Extension Cord Size Table
| Load (amps) | 25 ft | 50 ft | 100 ft |
|---|---|---|---|
| 0-2 | 16 AWG | 16 AWG | 16 AWG |
| 2.1-3.4 | 16 AWG | 16 AWG | 14 AWG |
| 3.5-5 | 16 AWG | 14 AWG | 12 AWG |
| 5.1-7 | 14 AWG | 12 AWG | 10 AWG |
| 7.1-12 | 12 AWG | 10 AWG | 8 AWG |
| 12.1-16 | 10 AWG | 8 AWG | 8 AWG |
| Gauge | Copper dia. | Typical loads it can carry |
|---|---|---|
| 16 AWG | 1.29 mm | Lamps, clocks, chargers, small electronics |
| 14 AWG | 1.63 mm | Vacuum, fans, drills (short runs) |
| 12 AWG | 2.05 mm | Space heater, circular saw, air compressor (25 ft) |
| 10 AWG | 2.59 mm | Big tools and long runs, welder input (short) |
| 8 AWG | 3.26 mm | Heavy loads at 50-100 ft - often cheaper to move the outlet |
An extension cord is a resistor you walk on: every foot of copper adds resistance, and the longer the run, the more of your appliance's voltage turns into heat inside the cord instead of work at the tool. That is why gauge and length must be read together - the chart below is the standard consumer pairing, the minimum wire gauge (AWG, where smaller numbers mean thicker copper) for a given load across 25, 50 and 100 foot runs. A 16-gauge cord is fine for a lamp at any length you would tolerate, carries a vacuum cleaner only to about 50 feet, and is genuinely the wrong tool for a space heater at any distance.
The physics is unforgiving but simple. Voltage drop is current times resistance, and resistance doubles when the run doubles or the gauge drops two steps; a 13-amp load on 100 feet of 16 AWG can strand more than ten percent of your voltage in the cord - which a motor feels as sluggishness and the cord feels as heat. Heat is the failure mode: cords are rated to stay cool in open air, which is why the single most dangerous thing you can do with an extension cord is leave it coiled while it carries load - the inner wraps cannot shed heat and the insulation cooks. The rules at the bottom of this page are the workplace ones (OSHA's flexible-cord provisions), and they are worth reading at home too: no staples, no doorways, no dangling by the cord, no cut ground pins.
How to use
- Start from the load, not the cord: read the appliance's amps (watts divided by 120 for US mains) - a 1,500-watt heater draws 12.5 amps, a vacuum 8-10, a lamp under 1.
- Pick the row, then the column: find your amp band, then your run length - the cell is the minimum gauge. When you land between cords, buy the thicker (lower-number) one; the price difference is a few dollars, the heat margin is real.
- Deduct for cordage you can't see: cord reels and wound-up cords count their full length even if only part is paid out - a half-spooled 50-foot reel behaves like 50 feet of cord wrapped around a hot core, not 25 feet in open air.
Frequently asked questions
Can I run a space heater on an extension cord?
Almost never. A 1,500-watt heater draws 12.5 amps continuously - hours at a stretch, which is the hardest duty a cord can see - and the chart puts that load at 12 AWG even for a 25-foot run. Most household extension cords are 16 AWG, rated for roughly 10 amps at short runs, and the failure is cumulative: the cord warms, insulation ages, a kink or a door across it concentrates the heat, and eventually the cord (not the heater's breaker) is the hottest object in the room. If the heater must be on an extension cord, use a short (under 25 feet), heavy 12 AWG cord rated for the load, plug it directly into the wall outlet, and never daisy-chain two cords - but the manufacturer instruction you are overriding says plug it straight in, and that remains the correct answer.
Why does a longer extension cord need to be thicker?
Resistance is proportional to length and inversely proportional to copper cross-section, so a 100-foot run has four times the resistance of a 25-foot run of the same gauge - and voltage drop is current times resistance, all of it becoming heat along the cord. Worked example: 13 amps through 100 feet of 16 AWG wastes roughly 30 watts as heat and leaves your tool around 105 volts instead of 120 - a motor slows, a heater element runs cooler, electronics with marginal supplies may brown out. Stepping up two gauge sizes (16 to 14, 14 to 12) roughly halves the resistance, which is why each doubling of length wants a thicker cord at the same load. The chart is that rule of thumb, tabulated.
Is it bad to leave an extension cord coiled up while in use?
Yes - it is the textbook failure mode, and OSHA's flexible-cord rules prohibit it explicitly for workplaces. A cord's ampacity assumes it can shed heat to open air along its whole length; a coil traps the heat of every wrap together, so a 10-amp load that is safe payed out can cook a spooled cord - the inner turns see no airflow and insulation melts where you cannot see it. This is exactly why cord reels ship with a warning to fully unwind under heavy load. The same family of rules: never staple or nail a cord to a wall (the fastener can cut insulation), never run it through a doorway or window where it gets pinched, never hang anything from it or dangle it by the cord from a hook, and never bind it with wire - all of them are ways of concentrating mechanical or thermal stress on a component designed for gentle duty.
What do the letters on an extension cord mean (SJTW, SJOOW, W)?
The jacket code is a duty rating. S = service (heavy-duty) cord, J = junior (lighter 300-volt jacket), T = thermoplastic insulation, O = oil-resistant outer jacket, W = rated for outdoor use (sunlight and water). For outdoor work you want a W-marked cord - SJTW is the common garden-variety outdoor cord - and a bright jacket, because the other outdoor killer is a lawnmower or trimmer finding an invisible white cord in the grass. Indoor-only cords harden and crack in UV and cold. The three-prong plug matters as much as the jacket: the round pin is the equipment ground, and cutting it to fit a two-slot outlet removes the only path a fault current has besides your body - use a proper adapter with a ground tab screwed to the outlet box, or replace the appliance cord.