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Old 28-07-2010, 10:51   #1
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Recommended Wire Size for Solar

It's a 12 volt system, 420 watts total. (2) 85 watt panels, and (2) 125 watt panels. Since the panels are in different locations on the boat, I wired the 85s together as one pair, and the 125s together as another, then where the cable enters the boat I spliced both pairs together.

I used 10 gauge for everything, because based on someone's recommendation, that should be sufficient. Now I'm hearing that due to the size of the array, I should have gone heavier.

If so what size, and do I need to increase the size on everything?

Or since 12 volt panels really output (in think 19ish volts?) sorry I don't remember the exact figure, do I just need to increase the size of the cable from the controller to the batteries?

If I do need bigger cable from the panels too, would 10 gauge from each pair be sufficient, and then go heavier after where I splice the 2 pairs of panels together?

Lastly, I have a ton of 12 gauge wire, could I double that up with the 10 gauge to get a heavier wire size, instead of ripping all the 10 gauge out and replacing everything with a single heavier gauge wire?
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Old 28-07-2010, 12:00   #2
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Ten guage will be safe, but it isn't the most efficient. Your 420 watts of panels will put out about 25 amps of DC at about 17 volts (the MPP) and that is safe for ten guage. But depending on the total length of the curcuit you could be wasting some of your watts on voltage drop. You will lose more than 10% of your watts due to voltage drop if your total to/from length is about 40'.

So it is a question of cost versus extra watts (and amps). Post your total to/from length and I can tell you how much you will loose for various wire guages.

David
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Old 28-07-2010, 19:37   #3
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If you want to use one of the MPPT controllers, you can increase the solar amp-hours by using heavier wire to the panels, because the MPPT controller can take advantage of the higher voltage. With a regular controller, you can size the wiring using the 10% loss tables in the WMP catalog.
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Old 29-07-2010, 04:18   #4
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From the point where you've joined the two arrays you're carrying quite a bit of current. You should calculate the maximum amperage going through this wire and the length of the run (ie if it's 6 meters to the solar controller the run is 12 meters).

You'll then need to refer to a wire sizing chart (Nigel Calder has one in his book) to determine the appropriate wire size. In your case, you're probably carrying about 25 amps which means that AWG 10 wire is good up to 4 meters. AWG 8 up to 6 meters, etc. If the wire is too small you'll lose a lot of power!!!

What I did was bring two arrays to the vicinity of the solar controller before combining them. This allowed me to use AWG10 for the small array and AWG8 for the big array. Maybe you can simply disconnect the junction between the two arrays and drag one more #10 through and join them close to the solar controller. It isn't ideal but it'll be much better than what you've got.
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Old 29-07-2010, 04:45   #5
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However, if using a MPPT, you could wire the panels in series (as long as the MPPT controller can handle the higher voltage). Put the MPPT close to the batteries (which you should anyway..most have a thermistor option or built tin to adjust charge currents)..this would decrease the amperage from the panels (due to the increase in voltage) and will dramatically decrease your loss.
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Old 29-07-2010, 04:46   #6
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6 would be safest / best, but also thickest and most expensive. My $0.02.
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Old 29-07-2010, 07:19   #7
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Originally Posted by djmarchand View Post
So it is a question of cost versus extra watts (and amps). Post your total to/from length and I can tell you how much you will loose for various wire guages.

David
I'm not 100% sure, but I'd say total length is about 40', but it's a 2 battery controller, so total length to each battery is probably closer to maybe 35'.
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Old 29-07-2010, 08:03   #8
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With a 40' round trip length here is the voltage drop (and power loss) for various gauge wires at 25 amps:

10 gauge 8.6%
8 5.4
6 3.4
4 2.1

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Old 02-08-2010, 20:41   #9
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With a 40' round trip length here is the voltage drop (and power loss) for various gauge wires at 25 amps:
Round trip? As in counting the length of the + and the - ? As in my 35'ish foot run would really be considered 70'?

If so, I'm guessing that would mean close to a 16% loss with 10 gauge?

Maybe I do need to go bigger...at least for part of the run anyway.
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Old 02-08-2010, 21:05   #10
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Regarding the round trip: Most of the tables I've referred to ask you to calculate the combined run of + and -, so a 35' run to the batteries is actually 70'.

That's not to say all the wire sizing tables do the calculation on round trip - just make sure you read the table carefully to make sure you're not undersizing!!

The tables often give wire sizes for 3% and 10% drops - the lower being the ideal because you'll get a further drop over time.
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