Wire Gauge Calculator — Correct Wire Size by Amperage & Distance
Find the correct AWG wire gauge for any circuit based on amperage, one-way distance, voltage, and conductor material. Includes voltage drop calculation and NEC guidelines.
How to Use This Calculator
- Enter the circuit amperage (should match your breaker size)
- Enter the one-way distance from panel to last outlet in feet
- Select voltage and conductor material
- Choose maximum acceptable voltage drop
- Click "Calculate Wire Gauge" to see the recommended AWG size
How the Calculation Works
// Voltage drop formula (single phase):
VD_volts = (2 × K × I × L) / CM
// K = 12.9 for copper, 21.2 for aluminum
// I = current (amps), L = one-way length (ft), CM = circular mils of wire
// Required circular mils to stay within VD limit:
CM_required = (2 × K × I × L) / VD_allowed
// Map CM to AWG gauge (NEC table)
// Actual voltage drop % with selected gauge:
VD_percent = (VD_volts / source_voltage) × 100 The NEC voltage drop formula calculates circular mils (CM) required to keep voltage drop within limits. K is the resistivity constant: 12.9 for copper, 21.2 for aluminum. The required CM is compared to the NEC wire table to find the minimum AWG gauge. A larger gauge (smaller AWG number) is always acceptable.
Variables:
- K: Resistivity constant: 12.9 for copper, 21.2 for aluminum
- I: Load current in amperes
- L: One-way wire run length in feet
- CM: Circular mils — cross-sectional area of the wire conductor
- VD_percent: Voltage drop expressed as percentage of source voltage
Pro Tips
- Always size wire based on the circuit breaker amperage, not the load — a 20A breaker needs minimum 12 AWG wire
- Longer runs lose more voltage — consider upsizing one gauge for runs over 50ft at 120V
- Aluminum wire is cheaper for large runs (200A service, sub-panels) but requires anti-oxidant compound at connections
- Copper wire is always safer for branch circuits in homes — aluminum expansion/contraction causes loose connections over time
- NEC recommends total voltage drop not exceed 5% (3% branch + 2% feeder) for optimal performance
Recommended Products
Frequently Asked Questions
What wire gauge do I need for a 20-amp circuit?
12 AWG copper is the minimum for a 20-amp circuit. For runs over 50ft at 120V or over 100ft at 240V, consider 10 AWG to reduce voltage drop. A 15-amp circuit uses 14 AWG minimum.
What is the NEC maximum voltage drop recommendation?
NEC recommends no more than 3% voltage drop on branch circuits and 2% on feeders, for a combined maximum of 5%. Higher voltage drop means lower efficiency, dimmer lights, and reduced motor performance.
Can I use aluminum wire for residential circuits?
Aluminum is acceptable for 240V circuits, service entrances, and feeders (typically 4 AWG and larger). For 120V branch circuits in homes, copper is strongly recommended due to aluminum's connection reliability issues at smaller gauges.
What AWG gauge should I use for a 240V dryer circuit?
A 30-amp dryer circuit requires 10 AWG copper (or 8 AWG aluminum) for runs up to about 75ft at 240V while staying within 3% voltage drop. Always use a 4-wire circuit (2 hots + neutral + ground) for dryers per current NEC code.
How do I find the amperage rating for a wire gauge?
14 AWG: 15A max. 12 AWG: 20A max. 10 AWG: 30A max. 8 AWG: 40-50A. 6 AWG: 55-65A. 4 AWG: 70-85A. These are NEC ratings for copper wire in conduit at 60°C. Actual ampacity varies by installation method and temperature.