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serpa4 asked

Wire gauge for MPPT 150/70? 2x435watt (Voc: 85.6v Isc: 6.43amps each) Marine use on 12 lead acid.

Good evening. I have a 12 volt boat with 6 group 31 AGM batteries (4 house/1 engine/1 thruster-generator).

I purchased 2 used SunPower 435watt panels for use on my small boat. A really good deal on 2 year old panels.

The panels are Voc: 85.6v Isc: 6.43amps each (not sure what they mean, but believe max volts and max amps while short circuited.

I purchased a Victron SmartSolar 150/70 controller. I believe I could have gotten away with the 150/60 per the Victron sizing calculator, but little over kill doesn't hurt. The boat will be in the tropics and likely pretty hot inside and outside the boat.

I will run them in parallel to make them more resistant to shading.

In parallel, I assume I'll have about 15 amps max and 90 volts tops out of the panels.

On the boat roof, I plan to wire the panels + to + and - to - before running the two individual wires into the boat. The 150/70 will be mounted about 30 feet from the panels. I looked on a page that said don't exceed a 2% voltage drop. I need 4 gauge marine wire (tinned for salt water exposure) to do 30 feet. BUT, I'm a car guy....4 gauge is for starting a car! 30 feet of 4 gauge seems ridiculous. Is this correct? Its only 15 amps, not starting a car.

On the battery side, I should size for 70 amps (150/70 charge controller rating) and 10 feet max run to reach the batteries. I should then use the same 4 gauge wire. Even if amps are higher, distance run is lower.

Is my wire sizing correct? Thanks all.


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2 Answers
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Justin Cook answered ·

Hi @serpa4, welcome to the Community!


You'll be relieved to know that, no, your calculations are off.

15A @ 90v @60' round-trip (you said distance of 30', so that's 60' round-trip) = 14ga to stay below 3% voltage drop (2.63%). Personally I'd recommend going to 10 (1.04% vdrop) just for the sake of it, but that's me.


On the output side, figuring 70A @ 12v @ 20' round trip (remember you have to calculate for both sides of a DC circuit) = 4ga minimum for 2.98% vdrop. Granted you won't be running at 12v, you'll be running an average of ~13.5v, so vdrop becomes 2.65%; still kind of high, strongly encourage going to 2ga, but that's ultimately your call.

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serpa4 avatar image serpa4 commented ·

Thanks for the good new, but can you tell me where I went wrong so I can learn. Also, what do you mean by round trip?

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serpa4 avatar image serpa4 commented ·

I found a different DC load/wire/legnth calculator that agrees with everything you said. Thanks for double checking and hence making me check again. I avoided a big mistake. I appreciate it.

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serpa4 answered ·

Thanks for the good new, but can you tell me where I went wrong so I can learn. Also, what do you mean by round trip? Can you explain that? Isn't Dc one way?

Also, you explain the 14gauge. All the charts i've googled, show much lower gauge/thicker.

Thank you again.

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