DC Wire Gauge Sizing Guide

American Wire Gauge (AWG) is a counterintuitive numbering system: lower numbers mean thicker wire. A 4 AWG wire is much thicker than a 14 AWG wire and carries far more current. The system tops out at 0 AWG, then continues with 1/0 ("one-aught"), 2/0, 3/0, and 4/0 for the largest battery cables.

For DIY off-grid solar, RV, and 12V projects, you will work mostly between 18 AWG (small lighting circuits) and 4/0 AWG (battery interconnects on a large bank). Anything thinner than 18 AWG is signal wire, not power wire.

If you sized a 30A 120V AC circuit with 10 AWG, you would be fine. Use 10 AWG for a 30A 12V DC circuit at 50 ft and your fridge will brown out before it ever cycles. The reason is voltage drop as a percentage of supply voltage.

Same current. Same length. Same physics of resistance. The difference is that a 6V drop on a 120V circuit is a rounding error, while the same 6V drop on a 12V circuit is half the supply voltage. DC equipment, especially motors, inverters, and refrigerators, will malfunction when input voltage sags below their cutoff (typically 11.0–11.5V on a 12V system).

This chart shows the maximum current each AWG can carry at a 3% voltage drop on a 12V DC system, for one-way wire lengths of 10, 25, 50, and 100 ft. For 24V systems you can roughly double these amperages; for 48V, quadruple them (covered in Section 6).

How to read it: find the row for the gauge you want to use, scan to the column for your one-way distance, and the cell shows the maximum amps. If your load exceeds that number, step up to a thicker (lower-numbered) gauge.

Most off-grid and RV circuits land on a handful of common amperages. Here is a fast reference, with deep-dive pages linked for each.

The industry standard for DC circuits is a maximum 3% voltage drop. On a 12V system, that is 0.36V — the threshold below which most equipment behaves identically to being connected directly to the battery. Above 3%, motors run slower, LEDs dim, and inverters start logging undervoltage events.

Voltage Drop (V) = (2 × L × I × R) / 1000

Where L is one-way length in feet, I is current in amps, and R is the wire's resistance per 1000 ft. The factor of 2 accounts for the round-trip path (positive and negative conductors).

Some applications tolerate higher drop. General lighting can run at 5%; sensitive electronics like Starlink or DC-DC chargers should stay under 2%. When in doubt, design for 3%.

Power equals volts times amps. To deliver 1200 watts, a 12V system pushes 100 amps; a 24V system pushes 50 amps; a 48V system pushes 25 amps. Half the current = a quarter of the resistive losses, and a much smaller wire.

Wire gauge is only one decision. Construction matters too — especially in mobile, marine, and outdoor installs where vibration and corrosion eat ordinary wire alive.

The fuse protects the wire, not the device. Size the fuse to the wire's ampacity, never above. A 10 AWG wire rated for 30A gets a 30A fuse — never a 40A "because the inverter draws a lot." If the inverter draws more than 30A, the wire is too small.

ANL fuses are used at the battery for high-current main runs. MIDI/MEGA fuses live on distribution feeds. ATC/blade fuses go on individual branch circuits. Each has a different physical mounting and interrupt rating — they are not interchangeable.

1. 1. Sizing for ampacity, not voltage drop.

   A 30A wire that drops 8% of system voltage will not catch fire — but the equipment it feeds will fail. Always run both checks; voltage drop wins on long runs.

2. 2. Counting one-way length instead of round-trip.

   A 25 ft "run" is 50 ft of copper resistance. Half the people who oversize a wire are correcting for this; the other half forgot and are running undersized.

3. 3. Reusing AC house wire on DC.

   Romex (NM-B) is solid-core wire designed for stationary 120V/240V circuits. It cracks under vibration and was never tested at low voltages where 1V matters.

4. 4. Crimping with pliers instead of a hex crimper.

   A bad crimp adds resistance equivalent to several feet of undersized wire — exactly where current is highest. Get a proper hydraulic or hex crimper for any lug 4 AWG and larger.

5. 5. Trusting the cheap CCA spool.

   If 100 ft of 4 AWG "copper" cable costs $30, it is copper-clad aluminum. Real 4 AWG marine-grade copper is $4–$8 per foot. Burn-down house fires from CCA in solar installs are well-documented.