Voltage Drop Calculator

1. Select your system voltage. Use 120V or 240V for standard US residential circuits. Use 12V or 24V for DC systems like RVs, solar panels, or marine wiring.
2. Enter the load current in amps. This is the expected draw of your device or circuit, not the breaker rating. For general circuits, use the actual load, not the breaker size.
3. Enter the one-way wire length. Measure from your panel to the outlet or device. The calculator automatically doubles this for the return wire.
4. Select the wire gauge (AWG). Lower AWG numbers mean thicker wire with less resistance. AWG 12 is common for 20A household circuits; AWG 10 for 30A circuits.
5. Choose the conductor material. Most residential wiring is copper. Aluminum is used in large service entrance cables and some branch circuits.
6. Read the results. The NEC (National Electrical Code) recommends keeping voltage drop below 3% for branch circuits and 5% total including feeder wires.

Voltage drop in a DC circuit or single-phase AC circuit is calculated using Ohm's Law applied to the total wire resistance in the circuit (both the hot and neutral/return conductors):

Wire Resistance (Ω) = (Resistance per 1000 ft / 1000) × Length × 2
Voltage Drop (V)    = Current (A) × Wire Resistance (Ω)
Voltage Drop (%)    = (Voltage Drop / Source Voltage) × 100
End Voltage (V)     = Source Voltage - Voltage Drop

The factor of 2 accounts for both the hot wire going to the load and the neutral wire returning. For three-phase systems, the multiplier changes to 1.732 (the square root of 3) instead of 2.

Example: AWG 12 copper wire (1.93 Ω/1000 ft), 50 ft one-way, 15A load, 120V system.

R = (1.93 / 1000) × 50 × 2 = 0.193 Ω
V_drop = 15 × 0.193 = 2.895 V
V_drop% = (2.895 / 120) × 100 = 2.41%  (within 3% NEC limit)
End voltage = 120 - 2.895 = 117.1 V