Transparent method
The formula
Worked example
Example inputs
How to interpret the result
Voltage drop wastes power as heat along the wire itself and can quietly undersize a circuit's real capacity, because a device at the far end of a long run receives less voltage than what left the source. A run long enough or thin enough can leave a device technically underpowered even though the breaker and source voltage both look completely fine.
Before you rely on it
What to check
Check the calculated drop against the wire run's actual length and gauge, not just the rated capacity of the breaker or source, since a long run is where the real loss accumulates.
The common error
Where people go wrong with voltage drop calculator
Sizing a wire run by the connected load alone without checking voltage drop over distance. A wire rated fine for the load at the source can still deliver meaningfully reduced voltage at the far end of a long run.
Sensitivity evidence
How current (amps) changes the voltage drop (volts)
| Current (amps) | Voltage drop (volts) | Power lost (watts) |
|---|---|---|
| 12 | 3.8 | 45.7 |
| 14 | 4.4 | 62.2 |
| 15worked example | 4.8 | 71.5 |
| 17 | 5.4 | 91.8 |
| 18 | 5.7 | 102.9 |
Every input, tested
Which input moves the voltage drop (volts) most
| Input | Tested from | To | Voltage drop (volts) at each end | Swing |
|---|---|---|---|---|
| Wire gauge | 14 AWG | 6 AWG | 7.6 to 1.2 | 6.4 (134%) |
| One-way distance (ft) | 90 | 110 | 4.3 to 5.2 | 0.953 (20%) |
| Current (amps) | 14 | 17 | 4.4 to 5.4 | 0.953 (20%) |
Two variables at once
Voltage drop (volts) by current (amps) and one-way distance (ft)
| Current (amps) \ One-way distance (ft) | 80 | 100 | 120 |
|---|---|---|---|
| 12 | 3 | 3.8 | 4.6 |
| 14 | 3.6 | 4.4 | 5.3 |
| 15 | 3.8 | 4.8 | 5.7 |
| 17 | 4.3 | 5.4 | 6.5 |
| 18 | 4.6 | 5.7 | 6.9 |
Step by step
The worked example, input by input
| Input | Value used | What it means |
|---|---|---|
| Current (amps) | 15 | |
| One-way distance (ft) | 100 | |
| Wire gauge | 12 AWG | Thinner wire (higher AWG number) has more resistance per foot. |
| Voltage drop (volts) | 4.8 | |
| Power lost (watts) | 71.5 | |
Inputs, definitions and assumptions
Current (amps)
One-way distance (ft)
Wire gauge
Thinner wire (higher AWG number) has more resistance per foot.
Where the inputs come from
Enter the current in amps, the one-way length of the wire run in feet and the wire gauge, which sets the resistance per 1,000 feet: about 1.59 ohms for 12 AWG copper, 2.53 for 14 AWG and 0.98 for 10 AWG. Measure the wire path, not the straight-line distance. The tool doubles the distance for the round trip. Use the actual expected current, not the breaker rating, for a realistic drop.
When the result stops applying
Resistance rises with temperature, and aluminum wire has higher resistance than copper. AC circuits with inductive loads add impedance. The tool covers single-phase two-wire runs. Motors starting draw more than their running current, so the momentary drop is larger. Electrical work should follow local code and be done by a qualified person.
What to compare it with
Relate power to current on the watts to amps calculator, see Ohm's law and power on the Ohm's law page, and combine resistors on the resistor calculator. The LED resistor and voltage divider pages handle low-voltage electronics, and the energy and power converters change units.
People also ask
Frequently asked questions
How much voltage drop is acceptable?
Generally no more than 3% on a branch circuit and 5% total for feeder and branch together, to keep equipment running efficiently.
How can I reduce voltage drop?
Use a thicker wire, shorten the run, lower the current or raise the supply voltage, since each of these reduces the loss along the wire.
Why is the distance doubled?
Current flows out and back through the circuit, so the resistance is that of twice the one-way length.