Wire size for 12v Solar Panels

[turfy77]

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

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

Vd for 8B&S (200w@4m) is 0.09
Vd for 6mm (200w@4m) is 0.16
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for the 160W panel with 12m cable run I get

Vd for 6B&S (160W@12m) is 0.13
Vd for 8B&S (160w@12m) 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.

[Sunking]

Voltage Drop Formula is:

CM = (22.7 x 1-way wire distance x maximum current) / VD

Where CM = Circular Mills
1-way wire distance is in feet
VD = voltage drop.
22.7 = constant for copper wire at 75 Celsius.

Note this is for Imperial lengths and not metric. The result is Circular Mills which you can work with, but you will have to convert meters to feet.

You combine panels where they are located, then use a single feeder to the controller. Not an individual run for each panel to the controller.

Try this on-line version. You need to do this yourself.

[turfy77]

Thanks for your reply.

I would like to learn the formula manually so I have converted the metric values to american.

You gave me the formula CM = (22.7 x 1-way wire distance x maximum current) / VD

wire distance is 39.27feet
maximum current is 18.8Amps
circular mills has been calculated as:
1mm2 = 1973.5CM
Therefor 13.6mm2 = 26839.6CM

VD= (22.7 x 39.37 x 18.8) / 26839.6 = 0.63V

hmmm very different from what I got before
I calculated it using the [http://www.redarc.com.au/handy-hints/calculator/voltage_drop_calculator/"] website
and I got (0.28 - 0.32)

Whos right?? I'm confused

[Mike90250]

There is a link in my .sig to a spreadsheet with a good solar wire calculator, a little complex, but thorough.

It covers arrays to combiner, combiner to controller, controller to battery

The resistance / power loss calcs seem to start off easy, but they are not simple to do by hand.

[turfy77]

Thanks Mike,

That calculator.zip file with xls spreadsheet was awesome, just what I needed. I get exactly the same results as I did in my original post so at least I was on the right track.

On the http://www.nooutage.com/vdrop.htm webpage one of the inputs there - you have to select between single set of conductors or 2 in parallel / 3 in parallel etc , what this all about, what do i choose here?

When calculating voltage drop of a 12v system, I'm assuming its a % of the 18V in the wires and not necessarily % of 12V, is this correct?

[inetdog]

Thanks Mike,

That calculator.zip file with xls spreadsheet was awesome, just what I needed. I get exactly the same results as I did in my original post so at least I was on the right track.

On the http://www.nooutage.com/vdrop.htm webpage one of the inputs there - you have to select between single set of conductors or 2 in parallel / 3 in parallel etc , what this all about, what do i choose here?

When calculating voltage drop of a 12v system, I'm assuming its a % of the 18V in the wires and not necessarily % of 12V, is this correct?

When using large wires (up in the 2/0 and 4/0 range) it is often easier to get a low resistance connection by using several conductors in parallel instead of a hard to get and hard to work with single conductor of larger size.
The NEC only allows conductors to be run in parallel if the individual conductors are 1/0 or larger. (310.10(H)(1) [2011])

[Sunking]

I am nor clear on your numbers but using 12 meters wire length, 18 volts @ 20 amps will require a #4 AWG copper cable. #6 AWG only gets you out to 9 meters at 3%

[turfy77]

Ive since learned that Ive been using the wrong wire length.
Ive had to double my wire length which includes the cable run to and from the solar panels to the battery, this doubles the voltage drop value.
Things are starting to make sense now.

[jony101]

I use 8 gauge on all my solar wiring that i have on my van. The amperage that my 240 watt panel puts out are not high enough to use anything bigger.

Most of the solar wiring they sale for solar is in the 10 and 12 gauge and these are for runs longer than what you plan on using.

The 200 watt panel on your roof, I would use 8 or 10 gauge. The 160 watt panel I would go with the 10 gauge.

The charge controller is going to get the voltage from the panel and reduce it down anyway. But definitely from the charge controller to the battery use 8 gauge, that way voltage drop wont be too much if any.

Only the expensive namebrand mppt controllers can handle any wiring over 8 gauge.

[SunEagle]

I use 8 gauge on all my solar wiring that i have on my van. The amperage that my 240 watt panel puts out are not high enough to use anything bigger.

Most of the solar wiring they sale for solar is in the 10 and 12 gauge and these are for runs longer than what you plan on using.

The 200 watt panel on your roof, I would use 8 or 10 gauge. The 160 watt panel I would go with the 10 gauge.

The charge controller is going to get the voltage from the panel and reduce it down anyway. But definitely from the charge controller to the battery use 8 gauge, that way voltage drop wont be too much if any.

Only the expensive namebrand mppt controllers can handle any wiring over 8 gauge.

Jony101. The OP will be using both a 200 watt and a 160 watt panel in parallel so his max amp will be higher then what your 240 watt delivers. So his wire size will also need to be larger due to the distance and higher amp load.