Cord safely.
Right extension cord gauge for any tool, appliance, or outdoor device. Accounts for amps, length, and voltage drop.
How we calculated this
Extension cord gauges are rated by how much current they can handle at a specific length while keeping voltage drop under 3%. The three key variables: amps (the appliance load), length (how far from the outlet), and whether outdoor conditions require weather-rated jackets.
Voltage drop math follows the same formula as the wire size calculator: VD = 2 × I × R × L / 1000, where R is the resistance per 1000 feet of that gauge. Lower gauge numbers mean thicker copper, less resistance, less voltage drop. A 12 AWG cord has about 40% less resistance than a 14 AWG cord.
Length matters enormously. At 25 feet, a 16 AWG cord handles 13 amps. At 100 feet, the same 16 AWG cord is only rated for 7 amps, the extra length means the same current produces more voltage drop. For long runs with heavy loads, always step up one or two gauges.
The appliance presets are practical compromises. 'Small electronics' (lamps, phones, laptops) are usually under 5 amps. 'Power tools' (drills, circular saws, small air compressors) are 5-10 amps. 'Heaters and large tools' (space heaters, portable ACs, table saws) run 10-15 amps. 'Heavy equipment' (RV loads, large compressors, industrial tools) can draw 15-20 amps.
Outdoor cords need jacket ratings that survive moisture and UV: SJTW or W-rated (e.g., '12/3 SJTW' means 12 AWG, 3 conductors, thermoplastic jacket, weather-resistant). Indoor cords (SJT, SVT) aren't rated for outdoor use and degrade quickly in sun and rain. Running an indoor cord outside is a fire hazard if the jacket cracks.
For continuous loads (tools running more than 3 hours straight, or appliances that cycle frequently), derate by 20%: pick a cord rated for 125% of the load. Also: never daisy-chain extension cords, every connection point adds resistance. Use one long cord, not three short ones.
Gauge and length are one decision, not two
A 16 gauge cord runs a circular saw beautifully at 25 feet and starves it at 100. Nothing about the cord changed; the length did. Resistance accumulates over distance, voltage arrives lower than it left, and the motor compensates by pulling more current and running hotter. That is the entire subject in one sentence, and it is why the only correct question is what gauge for this length at this amperage.
The calculator above takes the load, the distance, and where the cord will live, then returns a gauge. This page carries the full gauge chart, how to read the letter codes stamped on the jacket, why a cord left coiled on a reel is derated, what a generator needs that an ordinary cord cannot give it, and the specific rules for holiday lights.
How we calculated these numbers▾
Capacity figures reflect standard published cord set ratings consistent with UL 817 and OSHA 1926.405 practice for temporary wiring, derated for length to hold voltage drop near 3 percent. Jacket letter codes follow NEC Article 400 nomenclature. Manufacturer ratings on a specific cord always govern; where a cord is stamped with a lower amperage than this chart suggests, the stamp wins.
The extension cord gauge chart
25 ft | 50 ft | 100 ft | 150 ft | |
|---|---|---|---|---|
| 16 AWG | 13 A | 10 A | Not suitable | Not suitable |
| 14 AWG | 15 A | 15 A | 13 A | Not suitable |
| 12 AWG | 20 A | 20 A | 15 A | 10 A |
| 10 AWG | 30 A | 30 A | 20 A | 15 A |
Maximum continuous amps by gauge and length. Cords are usually sold as 16, 14, 12, and 10 gauge; the lower the number, the thicker the copper and the heavier the cord.
The practical reading: 16 gauge is a lamp and laptop cord, fine in a bedroom and out of its depth anywhere near a tool. 14 gauge covers most household use up to 50 feet. 12 gauge is the honest general-purpose jobsite cord and the right default if you are buying one cord to own. 10 gauge exists for generators, welders, and long runs, and it is heavy enough that nobody buys it by accident.
Typical draw | At 50 ft | At 100 ft | |
|---|---|---|---|
| String lights, laptop, radio | Under 3 A | 16 AWG | 14 AWG |
| Drill, sander, box fan | 4-7 A | 16 AWG | 14 AWG |
| Circular saw, pressure washer | 10-13 A | 14 AWG | 12 AWG |
| Space heater, table saw | 12-15 A | 14 AWG | 12 AWG |
| Compressor, large miter saw | 15-20 A | 12 AWG | 10 AWG |
| Generator feed, welder | 20-30 A | 10 AWG | 10 AWG |
Common loads and the gauge they need at 50 and 100 feet. Nameplate amps are on the tool; when only watts are listed, divide by 120 to get amps.
Indoor, outdoor, and the letter that separates them
Every cord carries a code stamped along the jacket, and the letters are not decoration. S means service grade at 600 volts and SJ means junior at 300. T is a thermoplastic jacket, O adds oil resistance, E is an elastomer that stays flexible in cold, and W means weather rated, which is the only letter that authorizes outdoor use. A cord without a W is an indoor cord no matter how heavy it feels or what the packaging photo shows.
The number after the gauge is conductor count. A 12/3 cord has three conductors, meaning hot, neutral, and ground, and that ground is why a three prong cord belongs on anything with a metal housing. Two conductor cords have no ground path at all. For outdoor and tool use the specification worth memorizing is simple: three conductors, a W in the code, and a gauge chosen from the grid above.
Cord reels and the coiling problem
Retractable reels are genuinely convenient and they introduce a hazard most people never hear about. A conductor under load generates heat, and an uncoiled cord sheds that heat to the air along its whole length. Wound on a drum, the inner turns are insulated by the outer ones and the temperature climbs. Reel manufacturers publish two ratings for exactly this reason, and the wound figure is often around half the unwound one.
The rule is to unwind the full cord before drawing any real load, even if you only need ten feet of it. If a reel gets warm to the touch during use, that is the failure mode announcing itself. This applies to cords coiled on the floor too, not just reels: a cord run in a tight loop behind a workbench is doing the same thing on a smaller scale.
Generator cords are a different product
A generator cord is not a heavy extension cord with a different label. Portable generators output through locking connectors, typically an L14-30 for 30 amp 240 volt or an L5-30 for 30 amp 120 volt, and the cord that mates with them is usually 10 gauge with four conductors for the 240 volt version. It is built to carry the full output of the unit continuously, and it locks so vibration cannot shake it loose.
Two hard rules go with them. Never backfeed a generator into a wall outlet with a double-male cord, which energizes the utility lines and can kill a lineman; a transfer switch or interlock is the only correct connection to house wiring. And run the generator outdoors only, well away from windows and doors, because carbon monoxide from portable generators kills people every storm season. The cord exists so the generator can stay far outside while the load stays inside.
A three day outage after a storm. A 7,500 watt portable generator sits 40 feet from the house, running a refrigerator at 6 amps, a window AC unit at 9 amps, and a few lights and chargers at 3 amps, all on 120 volts.
Combined draw is 18 amps, which needs 12 gauge at 50 feet and would be marginal on 14. The right answer is a single 10 gauge 50 foot generator cord to a powered distribution box near the house, then short 12 gauge cords from there to each appliance, rather than three long cords daisy-chained back to the unit. Total cost is about $180 in cords, which is the cheapest line item in a hurricane kit and the one people skip. In Florida humidity every cord in that chain needs the W rating, since afternoon rain is not optional.
Composite illustration based on typical project dimensions, regional contractor pricing, and 2026 material costs. Not a specific real project.
Christmas lights and daisy chaining
Christmas and other string lights are a low-draw load with a very high connection count, which makes them the seasonal exception worth stating separately. The limit is not the cord, it is the manufacturer's maximum number of strings connected end to end, printed on the tag: often three to five for older incandescent sets and considerably more for LEDs. Exceeding it overloads the wiring inside the first string, not the extension cord.
The cord rules for Christmas lights are otherwise ordinary. Use an outdoor rated cord with a W. Do not connect two extension cords together to reach further, because every junction is a resistance point and a place water gets in; buy the length you need instead. Keep connections off the ground and out of standing water, or use a cord connector cover. And a cord run under a rug or through a doorway is being crushed and abraded every time someone walks over it, which is how the jacket fails and the fire starts.
Where extension cords go wrong
Chaining cords to reach. Two 50 foot cords make a 100 foot run with an extra connection and the voltage drop of the full distance. If the chart says 12 gauge at 100 feet, two 50 foot 14 gauge cords do not satisfy it.
Using an indoor cord outside. Common, and the reason is that indoor cords are cheaper and feel adequate. Moisture and UV degrade a non-W jacket, and the failure shows up months later.
Ignoring warmth. A cord that is warm along its length is undersized for what it is carrying. That is not normal operation and it does not settle down.
Treating a cord as permanent wiring. Extension cords are for temporary use. A cord powering a freezer in a garage year round, run through a wall, or stapled to a baseboard has become permanent wiring, which is both a code violation and the scenario behind a large share of cord related fires. If a location needs power permanently, it needs a receptacle, which is a wire size calculator question.
The rest of the power question
A cord is a temporary answer to a wiring question. Permanent circuits size through the wire size calculator, a shop or garage circuit often sits alongside a garage door calculator opener install, and if the reason for all the cords is a solar or battery setup, the solar calculator sizes the system itself.
Match the gauge to the length and the load, buy one cord long enough instead of two that reach, and unwind the reel before you pull the trigger.
Frequently asked
What gauge extension cord do I need for a 15-amp tool?
Depends on length. For 25 feet: 14 AWG is sufficient (rated 15A). For 50 feet: 12 AWG is needed (rated 15A at 50 ft; 14 AWG drops to 13A rated). For 100 feet: 10 AWG (12 AWG is rated only 13A at 100 ft). Always size up for long runs: voltage drop reduces tool power and can overheat motors.
What happens if I use a cord that's too thin?
Undersized cords overheat under load, melting insulation and causing fires. Even before fire: voltage drop causes motors to run hot and lose power (circular saws bog down, refrigerators short-cycle), LEDs dim, and electronic devices may not power on at all. The cord itself may feel warm or hot: that's a warning sign to upgrade.
Can I use an indoor cord outside temporarily?
Not safely. Indoor jackets (SJT, SVT) crack when exposed to UV and moisture, which exposes the internal conductors and creates shock/fire risk. 'Temporary' use tends to become regular use. Outdoor-rated cords (SJTW, W) cost only 20-30% more and last years in weather. Always use the right cord.
What do the numbers on an extension cord mean?
A marking like '12/3 SJTW 300V' means: 12 = 12 AWG gauge, 3 = three conductors (hot, neutral, ground), SJTW = thermoplastic jacket, weather-resistant, 300V = voltage rating. Higher numbers after AWG mean more conductors (mostly you want 3 for grounding safety). Lower AWG numbers mean thicker wire.
Can I daisy-chain extension cords?
No, every connection point adds resistance and creates a potential arc point. If you need 100 feet, use a 100-foot cord, not two 50-foot cords. Never connect a smaller gauge cord after a larger gauge (defeats the voltage drop math entirely). For long distances with heavy loads, consider a temporary single cord of the right gauge.
How many amps does a typical power tool draw?
Corded drill: 6-10 amps. Circular saw: 12-15 amps. Reciprocating saw: 8-14 amps. Miter saw: 13-15 amps. Angle grinder: 6-12 amps. Small shop vacuum: 6-10 amps. Large shop vacuum: 12 amps. Always check the tool's nameplate: stated amperage is the sustained load, motors draw more during startup.
What about 240V extension cords?
For 240V equipment (welders, RV hookups, large shop tools), you need a dedicated 240V extension cord with the appropriate NEMA plug (10-30, 14-30, 6-50, etc.). Gauge math is similar but voltage drop at 240V is percentage-wise lower for the same cord length. Never use a 240V cord on 120V circuits or vice versa.
Should I use a surge protector on extension cord runs?
For sensitive electronics (computers, TVs, audio equipment), yes, at the OUTLET end of the cord. Don't run surge protectors on thin extension cords or daisy-chain them. For tools and appliances, surge protection isn't needed. For outdoor use, pick outdoor-rated GFCI protection at the source instead of surge suppression.
Sources
- UL 817: Cord Sets and Power Supply Cords - The safety standard cord sets are listed to, including jacket and rating requirements
- OSHA 1926.405: Temporary Wiring and Flexible Cords - Jobsite rules on cord condition, grounding, and temporary versus permanent use
- NEC (NFPA 70) Article 400: Flexible Cords and Cables - The jacket letter designations and the prohibition on cords as permanent wiring
- ESFI: Extension Cord and Seasonal Decorating Safety - Holiday light string limits, outdoor connections, and daisy chaining guidance
- CPSC: Portable Generator Safety - Carbon monoxide siting rules and the prohibition on backfeeding through an outlet
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