Cable Sizing from Ground Mount Array to 18Kpv in the garage

[RibarRanch]

My ground mount array is up and the new house is almost done, so I'll be trenching and running conduit / cable soon between the two. I've been using a couple of iPhone apps from Southernwire and want to do a check with you guys before I purchase the wires. This will be an underground trench from the array (using a Watts247 box for lighting / surge) to a PV Quick Disconnect on the side of the house at a distance not to exceed 200ft. My array has 4 strings of panels that I want to keep discrete into the 18kpv (MPPT #1 Option B - separate strings). Here's what I've calculated:

Cable Size: Assuming each of 4 strings will not exceed 600v / 15a, and my run is 200ft, southward is showing 10AWG with a resulting voltage drop of 1%. They also recommended 14AWG with a 3% drop.

Conduit Size: using PVC Sched 80, Copper THHN 10 STR, and 8 wires (2 ea string), southward shows 3/4" PVC with a 4% jam probability.

The 18kpv manual (section 4.4 PV Connection) states: 10AWG - 6AWG(max) with a 600v min insulation.

My gut says to use 10AWG THHN in 3/4" or 1" conduit.

Seems pretty straight forward. Thoughts?

Dan

 

[The Super Minion]

My advice is do 10 AWG pure copper or 8 AWG THHN. It's always better to oversize your wire by one size.

 

[hwy17]

Panel ISC x 1.56 = wire ampacity required

<15 amp 14 awg
15-20 amp 12 awg
20-30 amp 10 awg

E.g. my panels are 11.13 ISC x 1.56 = 17.36 wire ampacity required so I used 12 awg.

Distance and voltage drop are not wire sizing factors for PV DC. After doing the above calculation to determine your required wire sizing you can still do a voltage drop calculation out of interest to make an economic comparison of energy lost vs. additional wire cost to make a decision whether to upsize beyond that.

 

[hwy17]

1" conduit for sure. 9 x 10 awg is legal in schedule 40 3/4 but not schedule 80.

 

[RibarRanch]

Panel ISC x 1.56 = wire ampacity required

<15 amp 14 awg
15-20 amp 12 awg
20-30 amp 10 awg

E.g. my panels are 11.13 ISC x 1.56 = 17.36 wire ampacity required so I used 12 awg.

Distance and voltage drop are not wire sizing factors for PV DC. After doing the above calculation to determine your required wire sizing you can still do a voltage drop calculation out of interest to make an economic comparison of energy lost vs. additional wire cost to make a decision whether to upsize beyond that.

ISC at my panels is 11.08 x 1.56 = 17.28 wire ampacity required so 12AWG.
BISC at my panels is 12.12 x 1.56 = 18.9 so I think I'll stick with the 10AWG. Also looked and I have 1.5" conduit at both ends so that's where we'll end up: 10AWG in 1.5" conduit.

Thanks!

 

[hwy17]

ISC at my panels is 11.08 x 1.56 = 17.28 wire ampacity required so 12AWG. Thanks.

Yeah my conduit run distance is 260ft and the very rough math is that you'll lose like 80 watts per string on the wire at full output with 12 awg, and lose 40 watts per string on 10 awg. 100/50ish for me.

Basically up to your own disposition if you are inclined to pay a bit more up front to save those watts or not. I'm not, I think they're very low value watts, occurring mostly only during plentiful production. When the solar is scarce in winter, PV current will be lower and the difference becomes minimal, like 10 vs 20 watts.