Solar Wire & Cable Size Calculator

Find the right wire gauge for solar, battery and inverter DC runs — sized so the voltage drop stays within your target and the voltage actually arrives.

The continuous current the cable carries. Not sure? Use the Watts to Amps Calculator.
Distance from source to load, one way — the calculator doubles it for the return conductor.

A planning estimate using the standard DC voltage-drop formula (K = 12.9 copper / 21.2 aluminum). Real installs vary with temperature, cable type and connections — confirm against your wire's datasheet, and for very short high-current runs check that the cable is rated for the current. Have permanent or high-current wiring reviewed by a qualified electrician.

Wiring up the rest of the system? Find the current with the Watts to Amps Calculator, size the inverter and charge controller, or design the whole thing with the Off-Grid Solar Calculator.

How wire sizing works

A wire that's too thin wastes energy as heat — the voltage drops along the run, so less of it reaches your load. On solar, battery and inverter DC runs this is almost always the deciding factor, so the goal is to choose the smallest gauge that keeps the drop under your target.

1. Voltage drop

As current flows down a wire and back, the wire's resistance burns off some of the voltage. The DC voltage drop is:

Voltage drop = (2 × K × I × L) ÷ circular mils

where K is the resistivity constant (12.9 for copper, 21.2 for aluminum), I is the current in amps, and L is the one-way length in feet. The 2 is the round trip — current goes out on one conductor and back on the other. Express it as a percentage of system voltage, and you size the wire so that stays under your target:

Voltage drop % = voltage drop ÷ system voltage × 100

2. Short, high-current runs

Voltage drop sizes the wire on almost every solar and battery run, because low voltage over distance makes the drop the binding constraint. The one place it doesn't tell the whole story is a very short, very high-current cable — a battery-to-inverter link, say — where the run is too short for drop to matter but the current is large. There, pick the wire by its own current rating: check the gauge stamped on the cable or its datasheet and make sure it comfortably handles your current. This tool sizes by voltage drop, so on those short, heavy runs treat the cable's rated capacity as the deciding number.

Why 12V is so unforgiving

Voltage drop is about the share of voltage lost, so the same loss hurts far more at low voltage:

Low-voltage DC also carries high current for the same power, which increases the drop further. That's why a short, fat cable is normal on 12 V battery systems, and why raising the system voltage to 24 V or 48 V lets you use dramatically smaller wire.

A worked example

A 12 V charge controller sits 20 ft from the battery and carries 30 A. You hoped to use 10 AWG:

So that run wants 2–4 AWG, not 10. Getting this wrong is the difference between a battery that charges and one that never reaches full, with a warm cable to show for it.

Quick reference: copper circular mils

AWGCircular mils
1010,380
816,510
626,240
441,740
266,360
1/0105,600
2/0133,100
4/0211,600

Frequently asked questions

How do I calculate wire size for a solar or battery system?

Pick a gauge so the round-trip voltage drop stays under your target, usually 3%. The DC drop is (2 × K × current × one-way length) ÷ the wire's circular mils, with K = 12.9 for copper and 21.2 for aluminum. Choose the smallest gauge that keeps the drop under target. For very short, high-current runs, also check the cable's own current rating.

Why does voltage drop matter so much on 12V systems?

Voltage drop is the fraction of your system voltage lost as heat in the wire. Losing 0.36 V is only 0.3% at 120 V but a full 3% at 12 V. Low-voltage DC also carries high current, so the same run loses a much bigger share of its voltage — wire sizing is far more critical at 12 V than at 120 V or 240 V.

What voltage drop percentage should I aim for?

Three percent is the common target for DC solar and battery wiring. Use 2% or 1% for critical or high-current runs like battery-to-inverter or panel-to-controller, and you can allow up to 5% on non-critical loads such as lighting. Sizing up one gauge usually costs little and improves efficiency and safety.

Does this calculator size by voltage drop or by current capacity?

It sizes by voltage drop, which is almost always the deciding factor for solar, battery and inverter DC runs — low voltage over distance makes the drop test demand a thicker wire than current capacity alone. The exception is very short, very high-current runs like a battery-to-inverter cable; on those, also confirm the wire you buy is rated for the current using the cable's own datasheet.

Should I use copper or aluminum wire?

Copper is the standard for solar, RV, marine and battery wiring — it conducts better and handles vibration and flexing well, and most battery and inverter cables are fine-strand copper. Aluminum is cheaper and lighter and appears in long utility feeders, but its higher resistance means a larger gauge for the same job. For most off-grid and mobile systems, use copper.

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