Wire Size Calculator

Enter your voltage, current (amperes), one-way distance, and system parameters to find the correct wire gauge (AWG) for your circuit. Choose your conductor material (copper or aluminum), phase (single or three), and allowable voltage drop — the calculator returns the recommended wire size, actual voltage drop, and voltage at load so you can wire safely and within NEC guidelines. Also try the Voltage Drop Calculator.

Select the nominal source voltage for your circuit.

Single-phase is most common for residential; three-phase for commercial/industrial.

A

The load current in amperes. Obtain motor currents from manufacturer tables.

ft

One-way length of the conductor run in feet.

Copper is more conductive; aluminum is lighter and lower cost but requires larger gauge.

NEC recommends no more than 3% voltage drop for branch circuits.

Results

Recommended Wire Size (AWG)

--

Circular Mils Required

--

Actual Voltage Drop

--

Voltage Drop %

--

Voltage at Load

--

Results Table

Frequently Asked Questions

How do you calculate wire size?

Wire size is calculated using the formula: Circular Mils = (Resistivity × 2 × Amps × Distance) / Allowable Voltage Drop. For single-phase circuits use a resistivity of 12.9 for copper or 21.2 for aluminum (in ohms per circular mil-foot). The resulting circular mils value is then matched to the nearest standard AWG size that meets or exceeds that requirement. See also our calculate Current, Current (Milliamps) & Formula Used — Watts to Amps.

What is voltage drop and why does it matter?

Voltage drop is the reduction in voltage as current travels through a wire due to the wire's resistance. Excessive voltage drop can cause equipment to underperform, overheat, or fail. The NEC recommends a maximum of 3% voltage drop for branch circuits and 5% for feeder plus branch combined.

How do I calculate wire size for a 3-phase circuit?

For three-phase circuits, the formula is modified by a factor of √3 (approximately 1.732), so: Circular Mils = (Resistivity × 2 × Amps × Distance × 0.866) / Allowable Voltage Drop. This accounts for the phase relationship between conductors in a three-phase system. Select three-phase in the calculator and it handles this automatically.

Should I use copper or aluminum wire?

Copper wire is more conductive, allowing a smaller gauge for the same current, and is generally preferred for branch circuits and indoor wiring. Aluminum is lighter and less expensive, making it common for service entrance and large feeder runs, but it requires a larger gauge and special connectors to prevent oxidation issues. You might also find our Voltage (V) — Ohm's Law useful.

How do I calculate wire size for distance?

Distance directly affects voltage drop — the longer the run, the more resistance and the greater the drop. Use the one-way distance in feet in the formula. For long runs, you may need to use a larger wire gauge than the ampacity alone would require, in order to keep voltage drop within acceptable limits.

How do I calculate AWG wire size?

AWG (American Wire Gauge) sizes are standardized. After calculating the required circular mils, you select the AWG size whose circular mil area equals or exceeds your calculated requirement. For example, 14 AWG copper has 4,110 CM, 12 AWG has 6,530 CM, 10 AWG has 10,380 CM, and so on — larger AWG numbers mean smaller wire.

How do I calculate wire size for a motor?

For motor circuits, the NEC requires the wire to be sized at 125% of the motor's full-load current (FLC), not the nameplate amperage. Obtain the full-load current from NEC Table 430-248 (single-phase) or 430-250 (three-phase), multiply by 1.25, and use that value as your amperes input in the calculator.

What AWG wire do I need for a 20-amp, 120V, 100-foot circuit?

For a 20A, 120V single-phase circuit at 100 feet with a 3% allowable voltage drop, the required circular mils work out to approximately 17,208 CM for copper, which corresponds to 10 AWG (10,380 CM) being insufficient — you would need 8 AWG copper (16,510 CM is close, so confirm with 6 AWG at 26,240 CM for safety margin). Always verify against NEC ampacity tables for your specific installation conditions. Check out our calculate Minimum Box Length, Minimum Box Width & Minimum Box Depth — Junction Box Sizing as well.