Wire Size Calculator
Find the right AWG wire size for your circuit based on current, run length, voltage, and maximum allowed voltage drop.
Calculate Wire Size (AWG)
Result
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Recommended Wire Size
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Ampacity (copper, 60°C)
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Actual Voltage Drop
AWG Quick Reference (Copper, 60°C)
| AWG | Ampacity (A) | Typical Use |
|---|---|---|
| AWG 14 | 15A | Lighting, 15A circuits |
| AWG 12 | 20A | General outlets, 20A circuits |
| AWG 10 | 30A | Dryers, water heaters, 30A circuits |
| AWG 8 | 40A | Ranges, AC units, 40A circuits |
| AWG 6 | 55A | Large AC, 50A circuits |
| AWG 4 | 70A | Sub-panels, large appliances |
| AWG 2 | 95A | Large sub-panels |
| AWG 1/0 | 125A | Service entrances |
| AWG 2/0 | 145A | 200A service |
| AWG 4/0 | 195A | Large commercial service |
Example Calculation
Finding wire size for a 20A circuit with a 75ft one-way run at 120V, max 3% voltage drop:
Max allowed voltage drop = 120V × 3% = 3.6V
Try AWG 12 (1.98 Ω/1000ft): R = (1.98 ÷ 1000) × 2 × 75 = 0.297Ω
Voltage drop = 20A × 0.297 = 5.94V (4.95%) — too high
Try AWG 10 (1.24 Ω/1000ft): R = (1.24 ÷ 1000) × 2 × 75 = 0.186Ω
Voltage drop = 20A × 0.186 = 3.72V (3.1%) — marginal
Try AWG 8 (0.778 Ω/1000ft): R = (0.778 ÷ 1000) × 2 × 75 = 0.117Ω
Voltage drop = 20A × 0.117 = 2.34V (1.95%) — good
Try AWG 12 (1.98 Ω/1000ft): R = (1.98 ÷ 1000) × 2 × 75 = 0.297Ω
Voltage drop = 20A × 0.297 = 5.94V (4.95%) — too high
Try AWG 10 (1.24 Ω/1000ft): R = (1.24 ÷ 1000) × 2 × 75 = 0.186Ω
Voltage drop = 20A × 0.186 = 3.72V (3.1%) — marginal
Try AWG 8 (0.778 Ω/1000ft): R = (0.778 ÷ 1000) × 2 × 75 = 0.117Ω
Voltage drop = 20A × 0.117 = 2.34V (1.95%) — good
For a 20A, 75ft run at 120V with max 3%, AWG 8 is recommended.
Frequently Asked Questions
What wire size do I need for a 30 amp circuit?
For a standard 30A circuit, AWG 10 copper wire is the NEC minimum. For runs longer than 50 feet, consider AWG 8 to keep voltage drop under 3% at full load.
What is the 80% rule for wire sizing?
Wires should not carry more than 80% of their rated ampacity on a continuous basis (3+ hours). A 20A breaker circuit should carry no more than 16A continuously. This is built into NEC breaker sizing requirements.
Does wire length affect what size I need?
Yes. Longer runs increase resistance and cause more voltage drop. For runs over 50 feet, you often need to go up one gauge size (e.g., from AWG 12 to AWG 10) to stay within the 3% voltage drop recommendation.