Voltage Drop Calculator

Electricity behaves like water flowing through a pipe. If you run a massive, 500-foot hose to the end of a farm to water crops, the pressure at the end of the hose will be a miserable trickle because of the friction against the pipe walls. Copper wire works the same way. As electricity travels through a long wire, physical resistance causes the voltage to "drop." If you run a 200-foot extension cord to power a heavy saw, the voltage drop will starve the motor, causing it to overheat and catch fire. The Voltage Drop Calculator ensures your wiring architecture is mathematically sized to deliver safe power over extreme distances.

The Mathematics of Resistance

Voltage drop is dictated by Ohm's Law and the physical properties of the conductor. The calculation requires three variables: Amperage (Load), Distance, and Wire Gauge (Thickness).

The more amps the machine pulls, the harder the electricity is fighting through the wire. The longer the wire, the more physical resistance the electricity encounters. A thin wire (like 14 AWG) has high resistance, while a massive, thick wire (like 2 AWG) has very low resistance, allowing electricity to flow smoothly over long distances without losing pressure.

The 3% NEC Safety Limit

The National Electrical Code (NEC) dictates strict limits on voltage drop to prevent fires and equipment destruction.

For a standard branch circuit (from the breaker box to the wall outlet), the maximum legally allowed voltage drop is 3%. For a 120V household outlet, a 3% drop means the machine at the end of the wire must receive a minimum of 116.4 Volts. If the drop is 10%, the machine only receives 108 Volts. An electric motor trying to run on 108 Volts will pull massive amounts of extra amperage to compensate, melting the internal coils and starting an electrical fire.

How to Use the Calculator

Select your Conductor Material (Copper is standard; Aluminum has higher resistance and requires a thicker wire). Select your Phase (Single Phase for homes, Three Phase for industrial).

Input the starting Voltage (e.g., 120V, 240V), the exact length of the wire run (One-Way Distance), the Amperage of the machine you are powering, and the AWG Wire Size you plan to use. The tool will calculate the exact voltage lost over the distance, outputting the final remaining voltage and the Percentage Drop. To convert the basic Watts of your machine into Amps for this tool, use our Watts to Amps Converter.

How to Fix Excessive Voltage Drop

If the calculator flags your run as exceeding the 3% safety limit, you only have two options.

You cannot change the distance, and you cannot change the amperage of the machine. The only physical way to fix the problem is to increase the thickness of the wire (lower the AWG number). You may only need a cheap 12 AWG wire to handle 20 amps safely inside a house, but if you want to push that 20 amps to a shed 300 feet away, you will have to buy extremely expensive, massive 6 AWG wire to overcome the distance.