Amperage rating on extension cord

Recently, I bought a new extension cord for my electic lawnmower. I didn’t know there were different types of extension cords for different purposes, and I just bought the longest one I could find at my local hardware store that looked like an “outdoor” type.

When I got home, I used it with my lawnmower with no problems.

Later, when I was discarding the packaging for the cord, I noticed that there was a note on the package that said that the cord was rated for 10 amps, and some kind of warning related to that. I think it said it should not be used with tools that require more than 10 amps, to avoid damage to the tools. The next time I went to cut the grass, I checked, and my lawnmower has a sticker on it that says something like “12 A max”.

Is my lawnmower in danger? Am I? If it is only my lawnmower at risk, I’m not going to bother getting a new cord, because it is an old lawnmower, and I would much rather have one of the new-fangled battery powered ones so I don’t have to deal with the cord at all.

Thanks in advance for any info you can provide.

If there is a hazard, it’s for the cord, not the mower. Does the cord get hot when you mow? If so, maybe you should get a bigger cord. By the way, if the cord plugs into another extension cord, the capacity of both cords goes down, for reasons I cannot explain adequately.

I am not a fizzy cyst.

Resistance increases for longer cords, or linked cords, resistance decreases for thicker cords (the actual conductors, not the rubber stuff).

Resistance represents an exenditure of power, and that power produces heat.

So, your mower operates at a lower voltage, and has correspondingly less power. For a motor, ususally that is not terribly hard on it, just makes it work less efficiently. For electronics it can cause damage, in some cases.

Heat can melt the insulation, cause shorts, set stuff on fire, etc.


If you inquire with the manufacturer of the mower, they will probably tell you that you are not using the correct extension cord, but it is doubtful you could damage the mower.

The “12A max” referees to the maximum current the mower will draw. Most residential construction provides outside outlets that allow you to draw a max of 15 amps (this is dictated by the breaker your outlet is tied to). The extension cord you are using will not draw that much power because the size of the conductors and the length of the extension cord prevent it from doing so.

The US Government, the NFPA, and UL, all do a dance together to insure consumer product safety. The NEC published by the NFPA, provides tables that give guidelines for the “ampacity” of electrical cables. The AWG size of the individual conductors, the length, and the environment it is used in, dictate the “ampacity” rating of your extension cord - and again, it will most likely not allow you to draw more than 10 amps. Think of the conductors like water pipes - bigger pipes - more water. Bigger conductors - more amps.

The worst that will happen is your extension cord will wear out much faster if you consistently pull 10 amps or more, while bending it, and stepping on it, and dragging it around in the summer heat. The other draw back with using an extension cord that would not allow the mower to draw the amps it needs, is that ithe mower will simply not work as well as it could. Using an extension cord with a 15 or 20 amp rating would allow the mower to draw up to 12 amps when needed.

Many local fire departments have a display of items that have caused or been in fires. It’s not unusual to see one of those extension cord reels where the fire was caused by the cord delivering it’s rated, or higher, current while most of it’s still wound on the reel.

Always completly unwind the cord if you are going to be using the cord anywhere near it’s rated capacity.

This is true.

This is not true. If the motor wants 12 amps, it’ll get it. The cord will heat up and the voltage will drop, but you will still get 12 amps to run through that cord till it opens up like a fuse. In reality that cord could probably pull 20 amps through it continuously before it goes pooof and maybe even 40 amps for a short time.

If the cord wouldn’t allow more than 10 amps there would be no fire hazard.

E=IR, resistance is fixed then the voltage goes down as the current goes up. In a small cable like in the OP the effect will be more noticeable, but I would think that the manufacturer has some fudge built into the cable and that the UL listing on it means that it’s been tested and de-rated appropriately.

Your lawnmower may have a shortened life. If you load the machine down (high/thick grass) it won’t be getting the proper voltage it needs to run efficiently (voltage drop due to smaller cord). This may heat the motor windings and associated control parts up enough over time to shorten their life.

According to the description here, a motor will try to draw the same power, and thus draw more current to make up for the lower voltage. More current means greater resistive losses, which means the windings heat up more. This is likely the effect the warning label on the extension cord is referring to.

Every extension cord has a current limit. This is because the cord can become a fire hazard when too much current flows through it.


For a given length of 2-conductor extension cord, each copper conductor will dissipate power in the form of heat. The amount of power dissipated by each copper conductor is a function of the linear resistance of the copper conductor and the square of the current. This translates into a temperature rise for each copper conductor. And when the temperature rises for each conductor, the insulation surrounding each conductor will also rise (and this is where it becomes a fire hazard issue).

The amount of temperature rise in each conductor is a function of:

  • linear resistance (which is determined by wire gage) (Ohms)
  • RMS current through the conductor (amps)
  • The thickness of insulation around the conductor
  • The type of insulation around the conductor

The manufacturer has determined that, for a given ambient temperature, the temperature of the insulation surrounding each conductor is below a critical level when 10 A rms flows through each conductor. If more than 10 A rms flows through each conductor, the temperature of the insulation surrounding each conductor may rise above the critical level.

Note that a conductor operating at 12 A rms will dissipate 44% more power (in the form of heat) vs. operating at 10 A rms. It is wise to pay attention to manufacturers’ ratings.

One thing not mentioned here is that an electric motor will draw a lot of current when going from stopped to full speed but once it hits full speed, it’s only drawing a fraction of the initial start current.

The 12A max current marking is probably, therefore, the start current of the motor since it’s listed as a “max” value.

This start current problem is why most multi-speed motors, ceiling fans and so forth, have their first position after “off” as “high”. Starting the motor on high gets it spinning quickly and reduces the amperage draw more quickly. This lenghthens the motor’s lifetime since the coils don’t get a chance to get too hot for too long.

So, my suspicion is that you’ll be just fine. I’ll bet your operating current is well under 10A and the time your cord spends over 10A is small and well within tolerance.