| AWG | Ampacity | Resistance | V-drop | % |
|---|
wire_model.py.
NEC Table 310.16: the most continuous current a gauge carries before its insulation overheats, capped by the terminal rating (60 or 75°C, per NEC 110.14(C)), not the cable’s 90°C.
V = 2 × distance × current × resistance/1000 for single phase. Guidance: 3% on a branch circuit, 5% in total.
Current goes out to the load and back, so resistance acts on both legs: the effective length is twice the one-way distance. Three-phase uses √3 instead: about 13% less drop.
Ambient-temperature and grouping factors multiply, are applied to the cable’s own insulation column, then the result is capped at the terminal rating.
A wire can be perfectly safe from an overheating standpoint and still starve a motor or LED strip of usable voltage over a long run: two separate checks with two separate failure modes. A 12 AWG circuit safely rated for 20 A can show a 3%+ drop at 50 feet: safe and underperforming at once.
NEC’s voltage-drop language is informational in most jurisdictions, but ignoring it routinely causes dim lighting, slow motor starts and nuisance tripping, so it is followed almost universally in practice.
Distance is the biggest lever in the voltage-drop formula: doubling the run doubles the drop for the same wire and load, which is why a detached garage or EV charger circuit often needs one or two gauges larger than ampacity alone suggests.
Aluminium has roughly 61% of copper’s conductivity, so matching copper’s ampacity needs one to two AWG sizes larger; common for heavy feeders and service entrances where weight and cost matter more than compactness.
Published ampacity assumes 30 °C ambient and no more than three current-carrying conductors. Six conductors in a 45 °C roof space leave about 65%: enough to turn a correct 12 AWG answer into an undersized one. Heat is the shared cause: hot surroundings shrink the temperature difference driving heat out, and neighbouring conductors add heat and block the path. The factors multiply, and 0.82 × 0.80 is already a third gone.
The current you may draw is capped by the lowest-rated part of the path, almost always a 60 or 75 °C terminal. But you still derate from the cable’s own 90 °C column, which is what makes the better insulation worth buying: derate for heat and grouping first, then cap at the terminal column. The other way round punishes the cable twice. Heat flows into the lug as well as along the wire, so a 75 °C termination sets the temperature the junction may reach.
A bigger gauge number is a thinner wire: 14 AWG is thinner than 10 AWG, and beyond 1 AWG the numbering flips to 1/0, 2/0, 3/0, 4/0, each step thicker still.
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.
Releases in which this page changed, newest last. Derived from the archived copy of every release, not from notes written afterwards — so it reflects what actually shipped. Site-wide passes are left out; they are in the full changelog.