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Old 05-23-2018, 03:05 AM   #1
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Solar wire size issues.

I have contacted FR to see what size wire I have installed for the solar ready box on my Vibes roof. FR says it's 12 gauge wire from the roof to the controller and the same from controller to battery.

I have two 100 watt panels that I would like to use. However, after reading some of the posts on here, I'm not so sure I should. I called Renogy and they said it would be fine. Also, almost every chart I have found, including some on this forum, says I should be fine.

I don't want to worry about fire/excessive heat, but I would like to use both panels. So, I'm a bit confused about what I should do.

Am I over-thinking this? Should I go by what Renogy says?

Thanks for any input.

Eric
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Old 05-23-2018, 03:42 AM   #2
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Before the copper fanatics get here...there is no danger of fire whatsoever. 12 gauge is good for 30 amps and 2 100 watt panels will put out maybe 12 to 13 in parallel and 6 to 7 in series, both well within the capability of 12 Ga.

If the controller is PWM you will need to wire them in parallel and the losses will have very little bearing on what you get from your panels.

Go get some sun!
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Old 05-23-2018, 03:47 AM   #3
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Originally Posted by ScottBrownstein View Post
Before the copper fanatics get here...there is no danger of fire whatsoever. 12 gauge is good for 30 amps and 2 100 watt panels will put out maybe 12 to 13 in parallel and 6 to 7 in series, both well within the capability of 12 Ga.

If the controller is PWM you will need to wire them in parallel and the losses will have very little bearing on what you get from your panels.

Go get some sun!
Thank you for your input, I really appreciate it!
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Old 05-23-2018, 06:30 AM   #4
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Hi,

While I agree about fire risk potential, I don't think it falls into the fanatic category to suggest you research voltage drop through wire that size over the distance involved.

In a helpful spirit...

Rich Phillips
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Old 05-23-2018, 12:59 PM   #5
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I just purchased Zamps 200 Watt solar suitcase and they use 12 gauge stranded wire. You s/b fine.
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Old 05-23-2018, 01:09 PM   #6
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ScottBrownstein, I have to gently disagree with what may be a typo. 12 gauge wire is good for 20 amps, not 30 amps. It takes 10 gauge wire to go to 30 amps.
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Old 05-23-2018, 02:20 PM   #7
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You can trust Renogy. I have used their products and tech service for many years now. They know what they are talking about. #12 should do you just fine but I would go to #10 in case you want to add another panel or so later.
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Old 05-23-2018, 03:05 PM   #8
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Hi

I won't press my views too hard, but notwithstanding what was said on the phone, did the Renogy cable calculator https://www.renogy.com/calculators/ yield 12 gauge wire as your solution?

There must be very short wire runs involved. Or, I am missing something else pretty important -- which also could be the case.

FWIW.

Rich Phillips
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Old 05-23-2018, 03:24 PM   #9
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richp is right! I went out and checked and I ran #8 in my entire run for (2) 100w panels hooked to a Renogy 40w MPPT controller. I stand corrected and thank you sir.
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Old 05-24-2018, 01:11 AM   #10
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Quote:
Originally Posted by DaTraveler View Post
I have contacted FR to see what size wire I have installed for the solar ready box on my Vibes roof. FR says it's 12 gauge wire from the roof to the controller and the same from controller to battery.

I have two 100 watt panels that I would like to use. However, after reading some of the posts on here, I'm not so sure I should. I called Renogy and they said it would be fine. Also, almost every chart I have found, including some on this forum, says I should be fine.

I don't want to worry about fire/excessive heat, but I would like to use both panels. So, I'm a bit confused about what I should do.

Am I over-thinking this? Should I go by what Renogy says?

Thanks for any input.

Eric
If you are putting 2 -100 Watt panels, whether in series or parallel they will be fine on 12 ga.

They will usually be parallel if you are using PWM controller and that will be fine or if you decide to double up to 400 watts, you can go series if you are running a MPPT controller, and still be fine over the same wire
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Old 05-24-2018, 01:21 AM   #11
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Hi

I won't press my views too hard, but notwithstanding what was said on the phone, did the Renogy cable calculator https://www.renogy.com/calculators/ yield 12 gauge wire as your solution?

There must be very short wire runs involved. Or, I am missing something else pretty important -- which also could be the case.

FWIW.

Rich Phillips
From controller to batteries is more important because loss there can impact battery charge.

I usually run 10 ga from panels to controller but the small difference in loss 10 vs 12 with just 200 watts running from panels to controller will be negligible. 12 ga will be adequate and safe. Nonetheless for any wire size be sure to put fuses on the positive line. Renogy had nice ones that were INLINE MC4 plug ins.
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Old 05-24-2018, 03:12 AM   #12
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Thank you all for the advice and comments. I will be installing both panels this weekend. I feel much better about installing two panels now.
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Old 05-24-2018, 05:16 AM   #13
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ScottBrownstein, I have to gently disagree with what may be a typo. 12 gauge wire is good for 20 amps, not 30 amps. It takes 10 gauge wire to go to 30 amps.
You are correct, 20 amps is the NEC ampacity of 12 ga. He is way below that, however. Thanks for reading closely.
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Old 05-24-2018, 09:25 AM   #14
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30' of 12 gauge wire, carrying 15A at 12V (174W) will have 10.7W of power lost in the wire. This would allow the controller to be 15' (wire length) from the panel since you need two wires connecting the controler to the panels. This is over 6% of the power generated by the panel. At 7A, the power loss is less than 2.5W. Power losses are proportional to current squared, which means that, for multiple panels, either wire them in series and use an MPPT controller or use a larger guage wire between the panels and the controller. Of course, if you don't care about power losses between the panel(s) and the controller, then just use the smaller wire.

Don't forget that the same rule applies between the controller and the battery. This becomes very important when using an MPPT controler where you can have less than 15A from the panels providing 50A to the batteries.

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Old 05-24-2018, 10:07 AM   #15
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Good to start with Ohm's law and data. P = VI ... I = P/V. As power goes down, current goes down. As voltage increases, current goes down. Max current flow will occur when power is maximized and potential is minimized.

200 W. I'm not sure which voltage to use, the panel's operating voltage of 17.x VDC or the battery charging circuit that probably is in the 13-14 VDC range.

Let's take a worst case scenario and say that it's 100% of the 200W capacity and we're operating at the 12.000 VDC system. We know that supplied power will be something lower than 100% and the circuit potential will be something higher than 12. So, this is worst case.

That's a load of 16.7 Amps. As the actual power from the panel goes down and the voltage goes up, this number will get dynamically lower.

For example, assume that we get 80% efficiency from the panels, so that it's really supplying 160W of power and the line voltage is 13.5 VDC. We're down to 11.9 A on the line.

Whenever possible, trust data over random anonymous people on a forum!
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Old 05-28-2018, 08:50 AM   #16
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.......

200 W. I'm not sure which voltage to use, the panel's operating voltage of 17.x VDC or the battery charging circuit that probably is in the 13-14 VDC range.
.........
Panel output voltage will depend on the controller you're using. MPPT controllers will draw power from the panel at the panel's maximum output power. PWM controllers draw power from the panels at the current battery voltage.

If you check the manufacturer's specification sheets for any solar panel, you'll see that the output power from the panel, under the same incident light conditions, varies as the output voltage changes. There's a peak, usually around 17+V for 12V panels where the panel generates its maximum power.

MPPT contrrollers measure the power pulled from the panel and adjust the current draw to maximize power. The controller then converts this power to the voltage needed to charge the battery, As an example, a 200W panel might generate 11.5A at 17.4V. If the battery is being charged in the bulk charge phase, this will provide just over 14A at 14.1V or, if the charger is charging at a lower voltage, ie 13.3V, 15A.

When charging a battery, the current fed into it determines how fast the battery charges. I've seen some newer MPPT controllers that do not use fixed votlages during the various parts of the charge cycle but apply the same power calculations used on the input side of the controller to the output side. Instead of charging the battery at 14.1V, these controllers will lower the voltage to increase the current, charging the battery faster and making better use of the power coming from the solar panels.

Don't forget that batteries are not charged at a fixed current rate. If your battery is fully charged, the solar controller could be feeding it only 1A of current. This wold be less than 15W (at 13.6V) and a 200W 12V panel feeding an MPPT controller would show less than 1A coming from the panels.

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Old 09-28-2018, 02:57 PM   #17
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so is there a difference between regular 10 gauge and the solar 10 gauge wire?


is it just a ploy to buy more expensive wire?


thanks
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Old 09-28-2018, 03:11 PM   #18
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I don't think there's a difference... I could be wrong and I have no doubt those with more knowledge will chime in. I just used the 10 gauge from Home Depot. The stuff off the bulk rack.
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Old 09-28-2018, 03:12 PM   #19
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so is there a difference between regular 10 gauge and the solar 10 gauge wire?


is it just a ploy to buy more expensive wire?


thanks
None. One might be more flexible if it has finer strands but that won't make a difference in current carrying capacity.
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Old 09-28-2018, 04:36 PM   #20
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so is there a difference between regular 10 gauge and the solar 10 gauge wire?


is it just a ploy to buy more expensive wire?


thanks
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I used: BougeRV 30 Feet 10AWG Solar Extension Cable MC4 from Amazon.
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