Recommend a copper wire gauge (AWG) based on amperage, one-way run length, and acceptable voltage drop.
Usage Tip
Voltage drop adds up on long runs; upsizing one gauge is cheap insurance against dimming lights and humming motors.
K (copper) = 12.9, voltage drop = drop % × 120 V
Sizing assumes a 120 V copper circuit; also confirm the gauge meets ampacity for the breaker.
What wire gauge do I need?
Two things decide wire size, and the bigger of the two wins. First, ampacity: the wire has to carry the current without overheating, which sets a minimum gauge for a given amp load. Second, voltage drop: on a long run the wire’s own resistance bleeds off voltage, so a longer circuit needs a fatter wire than ampacity alone would suggest. A 20-amp circuit might be fine on 12 AWG for a short run, but stretch it to 80 feet and voltage drop pushes you to 10 AWG. This calculator checks both and recommends the gauge that satisfies whichever is stricter.
Voltage drop explained
Every conductor has resistance, and current through resistance loses voltage as heat. Over a long run that loss dims lights, slows motors and wastes energy. The common target is to keep it under 3 percent for branch circuits. The drop depends on four things: the current, the round-trip length (out to the load and back), the conductor material, and the wire’s cross-section. The formula:
Voltage drop = (2 × K × length × amps) ÷ circular mils
K is the resistivity of the metal — about 12.9 for copper and 21.2 for aluminum — and the 2 accounts for the current traveling out and back. That round trip is why one-way length matters: a 50-foot run is 100 feet of wire carrying the loss.
AWG wire size chart
Common copper branch-circuit sizes (ampacity is limited by code for the smaller gauges):
| AWG | Amps | Metric | Typical use |
|---|---|---|---|
| 14 | 15A | 2.1 mm² | Lighting circuits |
| 12 | 20A | 3.3 mm² | Outlets, kitchen |
| 10 | 30A | 5.3 mm² | Dryer, water heater, AC |
| 8 | 40-50A | 8.4 mm² | Range, sub-panel feed |
| 6 | 55-65A | 13.3 mm² | Range, EV, hot tub |
| 4 | 70-85A | 21.2 mm² | Sub-panel, large loads |
Copper vs aluminum
Copper carries more current per size and is the default for branch circuits. Aluminum is cheaper and lighter, common for service entrances and long feeders, but its higher resistance means you size up — roughly two AWG sizes larger for the same load. Aluminum also needs antioxidant compound and connectors rated for it. For small household circuits, copper is standard; aluminum makes sense mainly on big feeders where the cost saving is real.
Frequently asked questions
What wire gauge for a 20 amp circuit?
12 AWG copper for typical runs. On long runs, voltage drop may push it to 10 AWG.
What size wire for 50 amps?
6 AWG copper at 75 degrees C for most cases, or 4 AWG on long runs or with aluminum.
Is one-way or round-trip length used?
Enter one-way length. The voltage-drop formula doubles it automatically for the out-and-back path.
How much voltage drop is acceptable?
The common guideline is 3 percent for a branch circuit and 5 percent total including the feeder.
Can I use aluminum wire?
Yes for feeders and service, sized up about two gauges and with rated connectors. Copper is preferred for small branch circuits.
Why does my wire need to be bigger on a long run?
Resistance grows with length, so a long circuit drops more voltage. A fatter wire has less resistance and keeps the drop in range.
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Estimates use standard resistivity values and common NEC ampacity figures for planning and education only. Actual requirements depend on insulation type, ambient temperature, conductor bundling, termination ratings, continuous-load and overcurrent rules, and local code amendments. Size overcurrent protection to the conductor and terminations, and have wiring done and inspected by a qualified electrician.