Wire Size- 45- 385W Panels

[tarmac36]

Hello,
I have a mini excavator doing some work currently and while I have it there I am going to dig the trench for conduit for my solar system. My question is what size wire do I need for this? I understand regular electricity, but solar electricity is new to me.

System
45- 385W Heliene Panels, Mounted 15 panels each on a tiltable top of pole mount. The distances to the invertor will be 200ft, 215ft, and 230 ft, I will run the wire in conduit underground. It can get -40 degrees where I'm at. I was initially told to wire 7 and 8 panels for each mount, so 2 wires per mount, 3 mounts, 6 wires total I believe (somehow now 12 wires sticks in my mind). I was told I could use #10 wire. I ran it through a calculator and I'm getting a much different number, but I believe thats probably my inputs. Does any one have any suggestions or guidance?

Thank you

 

[pvgirl]

For most accurate information we are going to need to know the details of how the panels will be wired.

Best guess
2 strings per pole mount, 2 wires per string
1 ground
5 total wires per pole mount to inverter.
This would be 12 wires plus 3 grounds.

Best guest at panel Isc: 10.18A
NEC required wire ampacity: 15.9A

Assuming 3 separate conduit runs, one per pole mount
For less then 3% voltage drop 10 AWG copper required, or 8 awg aluminum.
With 4 current carrying conductors per conduit, 80% conductor derating required (19.9A ampacity required)
Both 10awg copper or 8 awg aluminum acceptable.
Both 10awg copper or 8 awg aluminum acceptable for ground as well.

Assuming 5 8AWG XHHW aluminum conductors per conduit, minimum conduit size is 1" per pole mount.

 

[n2aws]

If you don't already have the arrays setup and/or wired, I'd just focus on getting the conduits buried.

Since you don't currently have a wire plan, or number of conducters, I'd go overkill, since compared to the labor and equipment.. conduit is cheap.

My plan would be to bury 2 2" conduits for the entire run, and then maybe a single 2" conduit between the 3 arrays you have planned.
This way, you are flexible. you can decide later on to do higher voltage, and fewer amps.. (saves cost by using smaller wires) or, you may decide on equipment that forces you to go like 150v max, meaning higher amperage/bigger and more wires.

The 2 2" conduits give a lot of flexibility, and potentially future additions/upgrades.

If you already had the system planned out and were just getting some of the pre-work done because you had the excavator available, I'd probably suggest more specific ideas. But if its more of a nebulous idea, I'd bury the conduits that give you the most flexibility.

 

[tarmac36]

Wow that was fast. Thanks. Here are the specs on the panels.

Electrical Data (STC)Peak Rated Power Pmpp (W) 405 400 395 390Maximum Power Voltage Vmpp (V) 41.00 40.70 40.40 40.11Maximum Power Current Impp (A) 9.88 9.83 9.78 9.72Open Circuit Voltage Voc (V) 49.90 49.58 49.26 48.94Short Circuit Current Isc (A) 10.41 10.35 10.29 10.24Module Efficiency * Eff (%) 20.38 20.13 19.88 19.62Maximum Series Fuse Rating MF (A) 20 20 20 20Power Output Tolerance [- 3/+3 %]STC - Standard Test Conditions: Irradiation 1000 W/m2 - Air mass AM 1.5 - Cell temperature 25 ºC* Calculated using maximum power based on full positive output tolerance [-3/+3]%

Electrical Data (NMOT)Maximum Power Pmpp (W) 300 295 290 285Maximum Power Voltage Vmpp (V) 37.96 37.52 37.08 36.64Maximum Power Current Impp (A) 7.90 7.86 7.82 7.78Open Circuit Voltage Voc (V) 46.50 46.20 45.90 45.60Short Circuit Current Isc (A) 8.38 8.33 8.28 8.24NMOT - Nominal Module Operating Temperature:Irradiance at 800W/m2, Ambient Temperature 20ºC, Wind speed 1m/s

 

[n2aws]

Wow that was fast. Thanks. Here are the specs on the panels.

Electrical Data (STC)Peak Rated Power Pmpp (W) 405 400 395 390Maximum Power Voltage Vmpp (V) 41.00 40.70 40.40 40.11Maximum Power Current Impp (A) 9.88 9.83 9.78 9.72Open Circuit Voltage Voc (V) 49.90 49.58 49.26 48.94Short Circuit Current Isc (A) 10.41 10.35 10.29 10.24Module Efficiency * Eff (%) 20.38 20.13 19.88 19.62Maximum Series Fuse Rating MF (A) 20 20 20 20Power Output Tolerance [- 3/+3 %]STC - Standard Test Conditions: Irradiation 1000 W/m2 - Air mass AM 1.5 - Cell temperature 25 ºC* Calculated using maximum power based on full positive output tolerance [-3/+3]%

Electrical Data (NMOT)Maximum Power Pmpp (W) 300 295 290 285Maximum Power Voltage Vmpp (V) 37.96 37.52 37.08 36.64Maximum Power Current Impp (A) 7.90 7.86 7.82 7.78Open Circuit Voltage Voc (V) 46.50 46.20 45.90 45.60Short Circuit Current Isc (A) 8.38 8.33 8.28 8.24NMOT - Nominal Module Operating Temperature:Irradiance at 800W/m2, Ambient Temperature 20ºC, Wind speed 1m/s

Just a quick observation:

Your title mentioned 385w panels, but your specs don't show a 385w panel.
Additionally, since you don't mention what inverter or SCC it's connecting to (ie, it's maximum voltage). we don't know if we're limited to 150v, 300v, 600v, per string, etc. And one final thing, we'de need to know the lowest temperatures for your area, to see how much of a Voc buffer we need to pad in.

If you have an overall system plan, post the details so we can offer guidance. if not, go with at least 2 conduit runs of a large diameter conduit for now, and call it good.

 

[tarmac36]

I dont have the exact plan. But I was planning on a 15kw Sol Ark Invertor. It wont go up til next year. Just have the mini ex there right now and wanted to get the conduit in. .

 

[n2aws]

I dont have the exact plan. But I was planning on a 15kw Sol Ark Invertor. It wont go up til next year. Just have the mini ex there right now and wanted to get the conduit in. .

Yep, thats ok.


I'd personally stick with my initial suggestion then. bury the extra conduit, go large, give yourself plenty of flexibility for the system design. if you go small now, or don't bury an extra conduit, you may limit your options unintentionally.

 

[tarmac36]

Thank you guys. 2 Sections of 2" sch 40 conduit. Will do. Would you do aluminum or copper when I run the wire next year?

 

[pvgirl]

You will most likely need to use something other the sch40 conduit for sections above ground, where it will be subject to damage and can't be used, PVC sch 80, or RMC are options for those sections. Sch40 can still be used underground. Copper would be preferred, but depending on prices, aluminum could save some money.

 

[tarmac36]

You will most likely need to use something other the sch40 conduit for sections above ground, where it will be subject to damage and can't be used, PVC sch 80, or RMC are options for those sections. Sch40 can still be used underground. Copper would be preferred, but depending on prices, aluminum could save some money.

I think I will do the RMC for above ground. The elk tend to walk through there and I could see the pvc getting nibbled on.

 

[ghostwriter66]

Thank you guys. 2 Sections of 2" sch 40 conduit. Will do. Would you do aluminum or copper when I run the wire next year?

Either is fine BUT do your calculations ... just make sure that you get the right AWG of what you are working with and make sure you calculate your loses ... personally I go with whatever is the cheapest ... BUT again -- better to OVER-gauge then UNDER gage .////

 

[tarmac36]

I guess where I got confused on the calculator is whether to use the STC or NMOT numbers for the amps, and then I was multiplying by either 7 or 8 for the array. Relooking at the sheet the ISC (a) was 10.18 and IMPP (a) was 9.67. System voltage 1500v.

 

[tarmac36]

Another dumb question then. XHHW-2 or Photovoltaic PV Wire 2KV UL 4703 for wire?

 

[Spunky]

Another dumb question then. XHHW-2 or Photovoltaic PV Wire 2KV UL 4703 for wire?

I buried 10ga XHHW-2 in conduit, but used PV wire on the back of the arrays where exposure to the elements could be a factor.
Also about the string of 7&8 on the same array in to your Sol-Ark, I'd check into getting an extra panel designed onto those racks.
16 panels with two strings of eight, you'll like the math a lot better.

 

[n2aws]

I guess where I got confused on the calculator is whether to use the STC or NMOT numbers for the amps, and then I was multiplying by either 7 or 8 for the array. Relooking at the sheet the ISC (a) was 10.18 and IMPP (a) was 9.67. System voltage 1500v.

The system may allow 1500v, but the NEC doesn't. Always keep in mind that there is a huge difference between 'possible' and 'compliance'

 

[tarmac36]

I buried 10ga XHHW-2 in conduit, but used PV wire on the back of the arrays where exposure to the elements could be a factor.
Also about the string of 7&8 on the same array in to your Sol-Ark, I'd check into getting an extra panel designed onto those racks.
16 panels with two strings of eight, you'll like the math a lot better.

Nice. You got MT solar to do a 16 instead of a 15?
What do you mean by like the math better?
Thanks

 

[niktak11]

Nice. You got MT solar to do a 16 instead of a 15?
What do you mean by like the math better?
Thanks

You can't put strings in parallel with different numbers of panels. With 16 panel mounts I'd just run one set of wires for each array (plus a ground).

 

[Spunky]

Nice. You got MT solar to do a 16 instead of a 15?

I ordered the 15 and was going to omit a panel, so I'd have two strings of 7. After I put it together and seen that I could buy two un-cut rail pieces and squeeze another panel in the middle I made the call, mine is backed against some tall trees and hopefully won't see the max winds that the rack was designed for.
8 panels in series even at -40° should still be within the Sol-Ark's specs. But do check that, that's a hard limit that can't be crossed.

 

[Spunky]

Sol-Ark Solar Panel Sizing Tool

 

[tarmac36]

Yeah I think the wind might be a problem with mine. I get some serious wind and cold temps. I even used 8" sch 80 pipe cemented 10 feet in the ground.

Sol-Ark Solar Panel Sizing Tool

This is very helpful. Thank you