the 2014 language:

(B) Conductor Ampacity. PV system currents shall be considered to be continuous. Circuit conductors shall be sized to carry not less than the larger of 690.8(B)(1) or (2).

(1) One hundred and twenty-five percent of the maximum currents calculated in 690.8(A) before the application of adjustment and correction factors.

Exception: Circuits containing an assembly, together with its overcurrent device(s), that is listed for continuous operation at 100 percent of its rating shall be permitted to be used at 100 percent of its rating.

(2) The maximum currents calculated in 690.8(A) after the application of adjustment and correction factors.

 

[ggunn]

Here is the 2014 language:

(B) Conductor Ampacity. PV system currents shall be considered to be continuous. Circuit conductors shall be sized to carry not less than the larger of 690.8(B)(1) or (2).

(1) One hundred and twenty-five percent of the maximum currents calculated in 690.8(A) before the application of adjustment and correction factors.

Exception: Circuits containing an assembly, together with its overcurrent device(s), that is listed for continuous operation at 100 percent of its rating shall be permitted to be used at 100 percent of its rating.

(2) The maximum currents calculated in 690.8(A) after the application of adjustment and correction factors.

Is this language (which was 690.8(B)(2)(c)): "The conductor selected, after applications of conditions of use, shall be protected by the over current protective device, when required." still in there anywhere?

 

[SolarPro]

No, it's not in 690.8(B), but 690.9 has some new language:

690.9 Overcurrent Protection

(A) Circuits and Equipment. PV source circuit, PV output circuit, in verter output circuit, and sto rage 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). Circuits, either ac or dc, connected to current-limited supplies (e.g., PV modules, ac output of utility-interactive inverters), and also connected to sources having significantly higher current availability (e.g., parallel strings of modules, utility power), shall be protected at the source from overcurrent.

Exception: An overcurrent device shall not be required for PV modules or PV source circuit conductors sized in accordancewith 690.8(B) where one of the following applies:
(a) There are no external sources such as parallel-connected source circuits, batteries, or backfeed from inverters.
(b) The short-circuit currents from all sources do not exceed the ampacity of the conductors and the maximum overcurrent protective device size rating specified on the PV module nameplate.

(B) Overcurrent Device Ratings. Overcurrent device ratings shall be not less than 125 percent of the maximum currents calculated in 690.8(A).

Exception: Circuits containing an assembly, together with itsovercurrent device(s), that is listed for continuous operation at 100 percent of its rating shall be permitted to be used at100 percent of its rating.

(C) Direct-Current Rating. Overcurrent devices, either fuses or circuit breakers, used in any dc port ion of a PV power system shall be listed and shall have the appropriate voltage, current, and interrupt ratings.

(D) Photovoltaic Source and Output Circuits. Listed PV overcurrent devices shall be required to provide overcurrent protection in PV source and output circuits. The overcurrent devices shall be accessible but shall not be required to be readily accessible.

(E) Series Overcurrent Protection. In grounded PV source circuits, a single overcurrent protection de vice, where required, shall be permitted to protect the PV modules and the interconnecting conductors. In ungrounded PV source circuits complying with 690.35, an overcurrent protection device, where required, shall be installed in each ungrounded circuit conductor and shall be permitted to protect the PV modules and the interconnecting cables.

(F) Power Transformers. Overcurrent protection for a transformer with a source(s) on each side shall b e provided in accordance with 450.3 by considering first one side of the transformer, then the other side of the transformer, as the primary.

Exception: A power transformer with a current rating on the side connected toward the utility-interactive inverter output, not less than the rated continuous output current of the inverter, shall be permitted without overcurrent protection from the inverter.

 

[jaggedben]

... In 2011 690.9(C) Photovoltaic Source Circuits, the second paragraph indicates OCPDs shall be in one amp increments starting at one amp and up to and including 15A.

I had never noticed this before. But what is a 'supplementary-type overcurrent device'? This section seems to me like it could be referring to something that is not required.