How do I determine what gauge wire I need for a 12 volt system.

nobody928 · December 2013

I have 4 140 watt 12volt panels with. 7.6amps each. I have them wired parralel. I then have about a 20 foot drop from the roof to my charge controller.

What gauge wire should I be using? Am i ok with the standard 10 gauge pv wire?

Cariboocoot · December 2013

Re: How do I determine what gauge wire I need for a 12 volt system.

What you are looking at there is 12 Volts with a maximum current of (4 * 7.6) 30.4 Amps over a distance of 20 feet. So there are two things to consider: being able to handle the current and keeping the Voltage drop to a minimum over the distance.

This is where you use a handy V-drop calculator and wire gauge chart like this one: http://www.powerstream.com/Wire_Size.htm

First thing you notice is that 10 AWG is just about at its limit around 30 Amps (you have to interpret the current ratings somewhat; they are not straightforward). It is typically used for 30 Amp circuits in household wiring, for example.

Second thing is to plug the numbers into the calculator and see what you get. In this case it's a whopping 10% V-drop under maximum load. As such you should be looking at heavier wire for the connected run. Even at 6 AWG it becomes 4%, and usually we try to keep it around 3% or less. But at #6 the wire size is getting expensive and difficult to work with.

This is another advantage of the MPPT type controller: the array can be run at a higher Voltage than the system, thus reducing the loss over what is usually the longest wire run in the system. If you change the array Voltage to 24 the current then goes down to 15 and the V-drop on 10 AWG is now only 2.5%.

So which will cost you more? The very large wire or the MPPT controller? Usually it's the controller.

jcheil · December 2013

Re: How do I determine what gauge wire I need for a 12 volt system.

Cariboocoot wrote: »

So which will cost you more? The very large wire or the MPPT controller? Usually it's the controller.

But as I and we all have learned, solar gets addicting and with the MPPT controller it will be very easy and cheaper in the future to add panels without having to upgrade wiring.

A month ago we added 3 additional panels to my neighbor who went with the FM80 to start and it was literally as easy as adding another breaker in the combiner and connecting the panel wires together. Nothing else to do inside or out.

mahendra · December 2013

Re: How do I determine what gauge wire I need for a 12 volt system.

this is very true very simple and less back and bank breaking with mppt controller. i have 2 .midnite classics and the have saved me a lot of money the i would have to spend on wire.Not only because you can run thinner wires but you also use less wire runs by seriesing panels

Cariboocoot · December 2013

Re: How do I determine what gauge wire I need for a 12 volt system.

It's also more practical if you start out with the plan in mind. If you're trying to work with existing panels the financial issue is different.

The "GT" panels tend to be about half the cost per Watt of "conventional" panels: around $1 per Watt vs. $2. As such if you need >400 Watts you save >$400 buying the GT type, and that is the difference in price between the PWM and the MPPT controller. The larger the system the more practical this becomes (and the greater benefit of the MPPT flexibility in array design).

But when you already have four standard panels the cost of the controller is a flat-out expense. In this case the expansion possibility needs to be considered carefully before deciding what to do.

How do I determine what gauge wire I need for a 12 volt system.

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