Wire Size

xyzHollyxyz

I currently have 1 x 100W 12V flexible solar panel to hopefully charge my little travel trailer Group 31 battery. 
The only thing I need that battery to do is keep the fridge control board running while on propane. 
(and I suppose the CO/propane sensor...)

What gauge wire should be used from the panel to the controller (no more than 30 feet)? 
Controller is PWM. 

Would the wire size change IF I decide to add one additional 100W panel? 

Thanks for your help.

vtmaps

Welcome to the forum,

with 1 panel, 10 gauge copper wire will give you less than 2% voltage drop.   Two panels should have 8 gauge. 

If each of the two panels has its own 10 gauge cable (combine them near the controller) that would also work well.

--vtMaps

xyzHollyxyz

Thanks for the welcome and thanks for the reply!

As a follow-up, until, and if, I get the 2nd panel, can I use 8 gauge on the single panel so I don't have to re-make the extensions? For a rank amateur like me, that's no easy task!!

And a rank amateur question - a less than 2% voltage drop is a good thing, right? Will larger gauge wire (smaller diameter) give me a bigger voltage drop? That means a power loss going into the battery? 

vtmaps

xyzHollyxyz said:

 a less than 2% voltage drop is a good thing, right? Will larger gauge wire (smaller diameter) give me a bigger voltage drop? That means a power loss going into the battery? 

2% voltage drop is usually a cost effective design.   Regarding wire, shorter and thicker is better... but there is a law of diminishing return.  On a larger system, suppose you calculated that you were losing 100 watts in the cable between the panels and the controller, and the cable was 100 ft long.  If that bothered you, you might discover that its cheaper to buy another 100 watts of solar panel than to put in 100 ft of thicker cable.

Now having said all that, I'm having doubts about the numbers I mentioned earlier... I did the voltage drop calculations for an MPPT controller where the transmission voltage is Vmp.  On a PWM controller, I believe the transmission voltage is just a fraction of a volt above Vbatt (the battery voltage).  In that case, your percent voltage drop is a bit higher than I calculated. 

Does it matter?  Usually not... the volts you lose were never going to be harvested anyway... that's the nature of a PWM controller. 

--vtMaps