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Old 06-20-2012, 02:29 PM   #1
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Voltage loss in wiring to batteries?

Hello all. I've been lurking for a long time and gleaned lots of valuable information on the forums. Just registered 'cause I figure this is the place to get a knowledgeable answer to my solar system question.

A lot of advice on solar installations says to use as short a wiring run as possible and to connect the controller directly to the battery bank. Makes sense. However, the installation manual for the system I'm planning to use advises running the cable from the roof through the refrigerator vent, then through the furnace compartment to the 12V connection on the converter. Not a word about a direct connection to the batteries.

Connecting to the converter is by far the simplest way to go, and avoids the need to drill a hole in the roof and fish the wiring through the walls. However, my Wildcat is a rear galley design, which puts the converter and the batteries at opposite ends of the trailer -- nearly 30 feet apart.

I figured that was a long way to go, and that I would likely get a significant voltage drop in that long a wire run. But then I got to thinking about it and realized that the batteries are also connected to the converter over the same distance, and the converter seems to have no problem keeping the batteries charged from shore power.

So, what do the solar gurus say? Drill a hole in the roof and use 10 feet of cable, or follow the manual and connect to the converter?

Cheers to all.
Dave on Vancouver Island
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Old 06-20-2012, 07:07 PM   #2
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Dave,
What kind of solar system did you purchase? Don't know if you plan for parallel or series; I hear most solar panels on RVs are parallel to minimize the power loss from shadows. If you do parallel, then you are likely running 14.4 V down to a combiner or controller for your 12 volt system. Assuming your solar cells are less than 500 watts, 30 feet of 10 gage between panels and converter is a minor IR loss. Of course you can run larger wire to further reduce losses.
As to mounting the controller and a disconncect switch near the battery - if you can find a home for these things with the shorter wire and out of the weather, then you should be in good order. Sounds smart considering your configuraiton.

Good luck.
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Old 06-21-2012, 06:25 AM   #3
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if you have a smart phone, get a voltage drop calculator. 30' at 12.5 volts is a long long distance. even with a small current and #10 wire. Do the calculations. your loss with even a small solar system may be higher than desire. no sense spending your money on a solar panel if you're losing that power in the wire.

a common number is less than a 3% loss.

by the way, my system can produce 25 amps and 15 volts at max, 31' run from charge controller to battery.....#4 copper is just at 3% loss.
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Old 06-21-2012, 06:33 AM   #4
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what I'm trying to say is to run the solar where it works best for you. then run larger wire from the converter to the battery. this will help the converter run more effectively and lower the voltage drop to your appliances and lights. you'll notice the lights will dim less when the water pump kicks on or the furnace runs.
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Old 06-25-2012, 02:08 PM   #5
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Thanks to all. I've decided to bite the bullet and drill a hole in the roof.

On a related subject, the controller manual advises checking the voltage drop and rewiring with heavier gauge if needed.

It also suggests that the same result can be achieved by doubling up with lighter wire. That is, you could run two lengths of 10 gauge between the same terminals and get the same approximate resistance one length of 6 gauge.

Anybody tried this?
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Old 06-25-2012, 11:52 PM   #6
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Two #10 wires in parallel have the same resistance of a single #7 wire, more than a #6. If I were going to run parallel wires; I'd look at using two runs of 10/3 Romex and tying all three wires together in each run. Don't forget that you'll need to make sure you have very good connectors at both ends. The triple wire will have a resistance between a #6 and a #5 wire. see:
American wire gauge - Wikipedia, the free encyclopedia
for details.

Phil
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Old 06-26-2012, 08:33 AM   #7
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personally, if I was going to go thru the trouble of running new wire, I'd do it right the first time. Run the correct size wire and rest easy.
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Old 06-26-2012, 08:40 AM   #8
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You can NEVER over wire imo!
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Old 06-26-2012, 09:00 AM   #9
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Ron is correct. "Overwiring" is just peace-of-mind and potential for expansion or improvemment. Choose as SHORT a route as possible. The mfgrs recommend the over the roof, down the vent, through the fridge compartment, etc route because it involves the least amount of drilling and sealing. For me, that would have required over 40' of wire run, or relocating my batteries to an inside (under dinette bench) location - a bad idea!

Check out my detailed install at VERY LONG - My 12V Electrical Modifications and Solar Power System Installation . You will see that I ran my wires into a well sealed connenction box, then down through a closet to achieve the shortest possible run.

The wire from the panels should then run to a solar controller, matched to your panel outputs. There are a good variety of controllers out there, and after some research, I chose the Rogue, which I am VERY happy with! You then run out to the batteries, from there.

My factory converter is wired into the system, almost exactly the same as the way that it came from the factory. I just moved (shortened) the wires by 3' by bringing them inside to my junction box. This made the converter wire-run more effective (3'shorter - was 15% shorter) and it neatened-up my battery box (less cables and connections).

So think it out, just like you are doing. PLAN on going bigger, because once you see how effective it is, you'll want to take full advantage. Plan on a PSW (or similar) inverter to give yourself 120v on-demand..... if you plan on it, you'll wire for it, and you won't have to re-wire later.

You are learning about this exactly as I did... by researching and asking questions! It worked for me!!

Good luck with your project! Looking forward to seeing postings and photos!!!
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