Your panels are up on the roof. Or maybe they're still leaning against the garage wall while you sort out the wiring. Either way, you hit the same question every DIY solar build hits sooner or later: what size wire do I need?
It's the least glamorous part of the project, and the one that can quietly cost you the most. Buy wire that's too thin and you lose power as heat, or worse. Buy too thick and you've spent money on copper you didn't need.
The short answer
For most home rooftop systems, 10 AWG wire is the best default. If your panels run in a string with a short-circuit current under about 12 amps, and the cable run back to your inverter is under 80 feet, 10 AWG PV wire or USE-2 does the job. That's the standard installers reach for, and it's the standard for a reason.
But "most" is doing a lot of work there. Long runs, parallel strings and low-voltage battery setups all change the maths, so keep reading before you order anything.
Why thin wire costs you twice
Wire resists current, and that resistance turns some of your electricity into heat, showing up as voltage drop along the cable. Two things make it worse than people expect.
First, heat grows with the square of the current. Push twice the current through the same wire and you get four times the heat, not twice. That's how an undersized cable goes from warm to a fire risk.
Second, the loss is a percentage of your system voltage. A 3% drop on a 10 kW array throws away 300 watts: a whole extra panel's worth of output, gone into warming up a cable. The US National Electrical Code suggests keeping voltage drop under 2% on solar circuits; most designers treat 3% as the hard limit.
The three numbers that decide your wire size
Forget the charts for a minute. Three numbers decide everything.
- Current. Take your string's short-circuit current (Isc, printed on the back of every panel) and multiply by 1.25. That's the code's safety factor, because panels can briefly produce more than their rated current on a bright cold day.
- Distance. Measure the one-way run from the panels to the charge controller or inverter. The actual cable route, not as the crow flies, including the drop down the roof and across the loft.
- Voltage. A 360-volt string and a 12-volt battery bank are different worlds. The same volts lost in the cable are a rounding error at 360 volts and a disaster at 12.
Voltage drop in volts comes from one formula: V = 2 x L x I x R / 1,000, where L is the one-way length in feet, I is the current in amps, and R is the wire's resistance per 1,000 feet. For copper, that's 1.24 ohms for 10 AWG, 0.778 for 8 AWG, 0.491 for 6 AWG, and 0.308 for 4 AWG.
A worked example: the 12-volt trap
Say you've got a 300-watt array charging a 12-volt battery, panels 23 feet from the charge controller. That's 25 amps. Run the numbers on 10 AWG:
2 x 23 x 25 x 1.24 / 1,000 = 1.43 volts lost. On a 12-volt system, that's nearly 12%. Twelve percent of your power, gone.
8 AWG loses 0.89 volts, or 7.5%. Still too much. 6 AWG gets you to 0.56 volts, or 4.7%. You have to go all the way to 4 AWG, 0.35 volts, before you land under the 3% limit at 2.9%.
This is the trap. The wire handles the current fine: 10 AWG is rated for 40 amps, and you're only pulling 25. It still fails on voltage drop, because the voltage is so low. On a 12-volt system, distance is expensive, so keep the run short or go thick.
Same panels, 360 volts
Now take a normal rooftop string: short-circuit current 9.6 amps, run 67 feet to the inverter, string at 360 volts. On 10 AWG, the same formula gives 1.6 volts lost. That's 0.44% of 360 volts. Barely a rounding error.
Ampacity is easy too: 9.6 x 1.25 = 12 amps, against a 40-amp rating for 10 AWG.
Not the wire in your walls
One thing that catches people out: ordinary household cable isn't rated for this. The Romex or THHN in your walls is built for indoor AC mains wiring, while solar DC wiring sits outside in sun and rain, carrying direct current at up to hundreds of volts.
You need one of two types. USE-2 is the old workhorse: sunlight-resistant, moisture-resistant, rated to 90°C, fine for direct burial. PV wire is the newer designation, built for solar under the UL 4703 standard, rated to 90 or 105°C and up to 2,000 volts DC, and more flexible. The short leads already on your panels are PV wire. Don't swap them.
For most home installs, either is fine. PV wire wins on exposed runs and conduit; USE-2 wins on price. What matters is that the cable is actually rated for solar DC, not just thick enough.
Wire size at a glance
| Your setup | Wire to use |
|---|---|
| Rooftop string, under 12 A, run under 80 ft | 10 AWG |
| Same string, run 80–150 ft | 8 AWG |
| Parallel strings over 12 A combined | Size from Isc x 1.25; usually 8 AWG |
| 12 V battery run, 25 A, over 20 ft | 4 AWG |
| Charge controller to 12 V battery, 20 A output | 8 AWG |
| 1,000 W inverter on a 12 V bank (80+ A) | 4 AWG or 2 AWG |
UK readers: the same logic applies in metric. 4 mm² is the usual pick for home strings under 20 metres; longer runs or higher currents call for 6 mm² or 10 mm², under BS 7671 wiring rules.
Mistakes that cost real money
Undersizing a long run is the big one; see the 12-volt example above. Then there's forgetting the 1.25 safety factor and sizing to the panel's rated current instead of its short-circuit current, using AC cable because it was already in the shed, and skipping the fuse. Every solar circuit needs one at the battery end, rated for what the cable can carry, not what the panels produce.
And check the conductor is pure copper. Copper-clad aluminium has higher resistance, so every number in this post stops applying.
Check your own numbers
The maths above takes about a minute once you know your Isc and your run length. If you'd rather skip it, the solar wire size calculator does the same calculation. Put in your current, distance and voltage and it tells you the gauge and the expected drop.
If you're still sizing the system itself, the panel sizing guide works out how many panels your roof needs, and the battery sizing guide covers the storage side, including the thick cables between battery and inverter.
Questions people actually ask
Can I use 12 AWG wire for solar panels?
For one small panel on a short run, yes. A 100-watt panel at 12 volts pulls about 8 amps, and 12 AWG handles that over a few feet. Add a second panel or lengthen the run, though, and the voltage drop bites.
Is 8 AWG better than 10 AWG for solar?
Thicker is safer, but only up to a point. 8 AWG cuts voltage drop by about a third on the same run. If your run is long or your current is high, that's worth paying for. If your run is short and your string is 360 volts, you're buying copper to solve a problem you don't have.
Do I need an electrician to wire solar panels?
For the low-voltage DC side of a small off-grid setup, plenty of people do it themselves with the right cable and fuses. Anything grid-tied, anything feeding your home's consumer unit, and anything you're unsure about belongs to a licensed electrician. This post is general information, not installation advice, and local electrical codes have the final word. When in doubt, get a quote. An hour of an electrician's time costs far less than a roof full of panels wired wrong.
Buy the 10 AWG. Unless your numbers say otherwise, and now you know how to check. The thing to lose sleep over isn't the gauge on a normal rooftop string. It's a long 12-volt run with thin wire, quietly turning your panels' output into heat.