12 V wire size and voltage drop calculator

Enter your system voltage, the load, and the one-way cable length. The calculator picks the smallest copper wire that keeps voltage drop within your limit and is rated for the current, and shows the drop for every gauge.

How the calculation works

Current has to flow out to the device and back, so the length that counts is the round trip: twice the distance between the battery and the load. Enter the one-way distance and the calculator doubles it. Resistance values are the standard figures for copper wire at 68 °F (20 °C). Hot engine bays and attics raise resistance, so go one size up for cable in hot spaces.

Two checks have to pass:

• Voltage drop. Too much drop means the device sees a lower voltage than the battery supplies. Motors run hot, inverters shut down early on "low voltage", and chargers charge slowly. A common target is 3% or less. US boat-wiring standards (ABYC) use 3% for critical circuits and allow up to 10% for less sensitive loads such as cabin lighting.
• Current rating (ampacity). A wire carrying more current than it's rated for overheats, whatever its voltage drop. The calculator uses the 75 °C copper column of the US National Electrical Code's Table 310.16 as a conservative check. Your cable's own rating depends on its insulation and how it's installed, so check the manufacturer's data too.

In low-voltage systems, voltage drop usually decides the size on long runs, and current rating decides it on short, high-current runs such as battery-to-inverter cables.

Worked example

A 20 A load on a 12 V system, 15 feet from the battery (30 feet round trip), with a 3% limit. 3% of 12 V is 0.36 V.

All three gauges are rated for 20 A, but only 6 AWG keeps the drop under 3%. Now run the same 240 W load at 24 V instead: the current halves to 10 A, and even 12 AWG drops only 0.48 V (10 × 30 × 1.588 ÷ 1,000), or 2.0% of 24 V. That's why higher-voltage systems get away with thinner cable. Our series vs. parallel guide explains how battery banks reach 24 V or 48 V.

Fusing

A fuse or circuit breaker protects the wire, so it goes as close to the battery as practical. Its rating must be above the normal load current and no higher than the wire's current rating. The NEC also caps the overcurrent device at 15 A for 14 AWG, 20 A for 12 AWG and 30 A for 10 AWG copper in building wiring. The calculator shows this range for the recommended wire. Follow your equipment manufacturer's fuse recommendations where they give one.

Frequently asked questions

What size wire do I need for a 12 V, 30 A circuit?

It depends on the length. At 10 feet one way, 6 AWG keeps the drop under 3% (30 × 20 × 0.3951 ÷ 1,000 ≈ 0.24 V, or 2.0%), while 8 AWG gives about 0.38 V, or 3.1%. Longer runs need thicker wire. Enter your own length above.

Do I measure one way or round trip?

Enter the one-way distance from the battery (or fuse block) to the device. The calculator doubles it for the return path. If the negative goes to a chassis or common ground instead of running back to the battery, the return path is different, so measure that side too.

What size wire for an inverter?

Inverters draw large currents on the DC side. A 1,000 W load on a 12 V inverter draws roughly 1,000 ÷ 12 ≈ 83 A before losses, and more in practice. Use the inverter's continuous rating as the load, keep the battery cables as short as possible, and follow the cable size and fuse in the inverter's manual, which take priority over this calculator.

Can I use this for 120 V household wiring?

The voltage drop math is the same, but household wiring is governed by electrical codes and inspections. Use this calculator for low-voltage DC systems in vans, RVs, boats and off-grid setups, and hire a licensed electrician for building wiring.