Wire Gauge Calculator

Wire gauge selection determines whether a cable can safely carry a given electrical current over a given distance without overheating or wasting excessive power through voltage drop.

The AWG system (American Wire Gauge): AWG numbers run counterintuitively — a lower AWG number means a thicker wire. AWG 4 is much thicker than AWG 14. Each three-step decrease in AWG roughly doubles the wire’s cross-sectional area and halves its resistance.

Voltage drop formula:

V_drop = (2 × L × I × R) ÷ 1000

Variable definitions:

• V_drop = voltage lost along the wire (volts)
• L = one-way cable length (feet)
• I = current load (amperes)
• R = wire resistance (ohms per 1,000 feet, from AWG table)
• × 2 = accounts for the round-trip (positive and return conductors)

AWG copper wire reference table:

Worked example: A 20 A circuit runs 75 feet (one way) using AWG 12 wire: V_drop = (2 × 75 × 20 × 1.588) ÷ 1000 = 4.76 V On a 120 V circuit: 4.76 ÷ 120 = 3.97% voltage drop

Industry standards:

• ≤ 3% drop — recommended for branch circuits (NEC guideline)
• ≤ 5% total — maximum acceptable for feeder + branch combined
• Upgrade to AWG 10 in this example to bring drop under 3%

Aluminum wire note: Aluminum wire (common in older homes and service panels) has about 1.6× the resistance of copper. Always use the aluminum column of resistance tables when dealing with aluminum conductors.