Wire Size Calculator

Wire Gauge Chart by Amps

Wire is sized by its ampacity — the maximum current a conductor can carry continuously without overheating. The American Wire Gauge (AWG) system runs backwards: smaller numbers mean thicker wire. A 14 AWG wire is thin; a 4/0 ("four-aught") conductor is nearly half an inch of solid metal. The table below shows typical ampacities from NEC Table 310.16 at the 75°C column, which applies to common THWN/THHN conductors with standard breaker and lug terminations.

One important wrinkle: the NEC's small-conductor rule (240.4(D)) caps 14, 12, and 10 AWG copper at 15, 20, and 30 amp breakers respectively, regardless of what the ampacity tables might otherwise allow. That is why the familiar pairings — 14 AWG on a 15A breaker, 12 AWG on a 20A, 10 AWG on a 30A — are hard rules for branch circuits, not just conventions.

How Distance Changes Wire Size

Ampacity is only half the story. Every foot of wire has resistance, and resistance eats voltage. On a long run, a wire that is perfectly safe by ampacity can deliver noticeably less voltage at the far end — enough to dim lights, overheat motors, and slow EV charging. The NEC recommends keeping voltage drop under 3% on branch circuits and under 5% total from service to load.

Here is a worked example this calculator handles automatically. Say you're feeding a 50-amp, 240V sub panel in a detached garage 100 feet from the house. By ampacity alone, 8 AWG copper (50A rating) is the minimum. But check the voltage drop: VD = (2 × 12.9 × 50 × 100) ÷ 16,510 circular mils = 7.8 volts, which is 3.3% of 240V — over the 3% recommendation. Step up one size to 6 AWG (26,240 circular mils) and the drop falls to 4.9 volts, or 2.0% — comfortably within limits. So the right answer for that garage is 6 AWG copper, even though the ampacity chart says 8 AWG would carry the load.

Keep going farther and the sizes keep climbing: past roughly 145 feet, that same 50-amp circuit needs 4 AWG copper to stay under 3%. Distance is the reason two identical circuits in the same house can legitimately need different wire sizes, and it's the most common thing DIYers miss when they size wire straight off an ampacity chart.

Copper vs Aluminum Wire

Copper is the default for branch circuits: it conducts better per unit of cross-section, terminates more forgivingly, and every receptacle and switch is rated for it. Aluminum conducts about 61% as well as copper, so aluminum conductors must be roughly two gauge sizes larger to carry the same current — but aluminum is far cheaper and lighter, which is why large feeders and service entrance cables are very commonly aluminum even in new construction.

Modern AA-8000 series aluminum alloy conductors are safe and code-legal for feeders when installed correctly: use lugs and breakers rated AL/CU, apply antioxidant compound at terminations, and torque connections to spec. The aluminum wiring problems of the 1960s-70s involved old alloys on 15 and 20 amp branch circuits with devices not rated for aluminum — a different situation from a modern 4/0 aluminum SER feeder landing on rated lugs. This calculator applies aluminum's lower ampacities and its higher K factor (21.2 vs 12.9 for copper) in the voltage drop check automatically.

Common Circuits and Typical Wire Sizes

Two notes on this table. EV chargers are continuous loads, so a 48-amp charger must be wired at 125% (60 amps), which is why 6 AWG copper is the standard even though "48 amps" sounds like 8 AWG territory. And AC condensers are sized from the nameplate — the minimum circuit ampacity (MCA) and maximum overcurrent protection (MOCP) printed on the unit override generic rules.

What Happens With Undersized Wire

Undersized wire fails in two ways, one fast and one slow. The fast failure is overheating: a conductor forced to carry more current than its ampacity heats up, the insulation softens and degrades, and eventually it can arc or ignite surrounding material — often inside a wall where you won't see it until it's a fire. This is exactly what breakers exist to prevent, which is why the breaker must never be larger than the wire's ampacity allows.

The slow failure is voltage drop. The wire may never get dangerously hot, but the equipment at the end of the run suffers: motors draw more current to compensate and run hot, compressors struggle to start, LED lights flicker, and heating elements underperform. Chronic low voltage shortens the life of expensive equipment like well pumps and AC compressors. If a circuit is protected by the right breaker but the lights still dim when a load kicks on, voltage drop from a long or undersized run is the usual culprit — the fix is thicker wire, not a bigger breaker.