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Picking the Right Extension Cord Is a Safety Decision
An extension cord that is too thin for its load gets warm, drops voltage, and can damage tools or start a fire. A cord that is far too thick is heavy, stiff, and expensive. The right cord is the thinnest one that safely carries the tool current over the full cord length without excessive voltage drop. This selector finds it from three inputs: tool current, cord length, and supply voltage.
The selector checks two limits at once. First, the ampacity limit: each gauge has a maximum safe current, roughly 13 amps for 16 AWG, 15 amps for 14 AWG, 20 amps for 12 AWG, and 30 amps for 10 AWG. Second, the voltage drop limit: the selector requires the drop along the cord to stay at or below 3 percent of supply voltage. A cord passes only if it satisfies both limits, and the selector recommends the thinnest gauge that does.
How to Use the Selector
Enter the tool current in amps. You will find it on the tool nameplate, usually listed as amps or watts. If only watts are given, divide watts by volts to get amps: a 1,200 watt tool on 120 volts draws 10 amps. Then enter the cord length in metres, meaning the full length of the cord, not the distance to the nearest outlet minus slack. Finally choose 120 or 230 volts to match your supply.
The selector shows the recommended gauge, the estimated voltage drop, and the drop for every common gauge so you can see the trade-offs. If no common gauge meets both limits, it says so plainly. In that case, use a shorter cord, move closer to the outlet, or use a lower-current tool. Do not solve the problem by ignoring the result.
A Worked Example
Suppose you run a 12 amp circular saw on a 15 metre cord at 120 volts. A 16 AWG cord would drop about 4.7 percent, which fails the 3 percent limit even though its ampacity is technically enough for 12 amps. A 14 AWG cord drops about 2.5 percent and carries 15 amps, so it passes both limits, and the selector recommends 14 AWG. A 12 AWG cord would drop even less, but it costs more and weighs more, so it is not the recommendation unless you want extra margin.
Now run the same saw on a 30 metre cord. The 14 AWG drop doubles to about 5 percent, which fails. The 12 AWG drop is about 3.1 percent, which just fails too. The 10 AWG drop is about 2 percent, so the selector recommends 10 AWG. Length alone moved the answer two sizes thicker, which is why long cords need much more respect than short ones.
Why Voltage Drop Hurts Tools
Motors are the most sensitive loads. A motor running on low voltage draws more current to deliver the same power, which heats the motor and the cord together. The tool may run slowly, stall under load, or trip its thermal protection. Over time, chronic undervoltage shortens motor life. Heaters and lights are more forgiving: a heater just makes a little less heat, and a bulb just glows a little dimmer. But for power tools, voltage drop is a performance and safety issue, not a cosmetic one.
Also note that coiled cords trap heat. A cord rated for a given current when fully uncoiled can overheat when left coiled on a reel, because the heat has nowhere to go. Always fully uncoil a cord reel before running a heavy tool, and never run a cord under a rug or through a doorway that pinches it.
Common Mistakes to Avoid
The most common mistake is buying the cheapest thin cord for a heavy tool. The second is daisy-chaining two cords: two 15 metre cords do not behave like one 15 metre cord; they behave like a 30 metre cord, with double the drop. The third is ignoring the nameplate and guessing the current. A table saw, a welder, and a phone charger are not in the same universe, and guessing wrong by a factor of ten is easy.
This selector is an educational estimator. It uses typical ampacity and copper resistance values, but real cords vary, and local codes may require thicker gauges or specific cord types for outdoor or job-site use. Check your local electrical codes and consult a licensed electrician before doing any electrical work, especially for permanent or high-current installations.
Frequently Asked Questions
Can I use a thicker gauge than recommended? Yes, always. Thicker is safer and drops less voltage. The recommendation is the thinnest safe choice, not the only safe choice.
Does the selector cover outdoor cords? It covers the electrical sizing only. Outdoor use also needs a cord rated for outdoor conditions, with the right jacket and protection from water and damage.
What if my tool lists watts, not amps? Divide watts by your supply voltage. A 2,300 watt tool on 230 volts draws 10 amps.
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