Help With configuring and Wiring

[cbchamberlain]

I need help configuring my solar array and wiring. I am building a home that is entirely off grid and have purchased 32 - 375 watt REC solar panels. I am matching these with two 12k watt Growatt MPPT inverters and a battery bank of 6 - 48 volt LifePo4 batteries that can store 5k watts each. The array is a ground array about 125 feet from the house.

I was going to wire the panels in 4s4p for each inverter, but that would create strings of nearly 40 amps each and I think would require 6awg gauge wire to carry the load. I was trying to purchase combiner boxes, but could not find boxes that had fuses over 30 amps. Do I need to make 8 strings from each inverter and buy two combiner boxes?

 

[chrisski]

In this combiner box, you an install 300 amp breakers:

Product - MidNite Solar

MidNite Solar Product Page.

www.midnitesolar.com

Wire size is totally up to you. I’m not sure about this model, but my combiner from Midnite solar will take up to 2/0 for positive and negative from the combiner,

 

[FilterGuy]

What exact model Growatt are you using? I assume it is the SPF12000T DVM. That unit has a max PV input of 7000W so you should be fine putting 16 panels on each inverter.

The max voltage on the PV input is 250 volts so you should be able to get several panels in series, but tTo figure out the panel configuration, we need to know Voc and Isc of the panels.

​

 

[FilterGuy]

It is probably a good idea to have a separate combiner box for the array of each inverter.

This resource might help you calculate wire and fuse sizing.

Fusing & Wire Sizing guidelines for solar panels

To get the paper, click on the orange'' button at the top of the screen. This resource is intended to help the user understand when they need to add Over Current Protection Devices (OCPDs - Fuses or Breakers) to the solar panel array and what...

diysolarforum.com

 

[cbchamberlain]

What exact model Growatt are you using? I assume it is the SPF12000T DVM. That unit has a max PV input of 7000W so you should be fine putting 16 panels on each inverter.

The max voltage on the PV input is 250 volts so you should be able to get several panels in series, but tTo figure out the panel configuration, we need to know Voc and Isc of the panels.

​

You are correct, I am using the 12000t DVM. The VOC and ISC of the panels is 47.4 and 10.04 respectively. At a s4p4 I would be pulling 189.6 volts and I think I need 6awg wire. Not sure though.

 

[cbchamberlain]

What exact model Growatt are you using? I assume it is the SPF12000T DVM. That unit has a max PV input of 7000W so you should be fine putting 16 panels on each inverter.

The max voltage on the PV input is 250 volts so you should be able to get several panels in series, but tTo figure out the panel configuration, we need to know Voc and Isc of the panels.

​

Thanks for the resources!

 

[FilterGuy]

You are correct, I am using the 12000t DVM. The VOC and ISC of the panels is 47.4 and 10.04 respectively. At a s4p4 I would be pulling 189.6 volts and I think I need 6awg wire. Not sure though.

Yup..... 4S4P is probably the best configuration. Any more in series and you would have no headroom for cold temperature voltage rise.

For proper wiring on the home run from the combiner box back to the charge controller, it will need wire that is sized for 10.04A/panel x 4 panels x 1.55 = 62A.... That is 6AWG of 90deg C rated wire. A big wire! (See the resource I posted for an explanation of the 1.55 safety multiplier.)

The 250Ft round trip to the panels is pretty long so we have to figure out the size for voltage drop. The good news is that for voltage drop you do not need to use the 1.55 safety multiplier on the current. Furthermore, you can use Vmp and Imp which is slightly lower. I don't know Vmp or Imp, but using Voc x 4 and ISC x 4, in an online calculator, it looks like 5AWG (if you can find it) will get the voltage drop down to 3.3%. Try plugging Vmp x 4 and Imp x 4 into the calculator. With a little luck, you might be able to go up to a 6AWG and still be at ~3% voltage drop.

Note: You can safely use up to 8 AWG, but the system efficiency will go down. (8 AWG will give you around 6%-7% drop voltage drop and that directly translates into a 6% - 7% energy loss)

 

[cbchamberlain]

Yup..... 4S4P is probably the best configuration. Any more in series and you would have no headroom for cold temperature voltage rise.

For proper wiring on the home run from the combiner box back to the charge controller, it will need wire that is sized for 10.04A/panel x 4 panels x 1.55 = 62A.... That is 6AWG of 90deg C rated wire. A big wire! (See the resource I posted for an explanation of the 1.55 safety multiplier.)

The 250Ft round trip to the panels is pretty long so we have to figure out the size for voltage drop. The good news is that for voltage drop you do not need to use the 1.55 safety multiplier on the current. Furthermore, you can use Vmp and Imp which is slightly lower. I don't know Vmp or Imp, but using Voc x 4 and ISC x 4, in an online calculator, it looks like 5AWG (if you can find it) will get the voltage drop down to 3.3%. Try plugging Vmp x 4 and Imp x 4 into the calculator. With a little luck, you might be able to go up to a 6AWG and still be at ~3% voltage drop.

Note: You can safely use up to 8 AWG, but the system efficiency will go down. (8 AWG will give you around 6%-7% drop voltage drop and that directly translates into a 6% - 7% energy loss)

Thanks again. Plugged in the Vmp 40.1 and Imp 9.36 and got a drop of 2.3% for 6awg wire. Perfect.