Extension Cord Voltage Drop Calculator

Ensure your power tools and appliances get enough voltage to run safely without overheating.

Starting Outlet Voltage
Tool Current Draw 15 A
Check the sticker on your tool or appliance for its Amperage (A).
Cord Length 50 ft
Wire Gauge (Thickness)
Wall Outlet 120V
Appliance 115.3V
Safe Drop (Under 5%)
0.0V Lost Voltage
0.0% Percent Drop

Why Extension Cords Get Hot

When you run an extension cord over a long distance, the copper wire itself acts as a resistor. This causes a voltage drop-meaning the power coming out the end of the cord is lower than the power going in at the wall outlet.

  • The 5% Rule: Most electrical equipment is designed to handle minor voltage fluctuations. However, a drop greater than 5% is considered unsafe.
  • Motor Damage: When a motor (like in an air compressor, saw, or refrigerator) receives low voltage, it has to pull more current (Amps) to do the same amount of work. This excess current creates massive heat, which destroys the motor's internal windings.
  • The Solution: If your cord is too long or your tool draws too much power, you must step up to a "Heavy Duty" cord. Thicker wires (represented by lower AWG numbers, like 12 AWG or 10 AWG) have less electrical resistance and prevent voltage drop.
Safety Disclaimer: This calculator provides theoretical voltage drop based on standard copper resistance. In real-world scenarios, factors like damaged receptacles, ambient heat, and worn copper strands can increase resistance. Extension cords are intended for temporary use only. Never daisy-chain multiple extension cords together, and always follow the manufacturer's maximum amperage ratings.