Wire Size Calculator
Find the correct AWG wire size for any electrical circuit based on amperage, distance, and voltage. Calculate voltage drop percentage and wire resistance.
A
ft
%
Recommended AWG
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Voltage Drop —
Voltage Drop % —
Wire Resistance —
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ft
Minimum AWG
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Voltage Drop % —
Breaker Size —
NEC Notes —
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A
ft
V
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Wire Selection
Recommended AWG —
Voltage Drop % —
Ampacity & Derating
Derating Factor (bundled) —
Adjusted Ampacity —
Material & Cost
Total Wire Length (both conductors) —
Estimated Wire Cost —
How to Use This Calculator
- Enter the amperage of the circuit.
- Enter the one-way distance in feet from the panel to the load.
- Select the system voltage (120V or 240V).
- The calculator recommends the minimum AWG size and shows actual voltage drop.
Formula
Voltage Drop = 2 × L × I × R (for single-phase)
where L = one-way length (ft), I = current (A), R = resistance per foot (Ω/ft)
Voltage Drop % = (Voltage Drop / System Voltage) × 100
Example
Example: 20A circuit, 75 ft run, 120V, copper → Use 10 AWG (voltage drop = 2.5%)
Frequently Asked Questions
- A 20-amp circuit requires 12 AWG copper wire at minimum per NEC code. For long runs (over 50 ft) check the voltage drop — you may need 10 AWG.
- Voltage drop = (2 × Length × Current × Resistance per foot) for single-phase. The NEC recommends keeping voltage drop under 3% for branch circuits.
- Lower AWG numbers mean larger wire diameter. AWG 14 is common for 15A, AWG 12 for 20A, AWG 10 for 30A, AWG 8 for 40A, and AWG 6 for 55A circuits.
- Copper is preferred for most applications. Aluminum wire is cheaper and lighter but requires connectors rated for aluminum and is 2 AWG sizes larger for the same ampacity.
- Voltage drop % is how much voltage is lost along the wire. The NEC recommends max 3% for branch circuits and 5% total (feeder + branch). High drop causes dimming and inefficiency.