Solar PV wire sizing

[nwsolar]

Good day! I am sizing my PV wire for my small system. I am using 6 ea 400 watt panels in series that give me 222 open volts. The short circuit amps are 13.97. I live in the NW and rarely see temps above 80 degrees and will be running the wire in conduit under the ground. My total run is 300 ft. Can I use 10g or should I use 8? These panels will trickle feed my battery generator @00 amp 48v) that is used approximately once a week to run my well. My panel is a 10,000kw split phase and I will be turning it off during the winter months when not in use to prevent it from consuming all the power in standby. So can I get away with 10 gauhge or should I spent the extra money for 8?

 

[sunshine_eggo]

Good day! I am sizing my PV wire for my small system. I am using 6 ea 400 watt panels in series that give me 222 open volts. The short circuit amps are 13.97. I live in the NW and rarely see temps above 80 degrees and will be running the wire in conduit under the ground. My total run is 300 ft. Can I use 10g or should I use 8? These panels will trickle feed my battery generator @00 amp 48v) that is used approximately once a week to run my well. My panel is a 10,000kw split phase and I will be turning it off during the winter months when not in use to prevent it from consuming all the power in standby. So can I get away with 10 gauhge or should I spent the extra money for 8?

Best to learn to use a tool:

Voltage Drop Calculator

This free voltage drop calculator estimates the voltage drop of an electrical circuit based on the wire size, distance, and anticipated load current.

www.calculator.net

I filled in some rounded numbers.

For calculation purposes, you use Vmp for voltage and Isc for current. I estimated Vmp at 80% of Voc.

14A @ 178V with 10awg for 300' yields a 4.71% loss. This is above typical NEC spec of 3%, but some applications can tolerate up to 10%. 10awg vs. 8awg is purely your choice based on cost and benefit. Technically, you could use 14awg rated for 15A, but that would be around 12%.

This % loss exactly translates to a % loss of power.

Also worth noting that these numbers will be worst case and assume maximum performance from the array. When current is lower, % losses will be lower.

 

[murmeister]

I recommend 8AWG PV wire - even though you're using conduit. Wiring is something you want to do once and forget about it. There will be many other things that come up with your system and your wiring shouldn't be a concern. Also, your needs may grow over time and pulling new wires through 300 feet of dirty wet conduit is no fun. No matter how well you seal the conduit, plan on moisture and dirt somewhere in that 300 foot run. You might even want to consider running an additional pair of wires for future needs.

 

[nwsolar]

Best to learn to use a tool:

Voltage Drop Calculator

This free voltage drop calculator estimates the voltage drop of an electrical circuit based on the wire size, distance, and anticipated load current.

www.calculator.net

I filled in some rounded numbers.

For calculation purposes, you use Vmp for voltage and Isc for current. I estimated Vmp at 80% of Voc.

14A @ 178V with 10awg for 300' yields a 4.71% loss. This is above typical NEC spec of 3%, but some applications can tolerate up to 10%. 10awg vs. 8awg is purely your choice based on cost and benefit. Technically, you could use 14awg rated for 15A, but that would be around 12%.

This % loss exactly translates to a % loss of power.

Also worth noting that these numbers will be worst case and assume maximum performance from the array. When current is lower, % losses will be lower.

Thank you my friend! You have given me confidence to just buy the 10awg. It in it self is expensive.

 

[nwsolar]

I recommend 8AWG PV wire - even though you're using conduit. Wiring is something you want to do once and forget about it. There will be many other things that come up with your system and your wiring shouldn't be a concern. Also, your needs may grow over time and pulling new wires through 300 feet of dirty wet conduit is no fun. No matter how well you seal the conduit, plan on moisture and dirt somewhere in that 300 foot run. You might even want to consider running an additional pair of wires for future needs.

Adding panels in the future may be a good idea. Thank you!

 

[sunshine_eggo]

Adding panels in the future may be a good idea. Thank you!

8awg isn't much of a future-proofing step, but running a second set of wires is.

 

[nwsolar]

8awg isn't much of a future-proofing step, but running a second set of wires is.

Can I ground the solar panels at the array? Or do I need to ground at the invertor/All in one?

 

[murmeister]

8awg isn't much of a future-proofing step, but running a second set of wires is.

When you're right, you're right. For me, it's future proofing against pulling 8awg later because you wished you'd done that up front. Kind of a "wish I'd known then what I know now" kind of thing. I appreciate your activity in this forum and I enjoy reading your posts.

 

[hwy17]

10.

The best future proofing is the conduit to pull something else when you want it. Live for today and just buy the wire you need for now.

 

[sumpower]

Can I ground the solar panels at the array? Or do I need to ground at the invertor/All in one?

Grounding the panels at the array is optional, as in not neccessary. Pull a ground wire in the conduit with the array conductors and ground at the inverter.

 

[OzSolar]

To meet code when sizing wiring and breakers I use Imp x 1.56. I've read several briefs where that might be changing to 1.25 sometime soon.
1.56 x 13.87 = 21.8 amps = #10 wire to meet code.

To protect equipment I use the record low Voc.

For energy modeling and sizing the wire for voltage drop I use NOCT, Normal Operating Cell Temp. I've never seen a case where it penciled out moneywise out to step up the wire to a larger size that what I did to meet code in the first step. Give or take your array will spend 95% of its life at 10.5 amps or less. Why size the wire for anything other than that after you've met code?

 

[sunshine_eggo]

To meet code when sizing wiring and breakers I use it's Imp x 1.56. I've read several briefs where that might be changing to 1.25 sometime soon.
1.56 x 13.87 = 21.8 amps = #10 wire to meet code.

To protect equipment I use the record low Voc.

For energy modeling and sizing the wire for voltage drop I use NOCT, Normal Operating Cell Temp. I've never seen a case where it penciled out money out to step up the wire to larger size that what I did to meet code. Give or take your array will spend 95% of its life at 10.5 amps or less. Why size the wire for anything other than that after you've met code?

I forgot about that factor, so I dug a little deeper, and I have a different take.

For max current, NEC 2023 690.8(A) indicates 1.25X Isc. Checked 2020 and 2017, and they're the same. So the max current in the PV circuit is 13.97A Isc * 1.25 = 17.46A, so 12awg would work. The fusing is effectively 1.56X Isc, but the wire rating is only 1.25X, but no fusing is required on this single string.

Technically, you could use 14awg rated for 15A, but that would be around 12%.

I was wrong here. 14awg violates the code.

 

[timselectric]

I would also de rate the conductors to 80% for continuous load rating.
Unless you are somewhere that will never see 3 or more hours of production in a day.

 

[sunshine_eggo]

I would also de rate the conductors to 80% for continuous load rating.
Unless you are somewhere that will never see 3 or more hours of production in a day.

In addition to the 1.25X factor?

 

[dopeassjackson]

10.

The best future proofing is the conduit to pull something else when you want it. Live for today and just buy the wire you need for now.

You could say that but I have seen conduit fail making the "future proofing" a total failure.

 

[OzSolar]

The first 1.25 is for irradiance higher than STC due to cloud edge effect and the second 1.25 is for 3 hours or more of continuous current.

 

[hwy17]

You could say that but I have seen conduit fail making the "future proofing" a total failure.

Like one day you go to swap wires and you find the old ones won't pull out because the PVC's crushed in on it? Yeah, it can happen.

 

[hwy17]

The first 1.25 is for irradiance higher than STC due to cloud edge effect and the second 1.25 is for 3 hours or more of continuous current.

What a rabbit hole that is. It seems like the code basis for avoiding the second one with conditional considerations and OCP/termination ratings has existed since the 2017 code it's just poorly understood and rarely leveraged.

 

[timselectric]

In addition to the 1.25X factor?

Yes
They are separate requirements. And both apply to the situation.
But to be honest, I only use the 80% and voltage drop calculations.
But I don't apply the 1.25 to the OCP either. So, I guess that it's a wash.

 

[dopeassjackson]

Like one day you go to swap wires and you find the old ones won't pull out because the PVC's crushed in on it? Yeah, it can happen.

Exactly!