Generator Extension Cord Size Calculator
Estimate the minimum extension cord wire gauge needed for a generator based on load, voltage, cord length, amperage, and allowable voltage drop.
Generator and Load
Enter the total running watts expected to travel through this cord. Starting surge wattage should be checked separately against the generator, plug, cord, and connected equipment ratings.
Recommended Cord Size
Wire Gauge Comparison
| Gauge | Planning Ampacity | Voltage Drop | Result |
|---|---|---|---|
| Enter the load and cord length above. | |||
This calculator provides a conservative planning estimate. The cord, generator receptacle, plug, breaker, transfer equipment, and connected load must all support the required amperage. Follow manufacturer instructions and applicable electrical codes.
What Gauge Extension Cord Do I Need for a Generator?
The correct generator extension cord depends on both amperage and distance. A longer cord has more electrical resistance, which produces additional voltage drop. A heavier wire has a smaller American Wire Gauge number and reduces that resistance.
For example, 10-gauge wire is heavier than 12-gauge wire, and 12-gauge wire is heavier than 14-gauge wire. Never choose a cord based only on its physical appearance. Confirm the wire gauge, conductor count, voltage, and amperage printed on the cord.
How Generator Amperage Is Calculated
Current is estimated by dividing the running load in watts by the generator output voltage. The same wattage requires twice as much current at 120 volts as it does at 240 volts.
Why Extension Cord Length Matters
Electrical current travels from the generator to the load and returns through another conductor. Voltage-drop calculations therefore use approximately twice the one-way cord length. Longer cords may require heavier wire even when the connected equipment uses relatively little current.
Should I Use a 3% or 5% Voltage-Drop Limit?
A 3% limit is the more conservative planning choice, particularly for motors, refrigerators, freezers, pumps, furnaces, and electronic equipment. A 5% limit may permit a smaller wire but leaves less margin for startup current, connection resistance, and changing generator output.
Generator Cord Safety
Use a cord intended for outdoor generator service with the correct plugs, grounding arrangement, voltage, conductor count, and amperage rating. Fully uncoil the cord during heavy use and protect connections from water and physical damage.
Never connect a portable generator directly to a household receptacle. Supplying a building requires approved transfer equipment installed according to manufacturer instructions and local electrical requirements. When in doubt, consult a licensed electrician.