Correct wire gauge for a newbie.

jm0963

First time poster here....  I have scoured the forum and can't seem to find the information i am looking for.  I have recently begun getting acquainted with a simple solar set up and am having trouble discerning what wire gauge to use in my system.  I have (2) 135 watt panels that will run to a MPPT charge control (50 feet away) and that will run to a (4) marine/rv battery bank.  I've seen everywhere from 4 gauge all the way to 10 gauge.....any suggestions?

mike95490

It depends on how much loss you can stand.  Generally, less than 3% in the wires between PV and battery is the goal.
here's 2 links, first is a zipped, protected SpreadSheet with all sorts of user input and it tells you the loss,  (instructions at bottom of sheet) 2nd is some discussion

Calculator spreadsheet
http://www.solar-guppy.com/download/voltage_drop_calculator.zip
more calculator options and discussion
http://www.solarpaneltalk.com/showthread.php?2312-Wiring-interconnect-goodies-amp-volt-drop-loss-calculator

BB.

Here is simple voltage drop calculator:

http://www.calculator.net/voltage-drop-calculator.html

So, plug in:

• 135 Watt panel / 18 volt Vmp = 7.5 amps Imp
• 50 foot (one way run)
• 2x18 volts Vmp = 36 volt array (nominal)

Plug in numbers to array and get 1% and 3% drop settings (play with AWG):

http://www.calculator.net/voltage-drop-calculator.html?material=copper&wiresize=5.211&voltage=36&phase=dc&noofconductor=1&distance=50&distanceunit=feet&amperes=7.5&x=48&y=13

12 AWG
Voltage drop: 1.19
Voltage drop percentage: 3.31%
Voltage at the end: 34.81

8 AWG
Voltage drop: 0.47
Voltage drop percentage: 1.31%
Voltage at the end: 35.53

What if you used something like 14 AWG? (smallest AWG I would suggest for outside wire runs):

14 AWG
Voltage drop: 1.89
Voltage drop percentage: 5.25%
Voltage at the end: 34.11

You will lose about 5% of your systems output in the middle of the day (power=voltage*current, cut voltage by 5%, cuts power by 5%).

So there is your range--Would the system work with 14 AWG--Yes, just fine. But you will be losing around (5% * 270 watts=) 13.5 Watts in the middle of the day (full sun, full rated current).

Could use use heavier than 8 AWG cable--Yes, but you would be spending lots of money for very little return.

If you are burying the cable--Perhaps you would want a heavier cable if you eventually add two more solar panels. Or--Bury a larger conduit so you can pull more/new/heavier cable down the road and not worry about retrenching.

Just to give you an idea, every 3 AWG reduction in wire gauge is 2x more copper (and roughly 2x higher costs for cable). 14 AWG to 8 AWG is 6 change in gauge, or 4x more copper for 8 awg cable and 4x higher costs for cable.

-Bill