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Old 28-03-2017, 05:40   #1
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Solar wire sizing - advice please!

Just a quick question to see if I understand how to size the wire from my panel to controller. The setup is one 30v 265w panel and one 25a MPPT controller. The wire run from the panel to controller is 2 meters. The battery bank is 12v (210ah). I'm using this wire calculator program:

wire-sizing-calculator

In the data fields I entered 30v for the "solar system working voltage" which I understood as the panel voltage, and 265w as the "expected working peak power" and a run of 2 meteres. I also accepted a 2% drop in voltage (is that too much?). The result is a wire of roughly AWG 18 (or just a bit more than 1mm). This seems quite thin to me...

If I enter 12v (not 30v) for the "solar system working voltage" the wire changes to AWG 10 (or about 2.5mm). This seems to make more sense.
My question is: do I calculate the wire size from the panel to the controller based on the 30v panel voltage or should I use the 12v battery voltage? As I understand it, the panel sends 30v to the MPPT controller and the controller breaks this down to the charging voltage of the battery (13.2 to 14.8).

Any suggestions?
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Old 28-03-2017, 06:00   #2
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Re: Solar wire sizing - advice please!

What the wire will be carrying.

The panel range, 30V is a max output but in practice will be lower, I'd use 24. Also account for extra length for actual routing path

No harm in going thicker to reduce power loss, in practice buy wire in rolls and aux termination hardware in just a few sizes and round up.
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Old 28-03-2017, 06:05   #3
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Re: Solar wire sizing - advice please!

Thanks John. With those new numbers I get a wire suggestion of AWG 16 (or 1.3mm). I don't know why, but it seems awfully thin to me... If that's correct though, I'll use 2mm wire for a bit of overkill.
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Old 28-03-2017, 06:09   #4
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Re: Solar wire sizing - advice please!

I would suggest you check out the wiring and connectors ( MC4 ) designed for solar applications. Your voltage will be determined by the configuration and type of panels. I would suggest a little more reading here, there are many threads on solar wiring.

On a simpler note, you will probably do very well with 10 awg solar wire, in all but the most extreme installations. And don't forget to fuse the line to the batteries. Windynation.com is a good source.
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Old 28-03-2017, 06:18   #5
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Re: Solar wire sizing - advice please!

Quote:
Originally Posted by pesarsten View Post
I would suggest you check out the wiring and connectors ( MC4 ) designed for solar applications. Your voltage will be determined by the configuration and type of panels.
The connectors are P4. I'm buying wire with UV protection for solar panel installations. I have the configuration of the panel (specs listed in the first post). I'll keep reading, but I thought with the information provided it would be a simple calculation.
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Old 28-03-2017, 07:52   #6
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Re: Solar wire sizing - advice please!

I am not familiar with a P4 connector, but MC4 is the standard for solar and the connectors are quite good at sealing to prevent corrosion. Here is a link to the info on the cable. I have no affiliation with windynation , I have purchased from them and been very satisfied.

Solar Cable & Wire

Good luck with your project.
Paul
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Old 28-03-2017, 08:25   #7
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Re: Solar wire sizing - advice please!

The thicker the gauge of wire the lower the voltage drop. Also most Solar connectors (MC4) are sized for 10-12awg wire so its easier to use that gauge. If you're really only going 6ft then that much 10AWG isn't going to be expensive and it will be stronger than the 16awg and you'll have almost zero voltage drop which can't do anything but help your charging!
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Old 28-03-2017, 08:26   #8
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Re: Solar wire sizing - advice please!

Sorry Paul, I meant MC4!...
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Old 28-03-2017, 08:32   #9
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Re: Solar wire sizing - advice please!

Quote:
Originally Posted by pcmm View Post
The thicker the gauge of wire the lower the voltage drop. Also most Solar connectors (MC4) are sized for 10-12awg wire so its easier to use that gauge. If you're really only going 6ft then that much 10AWG isn't going to be expensive and it will be stronger than the 16awg and you'll have almost zero voltage drop which can't do anything but help your charging!
Thanks. I didn't know the MC4 connector was made for that AWG. It looks like that's what I'll use. I just hope it fits through all the runs in the boat!
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Old 28-03-2017, 08:32   #10
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Re: Solar wire sizing - advice please!

Your initial calculations of wire size does seem correct (using the panel's optimal voltage and current), but I would still go with the largest your connectors / controller will allow. You're talking only about a 2m run, so the price difference is immaterial. I think my MC4 connectors allowed 10 awg, so that's what I used.
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Old 28-03-2017, 08:49   #11
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Re: Solar wire sizing - advice please!

Quote:
Originally Posted by fallingeggs View Post
Your initial calculations of wire size does seem correct (using the panel's optimal voltage and current), but I would still go with the largest your connectors / controller will allow. You're talking only about a 2m run, so the price difference is immaterial. I think my MC4 connectors allowed 10 awg, so that's what I used.
Thanks! That's exactly what I'll do.
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Old 28-03-2017, 09:22   #12
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Re: Solar wire sizing - advice please!

Hi,
I have a 265w panel as well, the max VOC output is 38v my run to the controller is 3 metres or about 10 ft. I used #10 which is about 1 size over spec.
Works fine.
Good luck!
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Old 28-03-2017, 09:37   #13
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Re: Solar wire sizing - advice please!

Your 265w panel at 30v is ~9amps.

16awg will carry 9amps, but voltage drop will be an issue.

I would use minimum 12awg and go for 10awg if you can make it fit.
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Old 28-03-2017, 10:40   #14
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Re: Solar wire sizing - advice please!

Hi,
Also consider where you place the MPPT controller. The panels provide a 30V output (max) but the MPPT controller will convert this to 12V (nominal) for your batteries. Place the controller as close as practical to the batteries to reduce losses due to voltage drop because the current will increase. You need to make sure the wiring can accept this higher current. Ideally place the controller within about 3 feet of the batteries. This way you reduce the losses in your wiring (as said make this thicker than the minimum required) and get the best possible charging. We have a Moody 36 fitted with a 12v system and a 360AH domestic battery bank. Our two 130AH 24V panels are wired in series and provide a max 48V OP (we often see 50A plus here in the Med peak in summer). The run of AWG 10 solar wire from panels to MPPT controller is about 9m (28'). The controller is less than 1m from the battery bank and uses the same cable. We spend a of of time at anchor and we recharge our batteries (from 75%) by 1100h on most days even in cloudy conditions. Only on two occasions last year did we need to run the engine and those were on really stormy days. (we are a low electrical usage boat but still charge a computer daily as I am a writer). By using 24V panels and converting at the latest possible moment to 12V we have reduced the current flowing through the cables which are overrated. This gives us the maximum possible efficiency. This scheme was advise by the technical department of Midsummer Energy who supplied the system. Also use the MP4 connectors. They have now been out in all weathers for 2-years without any problem.

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Old 28-03-2017, 11:10   #15
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Re: Solar wire sizing - advice please!

Just placed the order for the panel, connectors and wires. I went with 6mm wire as it's the thickest the connector will take. Thanks for all the advice.
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