Newbie Questions Regarding EG4 Solar Panel Wiring

[SPNewbie]

My system will contain the following:

4 × EG4-LifePower4 Lithium Battery | 48V 100AH

2 × EG4 6.5kW Off-Grid Inverters​

4 × EG4 BrightMounts for Solar Panels (currently have 2 and will later expand to 4)​

16 x Bi-facial solar panels left to purchase (likely 450w to 550w each)​

2 x 500 foot spools of 8 AWG PV wire (uncertain whether 7 strand or 19 strand is preferred)

My solar panel mounts will be about 220 feet from the house. My understanding is that I will have to run 2 PV wires to each of the 2 (later 4) solar panel groups, 2 PV wires to each inverter.

Unless someone compelling talks me out of it, I plan to bury the PV wire deep enough (3 feet) to avoid needing conduit. Non-metallic conduit would run close to $1,000, as much or more than the already expensive PV wire.

Any guidance on the following questions would be greatly appreciated.

Is 19 strand PV wire (versus 7 strand) generally preferable for the above system?

Is it recommended that I use combiner boxes?

If so, do I need 1 combiner box for each group of panels for each inverter? (in my case, 2)

Thank you to anyone who knowledgeable and kind enough to provide me with some guidance.

SPNewbie

 

[Madco]

A couple of issues first.

Is the PV wire rated for direct burial? That stuff is expensive and may not be rated for your intended application.

If you were to install conduit, you can use much cheaper wire, I would use XHHW-2.

The size of the conduit will be small if XHHW-2 is used. The actual wire size would depend on viltage drop and derate because of fill.

Would like to have panel specs before going further.

 

[SPNewbie]

A couple of issues first.

Is the PV wire rated for direct burial? That stuff is expensive and may not be rated for your intended application.

If you were to install conduit, you can use much cheaper wire, I would use XHHW-2.

The size of the conduit will be small if XHHW-2 is used. The actual wire size would depend on viltage drop and derate because of fill.

Would like to have panel specs before going further.

Madco,

Thank you greatly for your help and quick response.

Yes, the wire is rated for direct burial and yes, at $410 a spool, it is expensive, for me at least. I'll have to purchase 2 spools.

8 AWG Solar Photovoltaic Wire 2KV UL 4703 (PV Wire), 500ft or 1000ft Spool

Solar Photovoltaic (PV) Wire XLP/USE-2 or RHW-2 or RHH 90°C - 2000 Volt Stranded Building Wire.

www.wireandcableyourway.com

I have read many who express a preference for a higher number of strands, especially if using wiring in tight spaces. I do not anticipate this will be an issue for me, but I have also read that more strands may mean better conductivity. My gut tells me 7 strand wire, but after reading comments my head tells me 19 strand wire.

I watched a video that indicated that with 8 AWG the loss might only be 6%. Whether that turns out to be true in the real world or not, loss with 10 or 12 AWG would be much higher.

It is my understanding that combiner boxes are only used when panels are wired in parallel and that wiring in parallel allows voltage to remain the same but amperage accumulates. I was going to start off with 4 panels per inverter, so did not think that would be an issue though I am not exactly certain of the threshhold even after looking at the specs. Thus, my tentative plan was to run them in series and use a pass through junction box rather than a combiner box. However, I understand that combiner boxes have fuses, which interests me because the aluminum panel mounts will just be installed in the middle of a field. We have had trees struck by lightening before so they concern me a bit.

I have attached the spec sheet for the solar panels I am considering.

Any correction to the above or better direct to consider would be most appreciated.

Again my thanks,
SPNewbie

 

[Madco]

I'll take a look at the panels specs and get back with an "optimal" design-may be able to save you some wire! BTW, I have the same inverters so I'll see about optimizing the panel strings.

On the stranding, it makes very very little difference on the wire's conductivity. A number 8 is a number 8. It does make a significant difference on flexibility though, so higher strand count is a lot easier to work with, but in your case, you don't need the added flexibility if it is going in a trench!

 

[Madco]

BTW, what is the extreme low temperature at your location?

 

[SPNewbie]

BTW, what is the extreme low temperature at your location?

-35 degrees. Alas, we moved here from sunny South Carolina. Thanks again!

 

[Madco]

Wow, big change from S Carolina! I was going to suggest two strings if 8 panels with each string feeding one MPPT input per inverter, but the -35 temp will probably limit you to 7 panels in each string with two being unused. I'll be able to check it later tonight.

 

[Madco]

Was also going to suggest aluminum USE but even it is has shot up in price.

 

[SPNewbie]

Wow, big change from S Carolina! I was going to suggest two strings if 8 panels with each string feeding one MPPT input per inverter, but the -35 temp will probably limit you to 7 panels in each string with two being unused. I'll be able to check it later tonight.

Again, I cannot thank you enough. While I wish running 4 copper lines (2 connecting each set of 8 panels for each inverter for a total of 4) wasn't so costly, we will bite the bullet and do so. But we will have to forgo conduit. In this state, that will fly since I will trench 3 feet down. Unfortunately, the ground is heavy clay. Having an alternative or backup system in place will give us some peace of mind. Thanks again!

 

[Madco]

Finally had time to review the panel specs! You can run two strings of 8 panels into one PV input on each inverter, even given the -35F temperature!

The PV input voltage limit is 500V. The 8 panels in series will exhibit an open circuit voltage of 466.5V at -35F.

That would take you down to a total of 5 wires in the trench (two for each PV string and a ground). I recommend a single point ground with the rod at the inverters with the panel frame ground run to it. There might be some nuances with the grounding depending on whether this is a completely off grid system or one where grid power is provided to the inverters.

I would still like to try to convince you to go with conduit. If you were to go with #8 XHHW-2, your wire cost would drop and the 4 conductors would go in 1" sch 40 pvc for an easy pull. That conduit would be around 1.40/ft, so about $280. With a little more pulling effort, the 4 wires will go in 3/4" for a little more savings! 3/4" would be about 1.08/ft, so $216 for the 200' run.

The ground wire run can just be chucked in the trench, outside the conduit.

The conduit is pretty cheap insurance given the wire cost!

BTW, I didn't run voltage drop calculations on the total 400' run with the #8 wire, but the currents would be 13-14amps if you want to double check that.

Hope this helped & sorry for the delay in getting back to you. Good luck with the project!

 

[SPNewbie]

Finally had time to review the panel specs! You can run two strings of 8 panels into one PV input on each inverter, even given the -35F temperature!

The PV input voltage limit is 500V. The 8 panels in series will exhibit an open circuit voltage of 466.5V at -35F.

That would take you down to a total of 5 wires in the trench (two for each PV string and a ground). I recommend a single point ground with the rod at the inverters with the panel frame ground run to it. There might be some nuances with the grounding depending on whether this is a completely off grid system or one where grid power is provided to the inverters.

I would still like to try to convince you to go with conduit. If you were to go with #8 XHHW-2, your wire cost would drop and the 4 conductors would go in 1" sch 40 pvc for an easy pull. That conduit would be around 1.40/ft, so about $280. With a little more pulling effort, the 4 wires will go in 3/4" for a little more savings! 3/4" would be about 1.08/ft, so $216 for the 200' run.

The ground wire run can just be chucked in the trench, outside the conduit.

The conduit is pretty cheap insurance given the wire cost!

BTW, I didn't run voltage drop calculations on the total 400' run with the #8 wire, but the currents would be 13-14amps if you want to double check that.

Hope this helped & sorry for the delay in getting back to you. Good luck with the project!

 

[zanydroid]

I believe you need 19 strand on #10 PV for final connection to panels and MLPE, since they will likely move during service. Minimum Strand count is definitely called out in NEC. There are some threads here with citations.

Use a voltage drop calculator app like from Southwire, not a video, for the voltage drop. You are running a 500V string, it’s probably not that much. Losses drop very quickly as voltage increases.

Since the best practice is to have a disconnect at the array you can use that to transition to XHHW for going into conduit.

 

[zanydroid]

If you are worried about lightning you should look into SPDs, that would be the cheapest insurance. Fuses are not designed to protect against lightning, and you could have high voltage situations either exceeding fuse voltage rating or below fuse current