PV power wiring question

[Rural]

My husband is installing a solar powered gate opener for us, and we don't know much about PV wiring. Do we actually need low voltage wire specifically between the solar panel and the battery, or can we use Romex that we already have? Edit: specifically outdoor-rated Romex; I know that much.

[BudgetSlasher]

Let me channel my inner lawyer . . . "it depends"

As far as my understanding goes (again channeling a disclaimer from my inner lawyer) wire is wire and an appropriately sized ROMEX cable should safely handle low voltage DC electricity.

Voltage drop will be a bigger concern. I found this online (my inner lawyer is tired, but we all know the dangers of the internet) it is tool that will output the wire size you need based on your inputs http://www.freesunpower.com/wire_calc.php. If the ROMEX in place is as least as good as what is output you should be good to go.

Based on your description I hope you are not planning on connection the solar panel directly to the battery; I would assume that you need wither a charge controller or diode in line to prevent the battery from sending power through the panel when the panel output is low at night.

[Rural]

^ Thanks. I know nothing about DC wiring (and virtually nothing AC), but yes, direct connection does sound like a bad idea - I don't have the vocabulary for all the parts, and he doesn't have an account here to ask questions. :-) Fortunately I'm not in charge of any wiring around here (and left to my own devices I'd hire out anything more complicated than changing out a fixture).


The big concern we had was voltage drop if we use much higher gauge wire, yes. But you're the second opinion that it would be fine, so that may be one part we don't have to buy. We will need conduit for whichever wire, though - don't have any more of that from the house build, and the line needs to be buried and needs to withstand potential driving over it along the edge of the driveway.

[Syonyk]

Wire is wire.  I can't speak to code issues, but as long as the voltage doesn't exceed the insulation value for the wire (with a reasonable safety factor thrown in), it's fine.  There's no need for "low voltage wire" - higher voltage wire will handle low voltage without a problem.

You're not going to be pushing enough power that you have to worry about AC vs DC skin effect issues, so as long as the wire is outdoor rated (or going to be in conduit, which it sounds like you're doing), it's fine.

My whole solar system is wired up with standard outdoor UF wire (underground feeder), because that's what I had laying around.  I really should bury it one of these days...

[Rural]

Thanks. No code here unless we build another house. Even then, the county was very surprised we pulled a permit for a noncommercial project. :-)


Sounds like we wire directly from the panel to the control box for the gate (found instructions online), so I'm assuming there's an integral charge controller in there.


http://www.mightymule.com/wp-content/uploads/FM121-Mighty-Mule-solar-panel.pdf

[Syonyk]

Sounds like there's a charge controller and battery in your gate kit, so you should be fine!

12 gauge or whatever will be overkill, but it's fine if you have it.

[CmFtns]

knowing nothing about wiring the solar panel I find myself wondering... What's the worst case? Is there even anything there to burn? if it's just a metal/brick/stone gate at the end of a driveway then what would happen even if it did overheat/melt/burn the wire?

[Rural]

knowing nothing about wiring the solar panel I find myself wondering... What's the worst case? Is there even anything there to burn? if it's just a metal/brick/stone gate at the end of a driveway then what would happen even if it did overheat/melt/burn the wire?

Forest fire, potentially thousands of acres. We'll be doing this right. :-)

[Syonyk]

knowing nothing about wiring the solar panel I find myself wondering... What's the worst case? Is there even anything there to burn? if it's just a metal/brick/stone gate at the end of a driveway then what would happen even if it did overheat/melt/burn the wire?

A small panel like their system needs doesn't have enough energy to do much bad.  It's not going to be able to heat up a dead short enough to cause problems.

Higher voltage/higher power systems, though, are capable of driving one hell of an arc fault.

[Rural]

knowing nothing about wiring the solar panel I find myself wondering... What's the worst case? Is there even anything there to burn? if it's just a metal/brick/stone gate at the end of a driveway then what would happen even if it did overheat/melt/burn the wire?

A small panel like their system needs doesn't have enough energy to do much bad.  It's not going to be able to heat up a dead short enough to cause problems.

Higher voltage/higher power systems, though, are capable of driving one hell of an arc fault.

I thought we were too low power for actual fire, but it's good you think so , too, because our property adjoins some significant acreage. :-)


Not that I have any wish to short out our $350 gate opener, either! Hence the asking and researching prior to doing. We're looking at a 10 or 15 watt panel/s.


We're not going to use the overpriced Mighty Mule solar kit though (the kit includes a compass for those too dumb to know which way is south on their own property, I mean really!), so making sure we set up right with our Harbor Freight panels.

[Syonyk]

The battery is more likely to cause a problem than the panel wiring.  If you're using house wiring, you're so far into overkill that it's not going to be an issue.