How to use this voltage drop calculator
- Enter the source voltage and the load current.
- Enter the one-way length of the run.
- Pick the wire gauge and conductor material.
Example
A 240 V, 30 A load at 150 ft on 10 AWG copper has a voltage drop of 11.18 V, which is 4.66% (above 3%).
What to know before you buy
What causes voltage drop
Every wire has resistance, and current flowing through it loses some voltage as heat. The drop grows with current and distance and shrinks with thicker wire. Long runs to garages, sheds, wells and outdoor lighting are where it matters most.
Effects of low voltage
Excessive voltage drop makes motors run hot and lose power, lights dim and electronics malfunction. Keeping drop under 3% protects equipment and saves energy.
Voltage drop, 120 V, 15 A, copper
| Wire and distance | Voltage drop | Percentage |
|---|---|---|
| 14 AWG, 50 ft | 4.71 V | 3.92% (above 3%) |
| 12 AWG, 50 ft | 2.96 V | 2.47% (OK) |
| 12 AWG, 100 ft | 5.93 V | 4.94% (above 3%) |
| 10 AWG, 100 ft | 3.73 V | 3.11% (above 3%) |
| 8 AWG, 150 ft | 3.52 V | 2.93% (OK) |
Formula
Voltage drop = 2 × K × I × L ÷ circular mils (single-phase) or √3 × K × I × L ÷ circular mils (three-phase). K = 12.9 (copper) or 21.2 (aluminum) ohm-cmil/ft.
Tips from the pros
- Doubling the voltage (120 V to 240 V) cuts the percentage drop in half for the same power.
- Motors and LED drivers are sensitive to low voltage; keep the drop small on those circuits.
Common mistakes to avoid
- Measuring voltage with no load: the drop only appears when current flows.
- Using the breaker size instead of the real load current in the calculation.
Frequently asked questions
What is an acceptable voltage drop?
The NEC recommends no more than 3% on a branch circuit and 5% total from the service to the load.
How do I reduce voltage drop?
Use a larger wire, shorten the run, or use a higher voltage.
How far can you run 12 gauge wire?
At 120 V and 15 A, 12 AWG copper stays under 3% drop up to about 60 ft; beyond that, use 10 AWG.
Does voltage drop waste electricity?
Yes. The dropped voltage turns into heat in the wire, which is wasted energy.