Voltage Drop Calculator
Calculate voltage drop in electrical wiring based on wire gauge, length, and current.
Voltage Drop = (2 × Length × Current × Resistance per 1000 ft) / 1000
This calculator determines how much voltage is lost along a wire run due to the wire’s natural electrical resistance. The factor of 2 accounts for both the supply and return conductors in a circuit.
What each variable means:
- System Voltage – your circuit voltage (typically 120V or 240V for residential)
- Current (Amps) – the electrical load being drawn by the device or circuit
- Wire Length – the one-way distance from the panel to the load (the formula doubles it automatically)
- Wire Gauge (AWG) – the thickness of the wire, which determines its resistance
Wire resistance reference (copper, per 1000 ft):
| AWG Gauge | Resistance (ohms/1000ft) | Typical Use |
|---|---|---|
| 14 AWG | 3.18 | 15A lighting circuits |
| 12 AWG | 2.00 | 20A general outlets |
| 10 AWG | 1.26 | 30A dryers, AC units |
| 8 AWG | 0.794 | 40A ranges, large appliances |
| 6 AWG | 0.500 | 50A+ sub-panels |
NEC guidelines: The National Electrical Code recommends no more than 3% voltage drop for branch circuits and no more than 5% total from the service panel to the furthest outlet. Excessive voltage drop causes lights to dim, motors to overheat, and appliances to underperform.
Practical example: A 20A circuit on 12 AWG wire running 100 feet at 120V: voltage drop = (2 × 100 × 20 × 2.0) / 1000 = 8.0V, which is 6.7% – too high. Upgrading to 10 AWG wire reduces the drop to 5.04V (4.2%).
Tips:
- If the calculator shows more than 3%, consider using a thicker wire gauge or shortening the run.
- Longer wire runs always need thicker wire to keep voltage drop acceptable.