Wire Size Calculator
The smallest wire that carries the current and stays inside the voltage drop limit.
Copper and aluminium at the stated insulation ratings, using NEC 310.16 ampacities and Chapter 9 Table 8 resistances. Local codes, installation method and derating rules differ; this is a sizing aid, not a substitute for them.
How it works
Two independent things size a cable, and the larger of them wins.
Ampacity is a heat question. Current warms the conductor, and the limit is the temperature its insulation can survive — which is why the same copper is rated differently in THHN than in TW. That base rating is then derated twice: once for ambient temperature above 30 °C, and once for the number of current-carrying conductors bundled together, because they cannot shed heat into each other.
Voltage drop is a distance question, and it has nothing to do with safety. It is about whether the far end of the run still works. The 3% branch-circuit and 5% total guidelines in the NEC are recommendations, not requirements, but they are where motors start running hot, LED drivers get unhappy and long runs stop behaving.
Which one governs depends almost entirely on length. On a short run inside a building, ampacity nearly always wins and the answer is the size the breaker implies. Past twenty or thirty metres, voltage drop takes over and grows without limit — a 20 A circuit at 100 m may need three or four sizes more copper than its current alone would suggest, and no amount of arguing changes the physics.
The small-conductor rule catches people out. Even though 14 AWG copper is rated 20 A at 75 °C, the NEC limits it to 15 A of overcurrent protection — and 12 AWG to 20 A, 10 AWG to 30 A — regardless of what the ampacity table says.
Wire size for a specific run
Common questions
Why is voltage drop separate from ampacity?
They answer different questions. Ampacity asks whether the cable gets dangerously hot; voltage drop asks whether enough voltage arrives at the other end. A cable can be entirely safe and still deliver too little voltage to run what is on the end of it.
Aluminium instead of copper?
It has about 61% the conductivity, so you need roughly two sizes up for the same performance — but it is much cheaper and lighter for long feeders, which is why utility service conductors are aluminium. It needs anti-oxidant compound and lugs rated for it.
How does derating work?
Multiply the table ampacity by the ambient correction factor and by the bundling factor. Four to six conductors in a conduit costs you 20%, seven to nine costs 30%. Both apply together, so a hot attic with a full conduit can halve the usable rating.
What if the length is enormous?
Raise the voltage if you can. Drop scales with current, and for a given power, current falls as voltage rises — doubling the voltage quarters the loss. That is the entire argument for high-voltage transmission, and it applies just as well to a long run down a field.