Electrical Wire Gauge Calculator
Determine the correct wire gauge for your circuit based on amperage and distance.
Avoid voltage drop issues with the right wire size.
How Electrical Wire Size Is Calculated
Choosing the correct wire gauge prevents overheating, voltage drop, and fire hazards. In the US, wire size follows the AWG (American Wire Gauge) system — lower numbers mean thicker wire and higher current capacity.
Voltage Drop Formula:
V_drop = (2 × L × I × R_per_meter) / 1000
Where:
- L = one-way cable length in meters
- I = current in amps
- R_per_meter = resistance of wire in mΩ/m (from wire tables)
- 2 = accounts for both live and neutral conductors
Maximum Allowable Voltage Drop:
- Branch circuits (NEC): 3% for branch, 5% total including feeder
- At 120V: max drop = 3.6V; at 240V: max drop = 7.2V
AWG Current Capacity (Ampacity) Reference:
| AWG | Diameter (mm) | Ampacity (copper, 60°C) |
|---|---|---|
| 14 | 1.63 | 15A |
| 12 | 2.05 | 20A |
| 10 | 2.59 | 30A |
| 8 | 3.26 | 40A |
| 6 | 4.11 | 55A |
Worked Example: A 20A circuit runs 25 meters to a workshop. Using 12 AWG (resistance ≈ 5.21 mΩ/m):
- V_drop = (2 × 25 × 20 × 5.21) / 1000 = 5210 / 1000 = 5.21V
- At 120V: 5.21/120 = 4.34% — exceeds 3% limit
- Solution: upgrade to 10 AWG (resistance ≈ 3.28 mΩ/m) → V_drop = 3.28V = 2.73% ✓
How we build and check this calculator
This calculator runs entirely in your browser, so the numbers you enter stay on your device. The math behind it is written by hand and tested against worked examples and standard references before the page goes live.
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