# Wire sizing

Registered Users Posts: 4

I'm sure this question has been asked and answered many times but I'm still confused.

I have 2 Canadian Solar panels ready to go onto my roof. They are 235 watt with Voc 39.6V, Isc 8.46A. My intention is to hook them up in series to give about 75 volts Voc at 8.46A, wire it to a 40Amp MPPT controller that's about 40 feet distance from the panels and feed the MPPT 12 volt output into 4 x 6 volt FLA batteries arranged in a series/parallel configuration..

My question is, do I calculate the safe amperage capacity of the wire from the solar panels going to the MPPT using the 8.46 amps?

I have used many on line calculators and they often come up with contradictory results, ranging from 12AWG to 02AWG.

Assuming your MPPT solar charge controller can manage the Voc-cold from the solar panels (need >100 Vpanel input rating--details matter here), then two calculations... First based on the voltage drop from the Array to the charge controller. We use 3% to 1% as typical values, but it can be more than 3% (MPPT controllers "don't care" as long as the minimum input voltage for your battery bank is met).

Assuming these are the specs:

Nominal Maximum Power (Pmax):235W Optimum Operating Voltage (Vmp):30.1V Optimum Operating Current (Imp):7.82A Open Circuit Voltage (Voc):37.2V Short Circuit Current (Isc):8.34A

First, the voltage drop:

• 2*Vmp= 2 * 30.1 volts Vmp = 60.2 volts Vmp
• 1x*Imp= 7.82 Amps Imp

Using 60.2 volts and 7.82 amps and 40 feet (one way run) for this calculator. For 3% maximum drop:

https://www.calculator.net/voltage-drop-calculator.html?material=copper&wiresize=8.286&voltage=60.1&phase=dc&noofconductor=1&distance=40&distanceunit=feet&amperes=7.82&x=58&y=27

14 AWG:

Voltage drop: 1.58

Voltage drop percentage: 2.63%

Voltage at the end: 58.52

And for 1% maximum drop:

https://www.calculator.net/voltage-drop-calculator.html?material=copper&wiresize=3.277&voltage=60.1&phase=dc&noofconductor=1&distance=40&distanceunit=feet&amperes=7.82&x=58&y=27

10 AWG:

Voltage drop: 0.62

Voltage drop percentage: 1.04%

Voltage at the end: 59.48

And from the panel specification, your maximum series fuse/breaker would be 15 Amps... So, a minimum of 14 AWG cable would be required (following US NEC code).

For single and 2x arrays, you do not need a series fuse/breaker(s). However, it is nice to use a breaker because you can use it as a switch when debugging/working on your system (turn off array without having to pull a power lead from the controller).

-Bill

Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
• Registered Users Posts: 4

Thank you so much.

Looking at various solar company sites I see many offering 50 foot extension cables with MC4 connectors already attached. So 2 of those in 10 AWG will be ordered.

Again, many thanks

My pleasure.

-Bill

Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
• Registered Users Posts: 3,621 ✭✭✭✭✭

Unless you have a need for mc4 connector on both ends of the extension, it may make more sense to get a 100' extention. The extension is then cut in half (or whatever is needed for each run), leaving a male and a female mc4 connector for the pv end, and a cut end for combiner or controller end.

Getting a single extension may be a bit cheaper, and can work better if the +ive and -ive ends of the array are a bit different wire distance from combiner/controller.

Off-grid.
Main daytime system ~4kw panels into 2xMNClassic150 370ah 48v bank 2xOutback 3548 inverter 120v + 240v autotransformer
Night system ~1kw panels into 1xMNClassic150 700ah 12v bank morningstar 300w inverter

As always, make sure you color wrap (colored electrical tape) your wiring (positive, negative, ground)...

Solar panels, if connected backwards, are simply "diodes", and can conduct large amounts of short circuit current (which will ruin the panel(s) if connected to other panels in parallel and/or connected directly to a battery bank).

-Bill

Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
• Solar Expert Posts: 1,296 ✭✭✭✭
edited March 19 #7

So, the Female MC-4 connector of the extension cable plugs into the Male, positive output of the panels. Some people make the mistake of assuming the male connector is always the positive. Yes as Bill said Get some red electrical tape, if you don't already have some and carefully label the +ive with red tape.

2.1 Kw Suntech 175 mono, Classic 200, Trace SW 4024 ( 15 years old  but brand new out of sealed factory box Jan. 2015), Bogart Tri-metric, 700 ah @24 volt AGM battery bank. Plenty of Baja Sea of Cortez sunshine.