For a 20-foot run, match the wire gauge to the device’s actual amperage: 16 AWG handles light loads near 10A, 14 AWG covers about 10–15A, 12 AWG suits 15–20A, and 10 AWG is the pick for a full 20A load.
Twenty feet is short enough that distance barely matters — current draw decides everything. Plug a 12-amp circular saw into a lamp cord and the saw bogs down, the cord warms up, and the motor strains on startup. Size the same saw with 14 or 12 AWG and it runs the way it should.
Amp rating is not the only number that matters. The device’s plug shape, its startup draw, and where you plan to use the cord all shape the decision, so the table below is a starting point, not the whole story.
What Gauge Fits Your Amp Draw at 20 Feet?
The industry sizing charts broadly agree on four bands, and the pattern holds at 20 feet as well as 25. More current needs thicker copper.
| Device Amperage | Recommended Gauge | Typical Use |
|---|---|---|
| Under 10A | 16 AWG | Lamps, chargers, fans, small electronics |
| 10–13A | 16 AWG (chart max) or 14 AWG | Light-duty tools, shop vacs on short runs |
| 13–15A | 14 AWG | Drills, smaller mowers, medium appliances |
| 15–18A | 12 AWG | Circular saws, table saws, space heaters |
| 18–20A | 12 AWG (20A on most charts) | Heavy shop tools, larger compressors |
| 20A continuous | 10 AWG | Compressors, welders, high-startup loads |
| Outdoor, wet, or heavy use | 12 AWG or 10 AWG, outdoor-rated jacket | Yard tools, pressure washers, patio equipment |
The three charts behind this table land in slightly different places on the edges. A second, more conservative U.S. rule at 120V and under 5% voltage drop gives 14 AWG for 13–15A, 12 AWG for 15–20A, and 10 AWG for 20–30A. When charts disagree, take the thicker wire.
What Decides the Right Gauge Beyond Amps?
Two devices with the same running amps can need different cords, because motors and compressors pull far more current in the first second than they do once spinning. That startup surge is what overheats an undersized cord and trips breakers.
- Plug and grounding: most household tools and appliances need a 3-prong grounded cord. A 2-prong lamp cord will not fit, and adapters defeat the safety ground.
- Breaker vs. device: a 20A breaker does not mean the device draws 20A. Size to the device’s running amps, not the circuit’s rating.
- Outdoor and wet use: an indoor light-duty cord used outside in rain or on a 15A load is a genuine shock and fire risk. Outdoor-rated jackets and thicker copper handle both.
- Voltage: these charts assume 120V U.S. circuits with copper conductors. Other voltages or cord constructions change the math.
Undersizing shows up as dimming lights, a tool that bogs down mid-cut, a cord that feels warm, or a motor that refuses to start. All four mean the copper is too thin for the job.
Mistakes That Lead to the Wrong Cord
The most common error is buying a thin cord because the run is “only 20 feet.” Length is rarely the deciding factor at this distance — current is.
Matching the cord to the breaker instead of the device, ignoring startup surge, and grabbing an indoor lamp cord for outdoor work round out the list. If you would rather skip the guesswork, a vetted set of tested 20 ft extension cord picks covers the common gauges and jacket types. For the underlying ratings, the gauge-to-amp sizing reference lays out the same bands in chart form.
FAQs
Is a 16 AWG cord safe for a space heater?
No. Most space heaters draw 12–15A continuously, which exceeds the 16 AWG range on standard sizing charts. Use 14 AWG at minimum and 12 AWG if the heater runs near 15A. A cord that feels warm during use is a clear sign the gauge is too small.
Does a 20-foot run really need a thicker cord than a 10-foot run?
Usually no. At 20 feet, voltage drop stays small enough that current draw, not distance, drives the choice. A 14 AWG cord handles the same load at 20 feet as it does at 10. Distance only starts to force a thicker gauge on much longer runs.
Will a 12 AWG cord work for a 20A tool?
It can, since many charts rate 12 AWG up to 20A at 25 feet. For 20A continuous loads or tools with heavy startup surge, 10 AWG gives real margin against voltage drop and heat. If the tool runs for long stretches, go with 10 AWG.
References & Sources
- Pro Tool Reviews. “Extension Cord Size Chart – Wire Gauge vs. Amps” Supports the 16/14/12/10 AWG amp bands at 25 feet.
- Safety in Numbers. “Extension Cords” Supports the conservative 120V, 5% voltage-drop sizing rule.
- Vantec Cable. “Extension Cord Gauge Chart: What Size Do You Need?” Supports the narrower 16–10 AWG amp ranges cited.
