What Size Extension Cord Do You Need for a Generator?
Figures last verified
The short answer
A 12-gauge copper extension cord is the useful default for many 120-volt generator loads because it carries more current and loses less voltage than 14- or 16-gauge cord. It is not a universal answer. The correct cord must pass four separate checks: load current, length, voltage drop, and connector rating.
Generator extension-cord voltage-drop calculator
Enter running watts and one-way cord length. This screens common copper gauges for both a conservative cord amp limit and calculated voltage drop. It does not validate plugs, receptacles, motor startup, wet-location protection, or the appliance manual.
Calculated running current
10.0 A
First passing gauge in this screen: 14 AWG copper (2.1% calculated drop).
| Gauge | Screen limit | Drop | Result |
|---|---|---|---|
| 16 AWG | 13 A | 3.3% | Does not pass |
| 14 AWG | 15 A | 2.1% | Passes inputs |
| 12 AWG | 20 A | 1.3% | Passes inputs |
| 10 AWG | 30 A | 0.8% | Passes inputs |
Resistance constants used: 16 AWG 4.016, 14 AWG 2.525, 12 AWG 1.588, and 10 AWG 0.999 ohms per 1,000 ft. Voltage drop = 2 x one-way length x current x conductor resistance. Use a listed outdoor-rated three-wire cord, keep every connector dry, and never use a male-to-male cord.
Wire gauge is only one limit
American Wire Gauge runs backward: a smaller AWG number means a larger conductor. Moving from 16 AWG to 12 AWG lowers resistance, which reduces voltage lost as current travels down the cord and back.
That still does not make every 12-gauge assembly a 20-amp cord. The molded plug, receptacle, jacket, temperature rating, number of conductors, and listing all belong to the finished cord. Read the rating printed on the cord and connectors. Never assign a cord a higher current rating merely because you recognize its conductor gauge.
The Consumer Product Safety Commission identifies minimum wire size, adequate strain relief, continuity, polarization, and an outdoor jacket as safety characteristics. Its consumer guidance also says high-wattage appliances need heavy-duty cords and outdoor work needs outdoor-rated cord.
Calculate running current before voltage drop
For a single-phase load:
current in amps = running watts / volts
A 1,200-watt load at 120 volts draws 10 amps. The same 1,200 watts at 240 volts draws 5 amps. Use the appliance nameplate current when available; watts divided by volts is a planning conversion, not a substitute for a manufacturer requirement.
Motor loads complicate this. Refrigerators, freezers, pumps, and compressors may demand several times their running current briefly at startup. The calculator above uses running current to screen the cord. You still must verify that the generator can supply the startup demand and that voltage at startup remains acceptable to the appliance.
Why length changes the answer
Current travels out and back, so a 50-foot extension cord creates roughly 100 feet of conductor path. The calculator uses:
voltage drop = 2 x one-way length x current x conductor resistance
Longer cord, more current, and smaller conductors all increase drop. A cord can remain below its printed amp rating and still deliver disappointing voltage at the appliance because the run is long. That is why a short ampacity chart and a voltage-drop calculation answer different questions.
The tool screens at either 3% or 5% calculated drop. Three percent is the more conservative planning target. The result is an estimate using standard copper resistance constants; connectors, temperature, coiling, damage, and conductor construction can change actual performance. Measure voltage under load when the consequence matters.
A practical selection sequence
- Read the appliance running amps or watts and its startup requirement.
- Confirm the generator receptacle voltage, amperage, and configuration.
- Measure the actual one-way cord route, including the distance needed to keep the generator outdoors and well away from openings.
- Choose a listed, grounded, outdoor-rated cord whose complete assembly rating covers the load.
- Calculate voltage drop and move to a larger conductor when necessary.
- Keep every connection dry, fully uncoil the cord, and inspect for heat or damage under load.
Do not place the generator closer to the building merely to use a short cord. CPSC says portable generators belong outside and far from doors, windows, vents, garages, basements, crawlspaces, and sheds. Use working carbon-monoxide alarms inside the home.
The cord cannot turn a receptacle into another circuit
A 120-volt cord cannot supply a 240-volt appliance. A 15-amp plug does not become a 20-amp connection because the conductors are large. Adapters can preserve a compatible wiring arrangement; they cannot add voltage, current capacity, grounding, neutral conductors, or overcurrent protection that the source does not provide.
Never use a cord with male plugs at both ends. CPSC warns that exposed prongs can become energized, and using such a cord to backfeed a home can electrocute people and start a fire. A house connection requires properly installed transfer equipment—not an improvised cord. See the transfer switch and interlock guide for that separate problem.
Evidence labels
- Confirmed: CPSC identifies adequate wire size and outdoor jackets as cord safety characteristics and instructs consumers to use heavy-duty, outdoor-rated cords for generator loads.
- Calculation: Current and voltage-drop results use the inputs and displayed formulas.
- Unknown until checked: startup current, connector condition, actual under-load voltage, cord assembly rating, and appliance tolerance.
Sources
- CPSC: Extension Cords Business Guidance, accessed 2026-08-12.
- CPSC: Generators and Engine-Driven Tools, accessed 2026-08-12.
- CPSC: Stop Using Male-to-Male Extension Cords, accessed 2026-08-12.
- OSHA: Grounding Requirements for Portable Generators, accessed 2026-08-12.
- Southwire: Bare Copper technical data, accessed 2026-08-12.
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