What gauge wire does your 12V DC circuit need?

Step 1: Size for voltage drop and ampacity, then take the larger

A DC wire has to pass two tests, and you use whichever needs the bigger gauge. Voltage drop governs long runs at modest current; ampacity (the heat the wire can shed without cooking its insulation) governs short, high-current runs like an inverter feed or a winch. A chart can only show ampacity because it cannot know your run length, and a calculator that checks only voltage drop will happily undersize a short 150-amp run, which is how wires start fires. This tool runs both and tells you which one won.

Step 2: Measure the round trip, not the one-way distance

Current flows out to the device and back through the ground, so the resistance that drops your voltage is the full there-and-back length of wire. The single most common mistake is sizing off the one-way distance, which halves the resistance in the math and leaves you a gauge or two too thin. Enter the one-way run from the battery or busbar to the device and the tool doubles it for you.

Step 3: Hold critical circuits to 3 percent, the rest to 10

Keep voltage drop under 3 percent on circuits where it matters: the solar array and its run to the controller, the controller-to-battery run, a 12V fridge, a water pump, and any inverter feed. Non-critical loads like cabin lights and USB outlets can run to 10 percent. A fridge or pump on a sagging 10 percent circuit underperforms exactly when the battery is already low, so when in doubt design to 3 percent. This tool uses the standard copper constant with margin for warm wire (K = 11.1, a step more conservative than the marine 10.75); a long run sitting in real heat near full load still drops a little more than any formula shows, so leave headroom there.

Step 4: Derate for heat and crowding

Wire carries less current when it is hot or packed in with other wires. Bundle four to six current-carrying conductors in a loom or conduit and each is derated to about 80 percent; seven or more drops to about 70 percent (NEC 310.15). A run through a hot engine bay, a sun-baked attic, or a wall cavity above roughly 50C loses about another 15 percent. For DC, both the positive and the negative count as current-carrying. Pick the routing that matches your worst stretch; the tool applies the derate to the ampacity side.

Step 5: Round up, never below 16 AWG, and use the right wire

Wire comes in steps, so when the math lands between two gauges you take the larger. Heavier wire runs cooler and drops less voltage for a few dollars more; there is no upside to shaving it. Keep a floor of 16 AWG for general RV and van wiring because thinner wire is too fragile for road vibration, and use fine-strand copper, not solid. Buy wire rated to 105C (battery and welding cable, quality marine wire); common house THHN is only 90C and carries less, and AWG-rated wire is not the same as cheaper SAE wire, which has about 10 percent less copper for the same gauge number.

Step 6: Fuse the wire, not the load

A fuse or breaker protects the conductor, so its rating sits at or below the wire's derated ampacity and at or above the current it serves. For a load that runs three hours or more (continuous), NEC sizes the protection and the wire to 125 percent of the load. Solar array-output conductors are a special case: size them to 156 percent of the panels' short-circuit current (NEC 690.8). Small conductors have hard caps regardless of their ampacity: roughly 10 A for 16 AWG, 15 A for 14, 20 A for 12, 30 A for 10 (NEC 240.4(D)). The golden rule: if your load needs more current than the wire can carry, upsize the wire, never the fuse.