# Maximum mc4 cable run

Registered Users Posts: 48✭✭
is it safe to run 4 run of mc4 10 awg the lowest I can find to make a 120 feet run to combiner box?
voltage for two 325 watts panel in series would be 94 volts, please advise

• Solar Expert Posts: 6,006✭✭✭✭✭
edited October 2017 #2
Not sure what "the lowest I can find to make a 120 feet run" means. Is this the shortest run?

The advise would be to combine at the panels and run a single line. I would guess you have 72 cell panels and the 94 volts is a VOC number? if you are calculating voltage drop I would use the VMP numbers.
Home system 4000 watt (Evergreen) array standing, with 2 Midnite Classic Lites, Midnite E-panel, Magnum MS4024, Prosine 1800(now backup) and Exeltech 1100(former backup...lol), 660 ah 24v Forklift battery(now 10 years old). Off grid for 20 years (if I include 8 months on a bicycle).
- Assorted other systems, pieces and to many panels in the closet to not do more projects.
• Registered Users Posts: 48✭✭
Photowhit said:
Not sure what "the lowest I can find to make a 120 feet run" means. Is this the shortest run?

The advise would be to combine at the panels and run a single line. I would guess you have 72 cell panels and the 94 volts is a VOC number? if you are calculating voltage drop I would use the VMP numbers.
Thanks for the comment I was looking for 8 gauge or better for the mc4 cables the panels will be on the roof and I plan to make the cables run to the combiner in the living room about 120 feet away, not safe to place combiner outside for fear of theft.
as far as the voc each panel voc is 47 volts I was going to make 2 string of 2 each 47*2=94 and run the two set of cables to the combiner
• Solar Expert Posts: 6,006✭✭✭✭✭
edited October 2017 #4
Your roof isn't safe from theft?

Actually for just 2 strings you don't need a combiner box, you can just use a couple branch connectors;

Home system 4000 watt (Evergreen) array standing, with 2 Midnite Classic Lites, Midnite E-panel, Magnum MS4024, Prosine 1800(now backup) and Exeltech 1100(former backup...lol), 660 ah 24v Forklift battery(now 10 years old). Off grid for 20 years (if I include 8 months on a bicycle).
- Assorted other systems, pieces and to many panels in the closet to not do more projects.
• Solar Expert Posts: 6,006✭✭✭✭✭
edited October 2017 #5
You would need to search a bit but I believe MC4's are rated for 8 gauge wire, USE2 rather than solar wire if you build your own, USE2 insolation is thinner. The VMP is the number you need to worry about when calculating voltage drop. I'd bet 8 gauge will bring it in under 2%, but don't want to work backwards right now...
Home system 4000 watt (Evergreen) array standing, with 2 Midnite Classic Lites, Midnite E-panel, Magnum MS4024, Prosine 1800(now backup) and Exeltech 1100(former backup...lol), 660 ah 24v Forklift battery(now 10 years old). Off grid for 20 years (if I include 8 months on a bicycle).
- Assorted other systems, pieces and to many panels in the closet to not do more projects.
• Registered Users Posts: 48✭✭
Photowhit said:
You would need to search a bit but I believe MC4's are rated for 8 gauge wire, USE2 rather than solar wire if you build your own, USE2 insolation is thinner. The VMP is the number you need to worry about when calculating voltage drop. I'd bet 8 gauge will bring it in under 2%, but don't want to work backwards right now...
so with the 8 gauge the 120 feet would be ok?
• Registered Users Posts: 474✭✭✭✭
edited October 2017 #7
To calculate if the loss on the wire would be acceptable, you need not just the Vmp (Voc is of no value) of the two panels in series, but what the Imp (current and maximum power). If you can give us the current of one panel (since the panels are in series, the current for the two is the same as for one), it's pretty easy to calculate or look-up the loss in 120ft.
Off-grid cabin: 6 x Canadian Solar CSK-280M PV panels, Schneider XW-MPPT60-150 Charge Controller, Schneider CSW4024 Inverter/Charger, Schneider SCP, 8S (25.6V), 230Ah Eve LiFePO4 battery in a custom insulated and heated case.
• Registered Users Posts: 48✭✭
Horsefly said:
To calculate if the loss on the wire would be acceptable, you need not just the Vmp (Voc is of no value) of the two panels in series, but what the Imp (current and maximum power). If you can give us the current of one panel (since the panels are in series, the current for the two is the same as for one), it's pretty easy to calculate or look-up the loss in 120ft.
The panel data

PERFORMANCE UNDER STANDARD TEST CONDITIONS (STC)* Maximum power Pmax

325 Wp Open circuit voltage Voc 47.0 V Maximum power point voltage Vmpp

37.7 V Short circuit current Isc 9.28 A Maximum power point current

Impp 8.68

• Solar Expert Posts: 6,006✭✭✭✭✭

so with the 8 gauge the 120 feet would be ok?
So working backwards...lol.

With a VOC of about 94 with 2 panels in series, I would assume a VMP of about 74. So the amps of a string of 2 would be wattage 650/74=8.8 amps 2 strings would be 17.6 amps at 74 volts, with a run of 120 feet on 8 gauge wire.

Now I have the needed number to drop into a voltage drop calculator, like this one;
http://www.calculator.net/voltage-drop-calculator.html

Rendered results above that which would be desirable, but not too ugly.

The voltage is high enough that you will still be able to charge a 48 volt battery bank with a PWM charge controller. Assuming the VMP is correct or close.

You can swap out numbers here;
http://www.calculator.net/voltage-drop-calculator.html?material=copper&wiresize=2.061&voltage=74&phase=dc&noofconductor=1&distance=120&distanceunit=feet&amperes=17.6&x=58&y=20

Using the NOCT (Normal Operating Cell Temperature) values for a 325 watt Canadian Solar panel, the voltage drop during real world use would be about 3%;

The voltage drops when panels are hot and normal wattage will be about 75% of the panel's rating;

Home system 4000 watt (Evergreen) array standing, with 2 Midnite Classic Lites, Midnite E-panel, Magnum MS4024, Prosine 1800(now backup) and Exeltech 1100(former backup...lol), 660 ah 24v Forklift battery(now 10 years old). Off grid for 20 years (if I include 8 months on a bicycle).
- Assorted other systems, pieces and to many panels in the closet to not do more projects.
• Registered Users Posts: 48✭✭
Photowhit said:

so with the 8 gauge the 120 feet would be ok?
So working backwards...lol.

With a VOC of about 94 with 2 panels in series, I would assume a VMP of about 74. So the amps of a string of 2 would be wattage 650/74=8.8 amps 2 strings would be 17.6 amps at 74 volts, with a run of 120 feet on 8 gauge wire.

Now I have the needed number to drop into a voltage drop calculator, like this one;
http://www.calculator.net/voltage-drop-calculator.html

Rendered results above that which would be desirable, but not too ugly.

The voltage is high enough that you will still be able to charge a 48 volt battery bank with a PWM charge controller. Assuming the VMP is correct or close.

You can swap out numbers here;
http://www.calculator.net/voltage-drop-calculator.html?material=copper&wiresize=2.061&voltage=74&phase=dc&noofconductor=1&distance=120&distanceunit=feet&amperes=17.6&x=58&y=20

Using the NOCT (Normal Operating Cell Temperature) values for a 325 watt Canadian Solar panel, the voltage drop during real world use would be about 3%;

The voltage drops when panels are hot and normal wattage will be about 75% of the panel's rating;

Photowhit said:

so with the 8 gauge the 120 feet would be ok?
So working backwards...lol.

With a VOC of about 94 with 2 panels in series, I would assume a VMP of about 74. So the amps of a string of 2 would be wattage 650/74=8.8 amps 2 strings would be 17.6 amps at 74 volts, with a run of 120 feet on 8 gauge wire.

Now I have the needed number to drop into a voltage drop calculator, like this one;
http://www.calculator.net/voltage-drop-calculator.html

Rendered results above that which would be desirable, but not too ugly.

The voltage is high enough that you will still be able to charge a 48 volt battery bank with a PWM charge controller. Assuming the VMP is correct or close.

You can swap out numbers here;
http://www.calculator.net/voltage-drop-calculator.html?material=copper&wiresize=2.061&voltage=74&phase=dc&noofconductor=1&distance=120&distanceunit=feet&amperes=17.6&x=58&y=20

Using the NOCT (Normal Operating Cell Temperature) values for a 325 watt Canadian Solar panel, the voltage drop during real world use would be about 3%;

The voltage drops when panels are hot and normal wattage will be about 75% of the panel's rating;

wow great info thank you, I will be using an mppt xantrex charge controller 60-150 i went trough tyour approach an factor in the whole picture 2 string of 2 panel 325 watts for a total of four panel, it seems the drop is faily hight
using the calculator i get

## Result

Voltage drop: 5.28
Voltage drop percentage: 7.88%
Voltage at the end: 61.72

Please note that the result is an estimation based on normal condition. The actual voltage drop can vary depend on the condition of the wire, the conduit being used, the temperature, the connector, the frequency etc. But, in most cases, it will be very close.

i will be charging a 24 volts battery bank would that work?
• Solar Expert Posts: 6,006✭✭✭✭✭
Photowhit said:

Photowhit said:

wow great info thank you, I will be using an mppt xantrex charge controller 60-150 i went trough tyour approach an factor in the whole picture 2 string of 2 panel 325 watts for a total of four panel, it seems the drop is faily hight
using the calculator i get

## Result

Voltage drop: 5.28
Voltage drop percentage: 7.88%
Voltage at the end: 61.72

Please note that the result is an estimation based on normal condition. The actual voltage drop can vary depend on the condition of the wire, the conduit being used, the temperature, the connector, the frequency etc. But, in most cases, it will be very close.

i will be charging a 24 volts battery bank would that work?
It sounds like you did something wrong, If you have a string of solar panels or batteries, the voltage adds, but the amperage remains the same. If you have 2 panels or strings of panels in parallel, the amperage doubles but the voltage remains the same.
I did my calculations working backwards from the 650 watt string of 2 panels;

"With a VOC of about 94 with 2 panels in series, I would assume a VMP of about 74. So the amps of a string of 2 would be wattage 650/74=8.8 amps 2 strings would be 17.6 amps at 74 volts, with a run of 120 feet on 8 gauge wire."

So my number represented all 4 panels.
Home system 4000 watt (Evergreen) array standing, with 2 Midnite Classic Lites, Midnite E-panel, Magnum MS4024, Prosine 1800(now backup) and Exeltech 1100(former backup...lol), 660 ah 24v Forklift battery(now 10 years old). Off grid for 20 years (if I include 8 months on a bicycle).
- Assorted other systems, pieces and to many panels in the closet to not do more projects.