Why Extension Cords Are Dangerous and What Causes Fires

Extension cords are dangerous mainly because they can overheat. A cord that is too thin, too long, damaged or overloaded resists the flow of electricity and turns some of it into heat, which can melt insulation and ignite nearby materials. Most cord fires come from misuse, so matching the cord to the load and using it correctly prevents most of the risk.

How a cord turns electricity into heat

Every wire resists electric current a little. The more current that flows, the more of that energy becomes heat in the wire. A thick wire has less resistance and stays cooler. A thin wire has more resistance and warms up faster under the same load.

Length matters too. A longer cord has more wire, so it has more total resistance. That is why a thin, long cord running a hungry tool is a classic setup for trouble. The heat builds inside the jacket where you cannot see it, and it can soften the insulation until bare conductors touch.

Plugs and sockets are weak points as well. A loose or dirty connection creates extra resistance at one spot, so the plug can get hot even when the cord looks fine.

The most common causes of extension cord fires

Fires rarely have a single mysterious cause. They usually trace back to one of a few habits or conditions.

Overloading: plugging in devices that draw more amps or watts than the cord is rated for, or putting several heaters or tools on one cord.
Wrong wire gauge: using a thin cord for a power tool, space heater or other heavy appliance.
Coiled or bundled cords: heat cannot escape from a cord left wound on a reel or piled in a heap while in use.
Damaged insulation: cuts, cracks, crushed spots, exposed wire or bent prongs.
Running cords under rugs, furniture or through doorways, where heat gets trapped and damage goes unnoticed.
Indoor-only cords used outside, where moisture and sun break down the jacket.
Daisy-chaining: joining several cords or plugging a power strip into a cord, which adds resistance and extra connections.
Permanent use: treating a temporary cord as fixed wiring for months or years.

Match gauge and ratings to your load

Wire gauge is the first thing to check. Lower AWG numbers mean thicker wire, and thicker wire carries more current safely. Each cord also lists rated amps and watts, which are your upper limits. Add up the watts of everything you plan to plug in and keep the total under the cord's rating.

On our best extension cord comparison, the 12 AWG cords are rated 15 A and 1875 W, while the 16 AWG cords are listed at 13 A and 1625 W or 10 A and 1250 W. All of these are rated for 125 V.

Rated watts equal volts times amps, so you can check the math yourself. A 12 AWG cord at 15 A and 125 V works out to 1875 W.

Wire gauge	Rated amps	Rated watts	Length in our data
12 AWG	15 A	1875 W	25 ft, 50 ft and 100 ft
16 AWG	13 A	1625 W	6 ft indoor cord and a 25 ft outdoor cord
16 AWG	10 A	1250 W	100 ft

Damage, moisture and shock risks

Fire is not the only danger. A cord with cracked insulation can expose live wire, which creates a shock risk, especially in damp places. Water and electricity do not mix, and a wet connection can trip a breaker or shock you.

Use cords only where they are rated. Indoor-only cords lack weather protection. For outdoor work, look for an outdoor or waterproof rating. Several of the outdoor cords on our comparison list IP65 water resistance and a minimum flexible temperature of -58°F, which helps the jacket resist cracking in the cold.

Even a waterproof cord has connection points. Keep the plug and socket ends out of puddles and standing water, and plug outdoor cords into a GFCI-protected outlet when you can. A GFCI cuts power quickly if it senses current leaking where it should not.

Safe habits that prevent most problems

Good habits cost nothing and remove most of the risk. Follow these steps each time you use a cord.

Check the cord before use. Look for cuts, cracks, bent prongs and warm or discolored plugs.
Add up the watts of your devices and confirm the total is under the cord's rating.
Uncoil the cord fully. Several manufacturers say to remove storage straps and avoid piling it up during use.
Use the shortest cord that reaches your outlet.
Plug the cord in fully so the prongs make solid contact, and never force a three-prong plug into a two-prong outlet.
Keep cords out of walkways, away from heat sources and out from under rugs.
Unplug the cord when you finish, and store it loosely coiled in a dry place.

What to look for when you buy

A good cord starts with the right specs. Choose a gauge that fits your heaviest tool, pick a length close to what you need, and match the indoor or outdoor rating to where you will use it. Our guide to choosing a best extension cord walks through these checks and puts the specs side by side.

Look for a lab mark such as ETL or ETL/cETL. It shows a recognized lab has listed the cord. On our comparison page, six of seven products list one, so check the spec sheet before you assume. A lighted end is a small extra that shows when power is on, and strain relief at the plug and socket helps protect the points where cords often fail.

If you often run heavy loads over long distances, thicker wire matters more than any extra feature. A 16 AWG cord can be fine for light, short jobs, but it has lower ratings than a 12 AWG cord of the same length.

When to stop using a cord

Retire a cord right away if the jacket is cracked, wire is showing, the plug is loose or the cord feels hot to the touch. Do not patch damaged insulation with tape and keep using it. Replace the cord.

If a plug or outlet smells like burning, unplug the cord if you can do so safely and stop using that outlet until an electrician checks it. If you see flames or smoke, leave the area and call 911. Never throw water on an electrical fire.

Frequently asked questions

Is it safe to plug one extension cord into another?

It is best to avoid it. Each joint adds resistance and a possible loose connection, and the total length can overload thin wire. Buy one cord of the right length instead.

Is it normal for an extension cord to feel warm?

A slightly warm cord under a steady load can happen. A cord or plug that feels hot is a warning sign. Unplug it, check the load against the cord's rating, and replace the cord if the heat continues.

Can I leave an extension cord plugged in all the time?

Cords are meant for temporary use. If you need power in the same spot every day, have an electrician add an outlet rather than relying on a cord.

Does a longer cord carry less power?

A longer cord loses more power along its length and runs warmer under the same load. That is why thicker wire matters on long runs.