wiring Size

[mparent]

Am adding four 5o watt solar panels. Run will be about 40 feet to battery. Any suggestions re wire size?

[hellosailor]

forty feet one way = 80' round trip. Look at any online voltage loss calculator, you are dealing with 200 watts, about 14 amps, and you want less than a 3% voltge drop. Since battery charging is a little more demanding about voltage losses, I'd go for a cable size that had 1% or less drop, if you can afford it.

[Paul Elliott]

Of course that 14 Amp number is for a 12V system. If you have a 24V bank, or an MPPT controller that lets you run a higher panel voltage, you may be able to reduce the wire size. Personally, I would look at the voltage-drop-calculated wire size as a minimum. You should consider running heavier wire (without being ludicrous about it) to improve the system efficiency, since every Watt is precious.

And remember that in a multi-panel boat system there are factors other than wire size to consider. Are you going to have these in series, parallel, or series/parallel? In a home rooftop system you usually run full-series (until you hit the maximum Voltage limit), but on a boat the unavoidable shadows can really complicate this decision.

An example: On my boat (12V battery, three parallel -connected 100W panels on top of the dodger, MPPT controller), I could have probably fit higher-power panels by running the long dimensions port/starboard. Unfortunately, my boom (and mast) usually casts a shadow that runs fore and aft, which would have essentially shut down all of the panels. By putting one panel to port, one to starboard, and one in the middle, I usually have one panel with shading but two at full-power. Your situation is probably different, but there are always other factors to consider.

But let's assume that you are running a 12V system, and the wire is carrying 14A. 80 feet of "0" (AWG) wire will give you about 0.0078 Ohms, which gives 0.11V drop (just under 1%). This is big wire, and in spite of my suggestion to go big, if wire size is a problem you might consider allowing more wire loss.