Choosing a wire gauge by guesswork either wastes money on copper or leaves your load starved of voltage. Enter the current, one-way distance, system voltage, and the drop you can tolerate, and this calculator walks the gauge table from thinnest to thickest to find the smallest commonly stocked wire that stays within your limit — then shows the drop it gives, the next size up, and the NEC reference ampacity for copper. It covers 4/0 to 30 AWG and IEC sizes up to 300 mm², DC and AC, and runs in your browser.
To check a wire you already have, use the voltage drop calculator. The Ohm's law calculator handles the basic current and power math, and the LED resistor calculator is handy for low-voltage lighting projects.
Frequently asked questions
What size wire do I need for my amps and distance?
It depends on two separate limits: ampacity (how much current the wire can carry without overheating) and voltage drop (how much voltage is lost over the run). For short runs ampacity usually decides; for long runs, and almost always at 12 V or 24 V, voltage drop does. This calculator finds the smallest gauge that keeps voltage drop within your limit and shows the NEC reference ampacity next to it — use whichever answer is larger. For example, 20 A at 120 V over 100 ft one-way needs 8 AWG copper to stay under 3 % at 75 °C, even though 12 AWG is enough for the current.
Does this tell me the code-required wire size?
No — it sizes for voltage drop only. Electrical codes set the minimum conductor for ampacity based on insulation temperature rating, the number of current-carrying conductors bundled together, ambient temperature, terminal ratings, and the overcurrent device. The ampacity figures shown are the NEC Table 310.16 copper 75 °C values for reference, and NEC 240.4(D) further limits 14, 12, and 10 AWG copper to 15, 20, and 30 A breakers. Local codes, other countries, and special occupancies differ; check with a licensed electrician or your inspector.
What voltage drop limit should I use?
Three percent is the common target for a branch circuit, with five percent total from the service to the load — the figures in the NEC informational notes. For 12 V and 24 V DC systems many installers use 3 % for electronics, chargers, and pumps, and accept up to 10 % for non-critical lighting. Tighter limits mean thicker, more expensive wire, so it is worth checking how sensitive your load actually is. The next-size-up card shows how much a one-step upgrade would buy you.
Why is 12 V wiring so much thicker than house wiring?
Voltage drop in volts depends only on the wire and the current, but the percentage is measured against the system voltage. The same 0.6 V loss is 0.5 % of 120 V but 5 % of 12 V. And for the same power, a 12 V load draws ten times the current of a 120 V one, so the drop is ten times larger as well. Together that makes low-voltage runs roughly a hundred times more sensitive, which is why RV, marine, and solar wiring uses such heavy gauges.
Can I use aluminum or metric wire?
Yes. Switch the conductor to aluminum and the calculator uses 61 % IACS resistivity, which needs roughly two AWG sizes larger than copper for the same drop. Switch to mm² to pick from the IEC 60228 metric series, 0.5 to 300 mm² in copper and 10 to 300 mm² in aluminum (smaller aluminum sizes are not standard). Reference ampacity is only shown for copper AWG, since that is the column the NEC table provides here.