Wire Size Calculator
The smallest conductor that meets your voltage-drop target and carries the load.
Calculator
How this works
This calculator solves the voltage-drop formula backwards. Instead of asking "how much does this wire drop?", it asks "how big a wire keeps the drop under my target?" — then walks the standard size ladder from 14 AWG up to 1000 kcmil and stops at the first conductor that passes:
K is 12.9 for copper or 21.2 for aluminum (ohm·circular-mil per foot, common 75°C design values), I is amps, L is one-way feet, and VDmax is your allowed drop in volts — 3% of system voltage by default.
Then comes the sanity check that generic calculators skip: the selected wire must also carry the current. The tool compares your load against commonly published 75°C ampacity values — with the widely applied small-conductor limits (14 AWG capped at 15 A, 12 at 20 A, 10 at 30 A of overcurrent protection) — and if the drop-based size can't handle the amps, it bumps the recommendation up and tells you ampacity made the call. Short run, big load → ampacity governs. Long run → voltage drop governs. You always see which constraint decided, plus the next size up as the conservative choice.
Copper wire size by load and distance (240 V, single-phase, 3% drop)
| Load | 25 ft | 50 ft | 100 ft | 150 ft | 200 ft |
|---|---|---|---|---|---|
| 20 A | 12 AWG | 12 AWG | 10 AWG | 8 AWG | 8 AWG |
| 30 A | 10 AWG | 10 AWG | 8 AWG | 8 AWG | 6 AWG |
| 40 A | 8 AWG | 8 AWG | 8 AWG | 6 AWG | 4 AWG |
| 50 A | 8 AWG | 8 AWG | 6 AWG | 4 AWG | 4 AWG |
| 60 A | 6 AWG | 6 AWG | 6 AWG | 4 AWG | 3 AWG |
| 100 A | 3 AWG | 3 AWG | 3 AWG | 2 AWG | 1 AWG |
Sized by the same engine as the calculator: voltage drop plus usable 75°C ampacity with small-conductor limits. 120 V circuits need larger wire at the same distance — enter your exact values above.
Worked example: 50 amps to a garage subpanel
A detached garage needs a 50 A, 240 V feed, and the panel-to-panel run measures 100 feet along the actual wire path. What copper conductor does the job at a 3% drop target?
Three percent of 240 V is 7.2 V of allowed drop. Required circular mils: (2 × 12.9 × 50 × 100) ÷ 7.2 = 17,917 CM. Now walk the ladder: 8 AWG is 16,510 CM — close, but short. Next rung is 6 AWG at 26,240 CM. That's the answer: 6 AWG copper, landing at an actual drop of 1.97%, or about 4.7 V.
Check the other constraint: 6 AWG copper is commonly rated 65 A at 75°C, comfortably above the 50 A load, so voltage drop governed this pick — ampacity alone would have allowed 8 AWG (50 A). Notice what that means: anyone who sizes this feeder from an ampacity table alone would pull 8 AWG and deliver 3.3% drop to a shop full of motor loads. Legal in many jurisdictions, but the table-saw-plus-compressor mornings will show it.
If the garage might someday host a welder or an EV charger, the conservative choice column already shows the next size up — 4 AWG at 1.2% — and on a one-time trench that upgrade is cheap insurance.
Practical tips and common mistakes
- Both constraints matter, always. Sizing by ampacity alone ignores distance; sizing by drop alone can, on very short runs, suggest a wire too small to carry the load safely. This tool checks both, but understand which one drove your answer.
- Derating can move the answer. Bundled conductors, hot attics, and rooftop conduit reduce usable ampacity below the published 75°C values used here. When conditions are harsh, assume at least one size up and verify properly.
- Terminations may limit you to 60°C on small circuits. Older devices and some breakers are rated for 60°C on 14–10 AWG circuits, which lowers allowable ampacity versus the 75°C figures. Check the hardware, not just the wire.
- Continuous loads need the 125% treatment. Anything running 3+ hours — EV chargers, water heaters, sign circuits — is sized at 125% of the load for breaker and conductor purposes. Our EV charger calculator does this automatically.
- Round trip is handled. Enter one-way distance; the 2× in the formula covers out-and-back. Doubling it yourself is the classic way to buy two sizes of wire you didn't need.
Frequently asked questions
What size wire do I need for a 50 amp circuit 100 feet away?
At 240 V with a 3% drop target, 50 A over 100 feet needs 6 AWG copper — 8 AWG would carry the current but drops about 3.3%. At 120 V the same run needs 4 AWG. Enter your exact numbers above; the answer changes quickly with voltage and distance.
What does "ampacity governs" mean in the result?
Two separate constraints set wire size: the conductor must carry the current without overheating (ampacity) and deliver acceptable voltage (drop). On short runs, drop is negligible and the minimum safe size wins. On long runs, drop usually demands a bigger wire than ampacity alone. The calculator checks both and tells you which one decided.
Should I use the recommended size or the next size up?
The recommended size meets your target; the next size up buys margin. Go bigger when the load may grow, when the run is buried or hard to replace, when motors start under load, or when your drop lands just under the limit. Wire is cheap compared to pulling it twice.
Why does 120 V need bigger wire than 240 V for the same amps?
The volts lost are identical, but they hurt twice as much as a percentage of 120 V. A run that drops 5 V is at 4.2% on 120 V — over the guideline — but only 2.1% on 240 V. Long runs are far cheaper to build at higher voltage.
Can I use aluminum instead of copper?
For feeders and larger circuits, aluminum is common and cost-effective — expect roughly two sizes larger than copper for the same performance. Select aluminum above and the calculator handles the higher resistivity and separate ampacity values automatically. Use connectors rated for aluminum and follow the manufacturer's termination practices.
Does this calculator account for temperature or conduit fill derating?
No. It checks commonly published 75°C ampacity values at face value. High ambient temperatures, more than three current-carrying conductors in a raceway, or rooftop conduit all require derating that can push you up a size. Treat this result as a starting point and verify against your locally adopted code.