Help with wire size for 12v solar

turfy77turfy77 Registered Users Posts: 1
To all the 12v Solar experts out there, I need your advice.

I'm in the process of ordering all the stuff I need to set up my solar panels on my caravan.

I've done a heap of research into voltage drops, wire sizes, etc etc. before I go ahead and

order the wire I just want to get advice Im doing the right thing.

* I will be mounting a 200W solar panel on the roof of my van which will have 4m of cable to

the battery.
* I will also be buying a separate 160W folding solar panel which will have a 12m Cable so I

can manually put in the sun in case of shade issues with the other panel etc.
* I have plans to possibly add another 160W folding panel later on.

I calculate I will need a 20amp solar controller for the first 2 panels and 30amps if I use 3

in the future so I have purchased a 30amp solar controller.

I have read that you shouldn't have more then 2% voltage drop in the solar wiring. Therefore

18V * 0.02= 0.36V

Im looking at 6B&s and 8B&s wiring fo the 12m cable.

(AWG6) 6B&S = 13.3mm2
(AWG8 ) 8B&S = 8.3mm2
(AWG11) 6mm = 4.2mm2

The formula Im using for Voltage drop is (from redarc website):

Vd=(length wire * Amps * 0.017) / wire mm2

So for the 200W panel with 4m cable run I calculate a voltage drop as follows:

Vd for 8B&S ([email protected]) is 0.09
Vd for 6mm ([email protected]) is 0.16
for the 160W panel with 12m cable run I get

Vd for 6B&S ([email protected]) is 0.13
Vd for 8B&S ([email protected]) is 0.22

By looking at the voltage drops all of them are under the 0.36V drop so my conclusion is I

could use either wire gauge for both cable runs.
For example the cost of 12m of 8B&S is $70 or for 12m 6B&S is $95. Would I be better of

getting the 6B&S for the little bit extra or is the 0.1V difference between the wires

negligible and should I just save the $25.

If I did get the 6B&s 12m wire if I added another 160w panel later (320w total) I calculate

the voltage drop of both these panels as being:

Vd=(12 * 17.8amps * 0.017) / 13.3 = 0.282v

This is also less than the 2% voltage drop (0.36v) that was recommended so my calculations

would say that 320w solar panel over 12m with 6B&S wire is ok. Is this correct or is that

voltage drop getting too high.

The other option Im looking at is going thicker cable again (3B&S) which starts getting very

expensive or running separate cable for each solar panel (Either 6B&S or 8B&S for each).

Any recommendations on wire size would be greatly appreciated and if any of my calcs are wrong

please let me know.


  • SolarPoweredSolarPowered Solar Expert Posts: 626 ✭✭✭
    Depends on the bussing of the inverter. Also the surge of the inverter and initial demand.

    Most 12v inverters will accept 6awg thwn, if it can accept 4awg use 4.

    Always engineer for DC expansion/coupling.
  • BB.BB. Super Moderators, Administrators Posts: 30,946 admin
    Voltage drop is a killer for low voltage DC systems... By the way, when you say 12 meter of cable, is that "round trip" or "one way" wire run?

    MPPT charge controllers can be really good for long wire runs (array to charge controller). You run 2x panels in series, and you can run much smaller diameter copper wire (1/4 the diameter or less--Note, for every 1/2 reduction of wire cross sectional area, the AWG gets smaller by 3 AWG. So a 1/4 smaller wire that may need a #6 would now be a #12 ).

    Run the solar array at Vmp-array~36 volts and the MPPT controller "efficiently" takes the high voltage/low current from the solar array and "down converts" to low voltage/high current needed to charge the battery bank.

    In your case, you could use standard "12 volt" (Vmp~18 volt) solar panels and PWM controller on the vehicle. And get a second (small) MPPT controller (wire both controllers to the same battery bank) that could take Vmp-array~36 volts (two panels in series) that will be run with a long "extension cord" back to the vehicle (of much smaller AWG).

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • nielniel Solar Expert Posts: 10,300 ✭✭✭✭
    don't forget to take into consideration the losses from the controller to the batteries as well as the losses from the battery to the inverter. the losses from the cc to the battery needs to be in with the losses from the pvs to the cc for the overall losses from pv to battery. it can get complex when using multiple strings. the losses to the inverter are not pv losses, but losses none the less that you need to watch out for. for example if your pv losses are say 2%, but the losses to the inverter are 3.5% then you have a problem as the overall losses from pv to load should not exceed 5%. this does not cover losses after the inverter in that 5%, but many often overlook that aspect too.
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    turfy77 wrote: »

    * I will be mounting a 200W solar panel on the roof of my van which will have 4m of cable to

    the battery.
    * I will also be buying a separate 160W folding solar panel which will have a 12m Cable so I

    First, what are the specs on those two panels?
    A 200 Watt panel is usually a 'GT' style with a Vmp of 30 whereas a 160 Watt panel usually has a Vmp of around 17.5.
    If the two panels aren't Vmp compatible your V drop calculations will be for nought.

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