American Wire Gauge is counterintuitive: a smaller number means a thicker wire. 10 AWG carries far more current than 20 AWG. Thicker wire has lower resistance, so it wastes less energy as heat and drops less voltage over a given run length.
Sizing runs one way — amps in, gauge out. The ampacity lookup runs the other: choose a gauge and see the maximum current it may carry under the conditions you have already set. Both directions use the same NEC Table 310.16 figures and the same 310.15(B)(1) ambient correction, so they cannot disagree about a conductor. The number shown is the derated one, and the panel says whether the table or the ambient temperature is what limits it. Copper and aluminum each use their own NEC Table 310.16 column, so an aluminum run is rated against aluminum figures — roughly 78% of copper for the same size. Two combinations the code does not rate are reported as such rather than as a number: aluminum below 12 AWG (copper is rated from 14 AWG, aluminum only from 12), and aluminum in free air.
Ampacity is a safety limit — the maximum current a wire can carry before its insulation overheats. Voltage drop is a performance limit — how much the wire's own resistance "eats" the voltage over distance, which can starve a device even on a wire that is otherwise safe. A wire can pass one check and fail the other, so this calculator checks both and recommends whichever gauge is larger.
Branch circuits (outlets, lighting): 3% max. Feeder circuits (subpanels): 5% max. Combined branch + feeder: 5% total. Sensitive electronics: 1% or less. These are NEC-recommended targets, not hard limits, but exceeding them risks dimming lights, sluggish motors, or misbehaving electronics.
Aluminum conductors have roughly 61% the conductivity of copper (higher resistance for the same size), so an aluminum run typically needs to be 1-2 gauge sizes larger than copper to carry the same current with the same voltage drop. Aluminum is lighter and cheaper for long, high-current runs, which is why it is common in service entrances and feeders.
Single-phase voltage drop uses a factor of 2 (current flows out and back on two conductors). Three-phase balanced loads use √3 (≈1.732) instead, because the phase currents partially cancel — so a three-phase circuit typically shows a lower voltage drop than a single-phase circuit carrying the same current over the same distance.
One unified screen: Quick calculation based on current, distance, and voltage — no mode to pick first.
NEC Compliance: Expandable section for temperature rating and installation method, checked against standard ampacity tables.
Advanced Environment: Expandable section for phase type and ambient temperature, with full derating and power-loss detail.
Material Options: Compare copper vs aluminum and select installation method (conduit, buried, free air).
Ampacity Lookup: Pick a wire gauge and see the maximum amps it is rated to carry, using the same conductor, insulation rating, installation method and ambient temperature — the reverse of sizing.
Home Electrical Projects: size wiring for new circuits or additions, ensure safe installation that meets code, and prevent voltage drop and equipment damage.
Professional Installation: calculate wire gauge for commercial systems, comply with NEC and local codes, and compare copper vs aluminum conductors.
Industrial & Heavy Duty: design high-current power distribution, account for temperature and installation method, and optimize for cost while ensuring safety.
Troubleshooting & Upgrade: diagnose voltage drop problems, upgrade undersized circuits, and plan capacity for future loads.
Ampacity Check: answer "how many amps can 12 gauge wire handle" for the wire you already have, with the NEC ambient derating applied rather than the raw table figure.
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