| AWG | Ampacity | Resistance | V-drop | % |
|---|
Wire sizing has to clear two independent hurdles. Ampacity is a safety limit — NEC Table 310.16 sets the maximum continuous current a gauge can carry before its insulation overheats, derated by which temperature rating your actual terminals support (per NEC 110.14(C), most residential equipment is 60°C or 75°C, not the 90°C a THHN cable itself is rated for). Voltage drop is a performance concern, not a safety one — resistance over distance (V = 2 × distance × current × resistance/1000, using the round-trip distance for a single-phase circuit) can starve a device of voltage even on a wire that's perfectly safe current-wise. NEC recommends keeping voltage drop under 3% for a branch circuit, 5% total — this tool checks both and reports whichever hurdle actually determines the minimum gauge.
A 20A, 120V circuit run 50 ft one-way needs at least 12 AWG for ampacity alone — but at that distance, 12 AWG's own resistance (1.98 Ω/1000ft) produces a 3.96V drop over the full 100ft round trip, or 3.3% — already at the recommended ceiling. Voltage drop, not ampacity, is what actually limits this run.
Current has to travel out to the load and back to complete the circuit — resistance acts on both legs of that path, so the effective length for voltage-drop purposes is always twice the physical one-way distance for a standard two-wire circuit.
The cable's insulation rating is only half the circuit — its ampacity is limited by whatever component in the whole path has the lowest rating, and that's almost always the breaker or terminal lug, which is commonly rated 60°C or 75°C even when the wire itself is rated higher.
A three-phase circuit's voltage-drop formula uses √3 instead of 2 in the distance multiplier, since the phase currents don't add directly the way they do in a single-phase circuit — the practical result is roughly 13% less voltage drop for the same wire and load.
Yes — American Wire Gauge runs backwards from what feels intuitive: 14 AWG is thinner than 10 AWG, and beyond 1 AWG the numbering flips again to 1/0, 2/0, 3/0, 4/0 (read "one-aught" etc.), each step getting thicker still.