Wire sizing and overall system schematic

Skysthelimit

New Member
Joined
Dec 25, 2019
Messages
52
Location
Cedar City UT
Greetings fellow DIY'ers:

My off-grid house is about 75% completed so it's about time for me to do my part, erect the solar array, wire everything up, and run the wires in the conduit to the power/fuse box. The only problem is, I really don't know the right way to do it. I don't know what gauge wire to use, if I need a fuse, or any of that stuff. The components I purchased are 32 370 watt panels, 2 Growatt 5k inverters, an auto transformer, and 6 GYLL 48 vdc 100 ah batteries. Does anyone recommend I add anything else to the build? I'm not sure what gauge wire I'll need to run from the array, which is ~50' from the large cement box I bought and installed in the ground for all the electrical components. From there, the output will go some 64' to the fuse panel in the garage. Are there any known schematics I can refer to? I welcome all constructive criticism and comments. Thanks all.
 

Samsonite801

Photon Sorcerer
Joined
Oct 15, 2020
Messages
1,297
You will want to use a voltage drop calculator to figure out proper gauge for the length / type of wire, and breakers should be sized to protect the wiring. On mine, 10 AWG was fine according to the calculator, but I opted for 8 AWG to go a bit beefier. I will have 3 pairs running in from the solar array (8x panels per run in my case), to the inverters...



I bought Midnight Classic DC combiner box to go outside by the solar array:


Normally, this box supports cutting the main combiner bus bar into 2 sections for 2 runs, I am going to do 3 PV runs, so I am going to do it custom and cut the combiner bus bar into 3 sections (2 breakers per string)... This box also provides external safety shutoff lever, which may be a code requirement in some jurisdictions anyways.
 
Last edited:

chrisski

Solar Boondocker
Joined
Aug 14, 2020
Messages
2,698
I'm a bit out of my league here but I do know that the circuit breakers are not just any, but need to be rated to at least the DC of the array and be temperature compensated for the coldest day of the year. I have midnite solar 150 VDC and 250 VDC for my build. Depending how you wire the panels, you may see more than that.

For the MNPV-6, I have that and I would recommend that as a combiner.
 

Skysthelimit

New Member
Joined
Dec 25, 2019
Messages
52
Location
Cedar City UT
You will want to use a voltage drop calculator to figure out proper gauge for the length / type of wire, and breakers should be sized to protect the wiring. On mine, 10 AWG was fine according to the calculator, but I opted for 8 AWG to go a bit beefier. I will have 3 pairs running in from the solar array (8x panels per run in my case), to the inverters...



I bought Midnight Classic DC combiner box to go outside by the solar array:


Normally, this box supports cutting the main combiner bus bar into 2 sections for 2 runs, I am going to do 3 PV runs, so I am going to do it custom and cut the combiner bus bar into 3 sections (2 breakers per string)... This box also provides external safety shutoff lever, which may be a code requirement in some jurisdictions anyways.
Thanks for your reply. I'll certainly use the voltage drop calculator. That should help.
 

Skysthelimit

New Member
Joined
Dec 25, 2019
Messages
52
Location
Cedar City UT
I'm a bit out of my league here but I do know that the circuit breakers are not just any, but need to be rated to at least the DC of the array and be temperature compensated for the coldest day of the year. I have midnite solar 150 VDC and 250 VDC for my build. Depending how you wire the panels, you may see more than that.

For the MNPV-6, I have that and I would recommend that as a combiner.
Yes, I remember reading that there is a difference between AC and DC breakers. Thanks for reminding me. I've got a lot of research to do and I too feel like I'm out of my league. I know it's not rocket science, but there just seems to be so many opinions on how to build a solar power system that choosing one that meets my needs and covers all the safety bases is hard to settle on. I appreciate your input. Thank you, sir.
 

Samsonite801

Photon Sorcerer
Joined
Oct 15, 2020
Messages
1,297
Yes, I remember reading that there is a difference between AC and DC breakers. Thanks for reminding me. I've got a lot of research to do and I too feel like I'm out of my league. I know it's not rocket science, but there just seems to be so many opinions on how to build a solar power system that choosing one that meets my needs and covers all the safety bases is hard to settle on. I appreciate your input. Thank you, sir.

Below is a link to the type of DC breakers that drop into that Midnight Classic MNPV6 combiner box, you need to figure out the size you need (15a, 20a, 25a, etc), and you'll need 6 of them if you want to fill that box up.

Example:

20a Midnight Classic DC Breaker:

I am going to have 24 solar panels total, so each 4 will be in series on mine to wire to 1 breaker, so with 6 breakers, that is 24 panels, then I am going to trim the connecting combiner bar on the other end of the breakers into 3 sections (2 combined, 2 combined, 2 combined), because I plan to have 3 PV wire runs back to the inverters, where I will use 3 of the 4 PV inputs across my 2x LV6548 inverters, as solar inputs. 4 panels in series (with 2x paralled) gives me the best PV input Voc voltage, and amperage capability without going over max on inverters PV input spec.

Hope this helps...
 

Samsonite801

Photon Sorcerer
Joined
Oct 15, 2020
Messages
1,297
First thing, you should do is lookup the max PV input volts spec on your Growatt... See how many PV inputs it has, figure out if the max spec is per input, or if some of the inputs are sharing one MPPT or multiple controllers.

Get the specs on your panels for Voc and Vmp, so you can figure out how many can be strung together in series without going over max PV input on the inverter (also calculating in for freezing temps add Voc), and how many can be paralleled without wasting max amps input on the inverter.

Then you can start drawing out pictures on paper and figure out how many PV wire home runs you need, and if 6 breakers will be enough for you.
 
Last edited:

Skysthelimit

New Member
Joined
Dec 25, 2019
Messages
52
Location
Cedar City UT
First thing, you should do is lookup the max PV input volts spec on your Growatt... See how many PV inputs it has, figure out if the max spec is per input, or if some of the inputs are sharing one MPPT or multiple controllers.

Get the specs on your panels for Voc and Vmp, so you can figure out how many can be strung together in series without going over max PV input on the inverter (also calculating in for freezing temps add Voc), and how many can be paralleled without wasting max amps input on the inverter.

Then you can start drawing out pictures on paper and figure out how many PV wire home runs you need, and if 6 breakers will be enough for you.
Thanks, all good information. I've got a week or two before my coarse plans are reviewed by the county office so I'll get busy crunching the numbers and figuring this out for my self. The gentlemen I spoke with and purchased the the inverters, panels, and batteries from assured me that the 32 Renasolar 370 watt panels would work fine if I configured them in two strings of 8 per Growatt 5k inverter, but I'll do the math as well to make sure. Another question that came to mind recently is: The 6 48vdc 100ah batteries are still in their original boxes and stored out on my land where the off-grid house is being constructed. At some point, I'll need to charge those packs up and make sure they are all balanced, won't I? I know the inverter has a charger built into it, but since I don't have grid power out there, I would have to build the array and rely on the solar panels/inverter charger to charger and equalize/balance the batteries? Does that sound ok to you, or do I need to unbox all the packs and bring them somewhere where I can connect a 48vdc charger to grid power and charge each pack that way?
 

Samsonite801

Photon Sorcerer
Joined
Oct 15, 2020
Messages
1,297
Are those actually 48v batteries? Or 12v batteries in sets of 4? Are they lithium or lead acid / AGM batteries?

If you really need a 48v charger, I purchased this one for charging my cells in testing my battery banks, before I get my solar array / inverter installed:

If they are lead acid, equalize charging can balance them. Lithium you will have to top balance them in parallel first, usually one might use a power supply to do it (where you can set specific constant voltage), but a charger could work if the voltage and battery type are proper enough...
 
Last edited:
Top