Reading an Extension Cord Label Before You Plug In a Heater

Every winter I get the same kind of message. Someone plugged a space heater into the thin white extension cord that used to run a lamp, went to bed, and woke up to a smell of hot plastic. Sometimes the cord is just warm and a bit discolored near the plug. Sometimes the insulation has softened enough to show the shape of the carpet underneath. Twice in the past few years, readers sent photos of a cord that had melted through its jacket in a spot where it was tucked under a rug.

In nearly every case the cord was not faulty. It was doing exactly what a cord of that size does when asked to carry more current than it was built for. The information that would have prevented it was printed on the cord itself, in a row of small letters and numbers most people have never been told how to read. This is how to read them, for cords sold in the United States and Canada, and the point at which no extension cord is the right answer.

The two numbers that matter most

Somewhere on the jacket, usually molded or printed in a long repeating line, you will find something like "16/3" or "14/3". The first number is the wire gauge, measured in AWG, American Wire Gauge. The second is the number of conductors inside, which for a grounded three-prong cord is three: hot, neutral and ground.

The gauge works backward from what most people expect. A smaller number means thicker copper. A 12-gauge cord is much heavier than an 18-gauge one, and it can carry far more current before the wire heats up. That heat is the whole story. Every wire has some resistance, current passing through it creates heat, and a thin wire carrying a big load creates more heat than its insulation was designed to shed.

The second number to find is the amp or watt rating, often printed on a tag near the plug or on the packaging. If the tag is gone, the gauge gives you a reasonable estimate.

What each gauge can safely handle

These are typical ratings for common household extension cords at 120 volts. Always defer to the rating printed on your specific cord if it differs, and treat these as upper limits for a cord laid out straight, not coiled.

To check a device, look at its own label for watts or amps. If it lists only watts, divide by 120 to get amps. A typical 1,500-watt space heater draws 12.5 amps continuously, which already puts it at the top of what a 16-gauge cord tolerates, and it runs for hours at a time. A hair dryer pulls similar current but for ten minutes. Continuous load is what cooks a marginal cord.

The letters on the jacket

After the gauge, many cords carry a string of letters such as SJTW or SJOOW. They describe the jacket, and they are worth knowing if the cord goes anywhere outside a dry living room.

• S means service grade, a heavy-duty cord rated for 600 volts. SJ means junior service, rated for 300 volts and thinner, which is what most household cords are.
• T means the jacket is thermoplastic, the common vinyl type. E stands for a thermoplastic elastomer, which stays flexible in cold weather.
• O means oil-resistant. OO means both the outer jacket and the insulation on the inner wires resist oil.
• W means rated for outdoor use, resisting moisture and sunlight.

A cord without a W should never be left outdoors, even under a covered porch. The vinyl on indoor cords cracks after a season of sun and cold, and cracked insulation on a damp deck is how people get shocked. Look also for a certification mark from a recognized lab, such as UL or ETL. Cords sold online without one have a poor track record.

Why "just buy a heavier cord" is not the answer for heaters

The usual advice, once someone learns about gauge, is to buy a 12-gauge cord and run whatever they like through it. For most tools that is sound. For space heaters it misses the point, and I disagree with it.

Space heater manufacturers and the U.S. Consumer Product Safety Commission both tell you to plug heaters directly into a wall outlet, not into any extension cord or power strip. The reason is not only the cord. Every added connection, the cord's plug, its socket end, any adapter, is a point where a slightly loose contact can heat up under a long, heavy load. A heavy cord removes one weak point and leaves two others. If the heater cannot reach a wall outlet, move the heater, not the outlet.

If a plug or cord feels warm to the touch after an hour, it is telling you the load is too high for that connection. Unplug it and find out why.

Power strips share the same limit. Most are built around a 14-gauge cord and a 15-amp rating, no matter how many sockets they offer. We covered how that plays out on a kitchen counter in why an overloaded power strip overheats.

Habits that keep any cord out of trouble

Gauge and rating set the ceiling, but how you use the cord decides whether it gets near that ceiling.

Uncoil it fully. A coiled cord traps its own heat, and a long reel left wound on the drum while running a heavy load can overheat even when its rating looks fine on paper.

Keep it out from under rugs and furniture. Covering a cord stops heat from escaping and hides wear from foot traffic. This is where most of the melted cords in my inbox were found.

Use the shortest cord that reaches. Longer runs lose voltage along the way, and many manufacturers reduce the recommended load for 50- and 100-foot lengths of the same gauge. Check the packaging.

Treat extension cords as temporary. If a cord has run to the same appliance for months, that outlet location is the real problem. An electrician can add an outlet where you need it, and the breaker on that circuit will then be sized for the job, as explained in how circuit breakers decide when to cut power.

Retire damaged cords. Cut jackets, flattened spots, a loose plug, or a missing ground pin all mean the cord goes in the trash. Electrical tape is not a repair.

One check to make this week

Walk through your home and find every extension cord that is in use right now. For each one, read the gauge on the jacket and the watt rating on whatever is plugged into it. If anything that heats, a space heater, kettle, toaster oven, or electric blanket, is running through an 18- or 16-gauge cord, unplug it today and plug the appliance straight into the wall. It takes about fifteen minutes to check a whole house, and it is the cheapest fire prevention you will do all year. For more on how the system behind your outlets works, see the home electrics section.