What size wire do I need for my solar panels?

It depends on the array's current at maximum power, the one-way distance to the controller, and how much voltage drop you are willing to lose. Enter those and this tool sizes it. There is no single answer that fits every array.

What voltage drop is acceptable for solar?

Common practice is around 2% on panel circuits and tighter between battery and inverter — tighter than the 3% branch-circuit convention, because at 12 or 24 volts there is very little headroom. These are conventions, not code requirements.

Why does solar need bigger wire than I expected?

Because the voltage is low. The same watts at 12 V draw twenty times the current they would at 240 V, and voltage drop scales with current.

Should I use the panel's short-circuit current?

Not for voltage drop. Use the current at maximum power (Imp), which is what the array actually delivers in operation. Short-circuit current has its own uses in overcurrent sizing.

Does this check ampacity?

No. This is voltage drop only. The conductor still has to be rated to carry the current, including any derating for temperature and how it is installed.

Solar Wire Size Calculator

Wire size for a solar array, where the usual 3% rule of thumb quietly stops working. Panel current is not the number on the label, and 3% of 12 volts is a third of a volt.

Which circuit are you sizing?

System voltage

Current

Work the current out from my inverter

Inverter continuous watts

Inverter efficiency

Lowest battery voltage

We divide by the LOWEST battery voltage, not the nominal 12 or 24 V. The inverter still has to deliver its full output when the bank has sagged, so that is when the current — and the drop — is worst.

One-way distance

Measure the distance one way only. We double it, because the current goes out and comes back.

Conductor

Voltage drop target

The pre-filled number is a commonly used starting point, not a code requirement. Change it to whatever your design calls for.

Why solar wiring isn't house wiring

1. The voltage is tiny, so the margin is tiny

3% of 240 V is 7.2 V — plenty of room. 3% of 12 V is 0.36 V. There is no headroom at low voltage, which is why solar practice runs tighter than the branch-circuit convention: roughly 2% on panel circuits, tighter still between battery and inverter.

2. Array voltage drop is lost yield

Every volt dropped in the wire is a volt that never reaches the controller. On a battery system it also eats the margin before the low-voltage cutout trips.

Use the right current, not the nameplate

For the panel-to-controller run, the current that matters is the array's Imp at Vmp — the current at the maximum power point — not the short-circuit rating and not a figure worked back from the panel's watt rating at nominal voltage. Nominal understates it.

The calculator derives the current per circuit rather than asking you to work it out, which is where most manual attempts go wrong.

What this page covers, and what it doesn't

This page is about the panel-to-controller run.

For controller-to-battery, size on the controller's maximum output, not its input. For battery-to-inverter, size on maximum continuous DC input at the low-voltage cutout. Nominal voltage understates that current because the inverter still has to deliver its rated watts when the bank has sagged; on a 12 V bank cutting out at 10.5 V, that is around 14% more current before inverter efficiency is counted. For those runs, use the full Solar & Battery Wire Size calculator — same engine, all three circuits.

Neither page covers the AC output side of an inverter. That is ordinary branch-circuit work: Voltage Drop and Ampacity Check.

An honest note on the targets

The drop targets this tool uses are starting points from common solar practice, not code requirements. There is no code rule that says 2%. Your equipment manufacturer may specify something tighter, and their number wins. The tool shows the target it used so you can change it knowingly rather than inherit it silently.

Ampacity is a separate question

This sizes wire for voltage drop. The conductor also has to be rated to carry the current without overheating, which is a different calculation with different tables. Outdoors, in conduit, and in sunlight, derating matters. Check it: Ampacity Check.

If the result is out of range, the tool says so explicitly rather than handing back the largest conductor it happens to know about.

Want to know why you should trust this result? Read how VoltDrop checks its numbers.

Frequently asked questions

What size wire do I need for my solar panels?: It depends on the array's current at maximum power, the one-way distance to the controller, and how much voltage drop you are willing to lose. Enter those and this tool sizes it. There is no single answer that fits every array.
What voltage drop is acceptable for solar?: Common practice is around 2% on panel circuits and tighter between battery and inverter — tighter than the 3% branch-circuit convention, because at 12 or 24 volts there is very little headroom. These are conventions, not code requirements.
Why does solar need bigger wire than I expected?: Because the voltage is low. The same watts at 12 V draw twenty times the current they would at 240 V, and voltage drop scales with current.
Should I use the panel's short-circuit current?: Not for voltage drop. Use the current at maximum power (Imp), which is what the array actually delivers in operation. Short-circuit current has its own uses in overcurrent sizing.
Does this size the wire from my battery to my inverter?: Not on this page. That run is sized differently, on maximum continuous DC input at the low-voltage cutout. Use the full Solar & Battery calculator.
Does this check ampacity?: No. This is voltage drop only. The conductor still has to be rated to carry the current, including any derating for temperature and how it is installed.