12V & 24V DC Wire Size Calculator

Voltage drop is what you can afford to lose, and it is measured as a percentage of the source voltage. A 3% drop on 120 V is 3.6 V, but on 12 V it is only 0.36 V — ten times less headroom. Because the wire carries the same current either way, a 12 V circuit needs roughly the same absolute voltage drop spread over far thicker copper. That is why a 20 A run that would be 12 AWG on 120 V household wiring jumps to 8 AWG at 12 V over the same distance.

In low-voltage DC systems it is almost always voltage drop that drives the gauge, not the conductor ampacity. A wire might legally carry 30 A without overheating, yet still drop far more than 3% of 12 V over a long run, leaving your lights dim and your pump weak. This calculator sizes purely on the voltage-drop target. You should still confirm the chosen gauge can carry the current and is protected by a correctly sized fuse or breaker close to the battery.

A 3% target is the common rule for circuits where performance matters — lights, pumps, inverters, fridges — and matches the ABYC guidance for critical marine circuits. A 10% target is sometimes used for non-critical loads where a little loss is harmless. Tighter targets give thicker, more expensive wire but a stiffer system; this tool defaults to 3% and lets you change it. Remember the length is one-way: the formula already counts the return conductor with its factor of 2.

Enter the one-way distance from the battery (or bus bar) to the load. The factor of 2 in the formula accounts for the current flowing out on the positive conductor and back on the negative, so the round trip is already included. A common mistake is to double the length yourself, which oversizes the wire by one or two gauges. If positive and negative take different paths, use the longer of the two one-way runs.

Copper is the standard for RV, van and marine DC wiring: it has lower resistance (K = 12.9 versus 21.2 for aluminum), so it needs a smaller cross-section, and it tolerates vibration and terminations better. Aluminum saves weight and cost on very large feeders but needs roughly 1.6 times the circular mils for the same drop and demands aluminum-rated lugs and anti-oxidant. For most DC runs in a vehicle, choose copper; the metal selector lets you compare both.

It sizes any DC run on voltage drop, including the heavy cable from the battery to an inverter or to a distribution bus. For an inverter, first convert its continuous wattage to amps at the battery voltage (watts / volts), then enter that current with the run length. Inverter feeds are short but carry very high current, so they almost always land on large gauges like 4 AWG, 2 AWG or 1/0. Always pair the result with a correctly rated fuse mounted close to the battery.

Two problems. First, excessive voltage drop starves the load — lights dim, pumps run weak, and sensitive electronics may misbehave or shut down on low voltage. Second, if the wire is also below the current rating, it heats up; in the worst case the insulation degrades and it becomes a fire risk. The fuse protects against the overcurrent case, but only correct sizing solves the voltage-drop case, which is why low-voltage DC runs are sized on drop first and ampacity second.