Voltage Drop Calculator
Calculate voltage drop in DC and single-phase AC circuits based on wire length, gauge, current, and system voltage.
ft
A
Results
Voltage Drop
4.76V
Percentage Drop
3.97%
Wire Resistance
1.588 Ω/1000 ft
Wire Gauge Reference
14 AWG
2.525 Ω/1000 ft
12 AWG
1.588 Ω/1000 ft
10 AWG
0.999 Ω/1000 ft
8 AWG
0.628 Ω/1000 ft
6 AWG
0.395 Ω/1000 ft
How is Voltage Drop Calculated?
Voltage drop in a DC or single-phase AC circuit is calculated using the formula: VD = 2 × L × I × R / 1000, where L is the one-way wire length in feet, I is the current in amps, and R is the wire resistance in ohms per 1,000 feet.
% Drop = (VD / System Voltage) × 100
Example
For a 120V circuit with 15A load running through 100 feet of 12 AWG wire: VD = 2 × 100 × 15 × 1.588 / 1000 = 4.76V, which is a 3.97% drop — exceeding the recommended 3% limit.
Common Use Cases
- Sizing wire for outdoor buildings, sheds, and workshops.
- Ensuring adequate voltage at the end of long cable runs.
- Designing low-voltage landscape lighting circuits.
- Verifying wire gauge meets NEC voltage drop recommendations.
FAQs
What is the recommended maximum voltage drop?
The NEC recommends no more than 3% voltage drop from the panel to the furthest outlet on a branch circuit, and no more than 5% total (panel to load, including branch and feeder). For critical or sensitive equipment, 2% or less is preferred.
Why does wire length matter?
Longer wire has more resistance, which causes a greater voltage drop. Doubling the wire length doubles the voltage drop. This is why wire gauge must be increased (lower AWG number) for longer runs.
What wire resistance values are used?
The calculator uses NEC Chapter 9 Table 8 values for uncoated copper conductors at 75°C: 14 AWG = 2.525, 12 AWG = 1.588, 10 AWG = 0.999, 8 AWG = 0.628, and 6 AWG = 0.395 ohms per 1,000 feet.