Voltage Drop Calculator

Comprehensive Guide to Voltage Drop

When you push water through a very long, narrow hose, the friction inside the hose causes the water pressure to drop significantly by the time it reaches the nozzle. Electricity behaves exactly the same way.

Every copper wire possesses a tiny amount of natural physical resistance. If you run a wire over a very long distance, this resistance acts like friction, causing the Voltage (electrical pressure) to drop. The Voltage Drop Calculator ensures that the electrical appliance at the end of the wire actually receives enough power to run safely, dictating exactly how thick your wires need to be.

How to Use the Voltage Drop Calculator

Failure to calculate voltage drop can cause motors to burn out or circuits to start fires.

1. Enter Source Voltage: Input the starting voltage at the breaker panel (e.g., 120V for a standard US outlet, or 12V for a car battery).
2. Enter Current (Amps): Input the maximum load the appliance will draw.
3. Enter Distance: Input the physical length of the wire run in meters or feet. (Note: The formula inherently accounts for the round trip of the positive and negative wire).
4. Select Wire Gauge (AWG): Choose the thickness of the wire.
5. Calculate: The tool will output the exact voltage lost over the distance, and the final percentage drop.

The Formula

The calculation relies on Ohm's Law ($V = I * R$) combined with the specific, measured resistance of copper wire per 1000 feet.

Voltage Drop = (2 * Length * Resistance * Current) / 1000

• Length: The one-way distance to the load.
• Current: The amps drawn by the load.
• Resistance: The specific ohms-per-1000-ft rating of the chosen AWG wire (e.g., 12 AWG copper has roughly 1.588 ohms per 1000 feet).
• Multiplier (2): Because electricity must travel down the hot wire and return via the neutral wire, the total distance the electrons travel is double the physical length.

Practical Examples

Scenario 1: The Long Landscape Light You want to run a 12 Volt halogen landscape light at the very back of your garden, 150 feet away. The light draws 5 Amps. You try to use cheap, thin 16 AWG wire.

• Result: The calculator shows a catastrophic voltage drop of 6.0 Volts (a 50% drop!). The light bulb will only receive 6V and will barely glow. You must upgrade to much thicker 10 AWG wire to reduce the resistance.

Scenario 2: Sizing a Garage Subpanel You are running a 240 Volt line to a new garage 100 feet away to power a heavy 40 Amp welder. You select thick 6 AWG copper wire.

• Result: The calculator shows a voltage drop of only 3.2 Volts (a 1.3% drop). The welder will receive 236.8V, which is well within safe operating limits.

Conclusion

Electricity is bound by the laws of physics. You cannot simply pull a wire as far as you want and expect it to work. By using the Voltage Drop Calculator, electricians and hobbyists can safely size their wires, ensuring expensive appliances receive the clean, powerful voltage they require to operate safely.