pv source circuit wire sizing - Mike Holt's Forum

Well i always thought I understood PV wire sizing, but as y'all know it gets a bit complex.
I am combining two PV strings on roof. Each string is 10.5 A Isc each = 21 A Isc combined.
There are existing #10 THHN conductors in conduit from roof to inverter. I would like to use those for labor saving.
Can I?

Overall, I would do the calcs like so:
De-rate wire ampacity down and multiply current up, then compare. Current must be less than equal to ampacity.

Specifically,
A. AMPACITY
Start with 40A ampacity to derate #10 THHN wire.
2 de-rates to consider:
-- Only 2 CCC in conduit, so no derate for # of wires in raceway.
-- Ambient TEMP: Average high Temps are just about 70-80F here, so no derate. (I recall it is average high temps that are to be used. Am i right on this? or record high Temp must be applied? Big difference!)

===> 40A x 1.0 x 1.0 = 40 A allowable ampacity (although ultimately 30A max allowed by NEC in end no matter what)

Now....can I further de-rate the 40A ampacity by 0.8 PV use factor - rather than multiply the Isc and continuous use factor by 1.25?
That gives me 32A allowable. This of course gets knocked to 30A absolute max allowable.

B. CURRENT
I multiply 21 A Isc by 1.25 continuous use factor = 26.25 A.
This is under the above 30A, so I am golden.

Otherwise if I must multiply by 1.56 (1.25 continuous and 1.25 PV use factors), then 1.25 x 1.25 x 21 A pushes me over 30A by a smidge. (1.56 x 21 A = 32.81 A) Ok.... by 2.81 smidges.

My recent reading says the 1.56 had been implied by the NEC, but not quite that simple..... and has been since clarified.
In other words, there is something about applying the 1.25 factors separately one at a time, but I never quite got that.
Any insight appreciated gents!

-- Ambient TEMP: Average high Temps are just about 70-80F here, so no derate. (I recall it is average high temps that are to be used. Am i right on this? or record high Temp must be applied? Big difference!)

There is an informational note to use ASHRAE as a data source for this. It isn't the record high, it is a statistical measure of a typical high, such that only 2% of the time will exceed it. You can get the data for high and low temperatures from solarabcs.com.

There also is a square root formula in the same section of the NEC, that occasionally has an advantage in its results when compared to using the table for looking up temperature correction factors. You can either use the table, or the square root formula, whichever is convenient for you.

Now....can I further de-rate the 40A ampacity by 0.8 PV use factor - rather than multiply the Isc and continuous use factor by 1.25?

The 0.8 is just an algebraic manipulation of the 1.25 multiplication that the NEC specifies. There are multiple 1.25 factors that may apply, and they can compound, depending on which calculation you are doing.

Derate factors are generally less than 1, so they generally reduce the ampacity of a conductor. Derates apply to wires, but not to terminations, which is the primary reason why there is value in 90C wire despite 75C terminations. It gives you headroom for your derate calculations. I know the NEC no longer uses the term derate, but it is the easiest way to explain it.

There is a 4-step calculation to determine the required ampacity of a conductor for "wild PV". I.e. PV circuits directly from the panels, or combined strings of panels, with no power electronics in between.
1. Calculate 1.56*Isc. Size your OCPD and terminations (generally 75C) from this value. An informational note tells you 1.25*1.25 becomes 1.56.

The reason for the two 1.25 factors:
The enhancement factor for more than 1kW/m^2 of sunlight. This applies for "wild PV", but not for outputs of current-limited devices.
The continuous load factor, this applies to sizing terminations and OCPDs, but does not apply to step 2 below.

2. Calculate 1.25*Isc/total derate. Look in the 90C column for a size that meets or exceeds this value.

3. Calculate the derated wire ampacity. 90C column value * total derate. This must "round up" to the OCPD you are using, if you are using one. If not, this step doesn't apply. See 240.4(B) for specifics of what I mean by "round up".

4. Confirm that the termination ampacity (usually 75C) also "rounds up" to the OCPD you are using.

It is the norm of ratings that terminations are 75C and wires are 90C, so adjust the above accordingly as applies to you. It is rare for today's wire types to be rated less than 90C. You may see 90C terminations, but they are a lot less common, and you have a burden of proof to confirm 90C terminations on both sides. 110.14(C) spells out the default termination ratings. The 60C part of the rule is more academic than practical, because most equipment is listed for the 75C rating. You still have a burden of proof, but it is rare to not meet it, for the 75C rating at 100A and less.

Remember, if part of equipment, the entire product needs to be listed for 90C terminations to take credit for it, and not just lugs marked AL9CU. To answer the question about MC4 connectors, as stand-alone connectors, they are 90C rated, but if built-in to a product, you have to follow the instructions of that manufacturer to know whether to use the 90C or 75C rating.

Do you have a reference for the idea that the continuous load factor applies to terminations? I don't see that in 110.14. Principally relevant for PV source conductors that have no OCPD.

Also, do you see a physics justification in applying a continuous load factor for the minimum ampacity of PV source conductors without OCPD? I don't. I only see the continuous load factor as coming from OCPD limitations. Ampacity is already a continuous rating.


So do PV panels typically come with +/- pigtails with MC4 connectors on them, and if so what temperature rating applies to those pigtails?

Cheers, Wayne

Read 690.8 and 690.9, that are the PV-specific sections. Part 1 and part 2 of my 4-step process, come directly from these sections.

As for the factory-installed MC4 connectors on PV panels, they usually are rated for 90C, but rarely (if ever) does this govern a design. Obviously, they have to have an ampacity rating for the product they are attached to, and unless you shine 2 suns of irradiance on them, you aren't going to overload those connectors from the ampacity.

Where you terminate your field-installed home runs on the other side of the wire, is more likely going to be what governs you to use the 75C rating.

I overlooked the exception to 690.8(B)(1), which has been in the NEC since at least the NEC. It reads:

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.

I assume the DC inputs on a string inverter are rated for continuous operation at 100% of their rating? If so, this exception eliminates the need for the 125% continuous factor on ampacity for PV Source Circuits without OCPD.

I am of course reading "together with its overcurrent devices(s)" as meaning "overcurrent device(s) if any," rather than indicating that there must be an OCPD. Because that is the logical requirement.

Cheers, Wayne

One place you would see this, is if you are using a SolarBOS recombiner for a central inverter, that has continuous duty rated breakers in it, and is listed as an assembly to allow you to take credit for this. I've only seen this where the OCPD's are breakers, and I haven't seen it where the OCPD's are fuses.

There is a burden of proof to see this in documentation, to allow you to forgo the 125% continuous load factor. If it isn't specified, you treat every fuse or breaker as a standard OCPD, and size the OCPD to 1.56*Isc for "wild PV", and also apply the continuous load factor to all other parts of the calculation where it applies. Likewise, if it isn't specified that terminations in equipment where OCPD doesn't apply aren't specifically rated for continuous duty, then you also apply the continuous load factor

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