Extension Cord Sizing: Wire Gauge, Length, and Amperage

Wire Gauge Basics

Extension cord wire gauge uses the AWG (American Wire Gauge) system, where lower numbers mean thicker wire. This is the counterintuitive part: a 12-gauge cord is thicker and carries more current than a 16-gauge cord. Thicker wire has less electrical resistance, which means less voltage drop and less heat buildup under load.

The common gauges for extension cords and what they handle:

• 16 AWG — light-duty. Lamps, fans, phone chargers, small electronics. Not for power tools.
• 14 AWG — medium-duty. Shop vacuums, sanders, jigsaws, and other tools drawing under 12 amps.
• 12 AWG — heavy-duty. Circular saws, miter saws, table saws, and other tools drawing 12 to 15 amps.
• 10 AWG — extra-heavy. Air compressors, welders, and tools drawing over 15 amps at long distances.

The gauge is printed on the cord jacket. Look for markings like "12/3 SJTW" where the first number (12) is the gauge and the second (3) is the number of conductors (hot, neutral, ground). The letter codes indicate the cord's construction and ratings. The S means it is a flexible service cord. The W at the end means it is rated for outdoor/wet use. For tool use, the gauge number is the one that matters most.

Length Changes Everything

A 12-gauge cord at 25 feet delivers clean power to a 15-amp saw. The same 12-gauge cord at 100 feet drops enough voltage that the saw bogs down on hardwood. This is the voltage drop problem. Electrical resistance is proportional to length: double the length, double the resistance, double the voltage drop. A cord that works perfectly from a nearby outlet causes real problems when you run it across the yard to a job site.

Voltage drop matters because electric motors are sensitive to supply voltage. A 15-amp circular saw designed to run at 120 volts performs differently at 110 volts. It spins slower, generates more heat, draws more current to compensate, and wears faster. On a long, undersized cord, the voltage at the tool end can drop to 105 volts or less under load. The motor compensates by pulling more amps, which heats the cord further, which increases resistance, which drops the voltage more. It is a feedback loop that ends with a tripped breaker, a melted plug, or a burned-out motor.

The practical rule: for every 50 feet of cord length, go up one wire gauge from the minimum. A 15-amp tool needs 14-gauge at 25 feet, 12-gauge at 50 feet, and 10-gauge at 100 feet. If the cord feels warm after running a tool for 10 minutes, the gauge is too light for that combination of current draw and distance. For more on the tradeoffs between corded and cordless, see our corded vs. cordless guide.

Amperage Ratings and How to Check

Every power tool has an amp rating stamped on the nameplate or printed in the manual. This is the maximum current the tool draws under full load. Typical ratings for common tools:

• Random-orbit sander: 2 to 3 amps
• Jigsaw: 5 to 7 amps
• Router: 8 to 12 amps
• Circular saw: 13 to 15 amps
• Miter saw: 12 to 15 amps
• Table saw (portable): 13 to 15 amps
• Air compressor: 12 to 20 amps

The extension cord must be rated for at least the tool's amp draw. Running a 15-amp saw on a cord rated for 10 amps overloads the cord, even if the cord is only 10 feet long. The amperage rating is about current capacity, not distance.

Extension cords have their amp rating printed on the packaging or molded into the plug housing. If you cannot find the rating, use the gauge as a proxy: 16-gauge handles up to 10 amps, 14-gauge handles up to 15 amps, 12-gauge handles up to 20 amps, and 10-gauge handles 25 to 30 amps. These ratings assume a 25-foot cord. Derate for longer runs.

Quick Reference: Gauge by Tool

Here is the practical summary. Find your tool's amp range and planned cord length, then use the corresponding gauge.

• Sanders, jigsaws, rotary tools (2 to 7 amps) — 16-gauge up to 50 feet, 14-gauge up to 100 feet.
• Drills, routers, small grinders (7 to 12 amps) — 14-gauge up to 50 feet, 12-gauge up to 100 feet.
• Circular saws, miter saws, table saws (12 to 15 amps) — 12-gauge up to 50 feet, 10-gauge up to 100 feet.
• Air compressors, welders (15+ amps) — 10-gauge up to 50 feet. For longer runs at this amperage, consult an electrician about a dedicated circuit.

When in doubt, go one gauge heavier than you think you need. A 50-foot 12-gauge outdoor cord costs about $30 to $50, roughly $10 more than a 14-gauge cord of the same length. That $10 handles everything up to 15 amps at 50 feet and eliminates the guesswork. Buy one and you are covered for most workshop and job-site situations.

Indoor vs. Outdoor Cords

Indoor extension cords are not rated for moisture, UV exposure, or abrasion on rough surfaces. Using an indoor cord outside can lead to insulation cracking, shorts, and shock hazards. For any outdoor power tool use, buy a cord marked with "W" in the type code (for example, "SJTW"). The W means it is rated for wet locations and has a tougher jacket that resists sunlight and abrasion.

Outdoor-rated cords also work fine indoors. If you are only going to own one extension cord for tool use, buy an outdoor-rated 12-gauge, 50-foot cord. It handles the widest range of tools and conditions. The thicker jacket is a little stiffer to coil, but the versatility is worth it.

Signs of a Bad Cord

Inspect your cords before each use. It takes 10 seconds and can prevent serious problems.

• The cord is warm or hot to the touch after running a tool. This means the gauge is too light for the load or the cord has internal damage increasing resistance.
• The tool runs noticeably slower than when plugged directly into the wall outlet.
• The plug or receptacle end shows melting, discoloration, or a burnt smell. This is usually caused by a loose connection generating heat at the contact point.
• The cord jacket is cracked, cut, or has exposed wire anywhere along its length.
• The ground prong is missing or bent. Never remove the ground prong to fit a two-prong outlet.

Any of these means the cord is undersized, damaged, or both. Replace it. A $30 to $50 cord is cheaper than a burned-out motor or a house fire.

Frequently Asked Questions

Can I use an indoor extension cord for outdoor power tools?

No. Indoor cords are not rated for moisture, UV exposure, or abrasion on rough surfaces. Use cords marked "W" in the type code (for example, SJTW) for outdoor use. The W means the cord is rated for wet locations. Indoor cords used outdoors can short, shock you, or degrade rapidly.

Can I daisy-chain two extension cords?

Technically yes, but it doubles the effective length and the voltage drop. Two 50-foot 14-gauge cords in series perform like a single 100-foot 14-gauge cord, which is not enough for a 15-amp tool. If you need 100 feet of reach, buy a single 100-foot cord in the right gauge. A single cord also eliminates the connection point between the two cords, which is where most cord failures happen due to loose contacts and moisture entry.

How long does an extension cord last?

With normal use, 5 to 10 years. Inspect the cord, plug, and receptacle end before each use. Replace any cord with cracked insulation, bent prongs, loose connections, or visible damage. The cord ends are the first failure point because they get stepped on, driven over, and yanked out of outlets by the cord instead of the plug. Treat the ends gently and the cord lasts.

Related Reading

• Corded vs. Cordless Tools
• What Is Wire Gauge?
• What Is Voltage Drop?
• What Are Amps?