Where the volts go
An extension cord is a resistor you have deliberately placed between the receptacle and the tool. The current goes out along one conductor and back along another, both of them drop voltage, and the drop is the same circular-mil arithmetic used for building wiring:
Volts dropped = (2 × 12.9 × amps × feet) ÷ circular mils
The 12.9 is the resistivity of copper expressed in ohm-circular-mils per foot at a typical warm operating temperature, and the 2 is because the current makes a round trip. Circular mils are the conductor area: 16 AWG is 2,580, 14 AWG is 4,110, 12 AWG is 6,530. Each step down in gauge number is roughly a 60 percent increase in area, so 12 AWG drops about 40 percent of what 16 AWG drops at the same length and load.
Everything else follows from that one equation. The drop is proportional to length, which is why a 100 foot cord is four times as bad as a 25 foot one. It is proportional to current, which is why a drill is fine on a cord that ruins a table saw. And the power turned into heat is the drop times the current, which grows with the square of the load.
The chart people actually want
Percentage drop at 120 volts, calculated with the formula above. Anything at or under 3 percent is invisible in use; 5 percent is the point where a motor starts working harder for the same result.
| Load | 25 ft | 50 ft | 100 ft | 150 ft |
|---|---|---|---|---|
| 5 A on 16 AWG | 1.0% | 2.1% | 4.2% | 6.3% |
| 10 A on 16 AWG | 2.1% | 4.2% | 8.3% | 12.5% |
| 15 A on 16 AWG | 3.1% | 6.3% | 12.5% | 18.8% |
| 15 A on 14 AWG | 2.0% | 3.9% | 7.9% | 11.8% |
| 15 A on 12 AWG | 1.2% | 2.5% | 4.9% | 7.4% |
| 15 A on 10 AWG | 0.8% | 1.6% | 3.1% | 4.7% |
The row that explains most job-site frustration is 15 amps on 16 AWG at 100 feet. Twelve and a half percent means the saw sees 105 volts. It will run. It will run badly, it will run hot, and the operator will conclude the saw is worn out.
Heat, and why coiling is the real hazard
Voltage drop is an annoyance. Heat is the safety problem. The power dissipated in a cord is the volts dropped times the amps flowing, and at 15 amps through 100 feet of 16 AWG that is roughly 225 watts spread along the length of the cord. Run out flat on a floor, that is about two and a quarter watts per foot, which the cord sheds to the air without trouble.
Left on the reel, the same 225 watts is concentrated inside a tightly wound coil where the inner turns are insulated by the outer ones and there is no airflow at all. The temperature in the middle of that coil rises until something gives, and what gives is the insulation, which softens, sticks, and eventually allows conductors to touch. This is why cord reels carry two ratings, one for coiled use and one for fully unwound, with the coiled figure sometimes less than half. It is also why a cord that is warm along its whole length is a warning rather than a curiosity, and why a cord that is hot in one spot is worse — a localised hot spot usually means a damaged conductor or a failing connection rather than distributed resistance.
Chaining cords, and what the gauge does not tell you
Two 50 foot cords are electrically 100 feet, plus one more plug-and-socket junction. The junction adds resistance of its own, and worn or corroded contacts add a great deal more, concentrated in a very small volume where it does the most damage. Chaining a heavy cord to a light one is worse than either: the run behaves like the lightest section for heating purposes, and people forget which section is which. If the run needs 12 AWG, all of it needs 12 AWG.
Gauge is also not the whole specification. The jacket determines where the cord may be used — outdoors, in wet conditions, on a job site where it will be dragged across concrete — and the temperature rating matters near heaters and engines. A cord with a cut jacket, a missing ground pin or a repaired splice is a hazard whatever the arithmetic says. And no cord is a substitute for a receptacle where the load is permanent; a run of cord feeding a freezer in a garage every day is a wiring problem wearing a temporary disguise.
Electrical work on a building generally needs a permit and an inspection, and many jurisdictions restrict who is allowed to do it at all. An error in this part of a house burns houses down and kills people. This page sizes and estimates. It does not replace the judgement of a licensed electrician, and it gives no instruction for opening equipment or making connections.
See also the voltage drop calculator for permanent runs, generator sizing calculator for what feeds the cords on a site, and appliance running cost calculator for what the wasted heat costs.
Questions people ask
What gauge extension cord do I need for 100 feet?
It depends on the load, which is why the gauge charts printed on cord packaging carry both. At 100 feet on 120 volts, staying under about 5 percent drop needs roughly 16 AWG up to 5 amps, 14 AWG up to about 9 amps, 12 AWG up to about 15 amps, and 10 AWG beyond that. The practical answer for anyone buying one cord to keep in the truck is 12 AWG at 100 feet, because it covers everything up to a full 15 amp tool with margin and it costs less than finding out the hard way. It is heavy and it is worth it.
Is it dangerous to leave an extension cord coiled?
Under load, yes, and it is the most common cord hazard there is. The cord dissipates heat in proportion to the current squared, and a coil has no way to shed it — the inner turns are surrounded by insulation on all sides. The insulation softens, adjacent turns fuse, and eventually conductors make contact. Cord reels are rated twice for exactly this reason, with a much lower figure for coiled use, and the rating on the drum is the one that applies when the cord is not fully unwound. For anything drawing serious current, pull the whole cord off the reel even if you only need ten feet of it.
Can I plug two extension cords together?
Electrically it works and the drop is what the combined length gives, which the calculator above handles with the cords-in-series field. Two practical cautions. The junction is an extra pair of contacts, and contact resistance in a worn or corroded connector produces heat in a very small volume, which is exactly the condition that melts plug faces. And mixing gauges is worse than using the lighter one throughout, because the run heats according to its weakest section while people assess it by the heaviest. Many job-site safety policies prohibit chaining outright, and where a run genuinely needs to be that long, a single cord of the right length and gauge is the correct answer.
Why does my saw bog down on a long cord?
Because a motor is roughly a constant-power device. Give it less voltage and it draws more current to produce the same mechanical output, and that extra current deepens the voltage drop, which lowers the voltage further. The result is a motor running hot at reduced torque, and a saw that stalls in cuts it should walk through. Resistive loads behave much more gently — a heater on a long cord just runs cooler and nothing is damaged — which is why the same cord that is fine for a work light ruins a compressor. If a tool bogs on a cord and runs correctly plugged into the wall, the cord is the diagnosis.
Does the length printed on a cord include both conductors?
The printed length is the physical length of the cord, and the calculation doubles it, because current flows out along one conductor and back along another and both drop voltage. A 100 foot cord contains 200 feet of conductor in the circuit path. This is the single most common arithmetic error people make when checking a cord against a chart, and it always errs in the dangerous direction — halving the effective length makes the drop look half what it is.