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I have an electric lawnmower that's about 8 years old. I use a 100-foot extension cord with it that's probably 20 years old. While mowing the other day, I smelled a faint electrical smell. When I finished, I noticed that one of the holes on the end of the extension cord where it plugs into the mower was black and slightly melted (see photo). The corresponding prong on the mower was a bit blackened as well (see photo, though it's kind of hard to see).

Melted extension cord:

Melted extension cord

Lawnmower plug:

Lawnmower plug

I assume that either the mower or the extension cord has a major problem. My main question is which one? Is there an easy way to tell? Is one inherently more likely than the other? Is there some kind of test I could run that wouldn't be dangerous to whatever else I was plugging in?

Obviously, I'd rather replace the extension cord than the mower, but I don't want to get a brand new extension cord and have that melt too, the first time I use it.

Thanks again for your help!

  • P.S. The CPSC would be interested in hearing about this -- you may not be the first person who's had this problem with this mower before, even! – ThreePhaseEel Sep 25 '16 at 21:32
  • What's the amperage of the mower and what's the gauge of the extension cord? – mmathis Sep 26 '16 at 0:07
  • Had the exact same problem. Ruined my last two extension cords. – Daniel Oct 28 '18 at 16:26
  • Very high probability the cord gage is inadequate for that length and amperage.Foe example a 1/2 Hp would need 16 ga MINIMUM, I would use 14 ga – blacksmith37 Oct 28 '18 at 22:00
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The connector is worn and/or damaged.

The best solution is to replace both connectors (male and female). This is easy for the extension cord but you may need to order a spare-part for the lawnmower connector.


As an alternative, You could probably hard-wire a line connector rated for outdoor use (wet conditions) but you'd need to open up the lawnmower connector and make modifications to remove the existing connector and provide strain relief for the new short cable to the line-connector.

enter image description here

Typical outdoor-rated unearthed line-connector for (double-insulated) lawnmowers etc used in the UK. There will be equivalents for US and other countries.

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This is a design flaw in the mower -- the use of a chassis mounted inlet here does not provide effective strain relief for the connection, resulting in the extension cord working loose, arcing, and eventually burning as you see here. The better solution is to take out the chassis mount inlet and replace it with a plug attached to a short length of flexible cord and properly strain relieved both at the plug and where it enters the chassis (you'll need something like a 14/2 SOOW cord and a field-fitted NEMA 5-15P for this, as well as appropriate strain relief parts).

  • Most mowers with a frame or body mounted plug have a built in method for extension cord strain relief, if they didn't they would come unplugged constantly during use. I agree with potential worn/loose plug/connector set but what about voltage drop? What gauge is the 100' cord? Less voltage = more amps = more heat on undersized wire/plug set, yes? – Jimmy Fix-it Sep 25 '16 at 22:38
  • @JimmyFix-it -- they have something there, but my experience with frame mounted inlets on outdoor appliances is basically the OPs, only with a leafblower instead of a mower. The strain relief you can get by trying to grab an extension cord and hang onto it just isn't nearly as good.... – ThreePhaseEel Sep 26 '16 at 0:32
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    Yes, I totally like your proposed solution, secure pigtail +1. I just don't want OP to discount problems related to cord length vs. wire size for electric motor. It can fubar the motor too. – Jimmy Fix-it Sep 26 '16 at 0:36
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You can replace the end of the extension cord. The picture of the mower is blurry so it's hard to see the condition of the prong; you may be able to clean it up without having to do major surgery on the mower.

This problem generally happens because the extension cord isn't secured in place. Tension on the cord wiggles the connection and tries to pull it apart; it may even loosen the contact pressure inside the extension cord. Any time it makes a momentary break, you get arcing. Once the arcing pits the contacts, you get a worse connection and it exacerbates the problem.

Once you get the electrical connection fixed, use a provision to secure the cord. If there is no cord lock on the mower, knot the cord to the handle so that there is a little slack on the connection, and any tugging on the cord won't affect the connection.

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I find molded extension cords regularly fail at this location (the female end) , I replace the cord cap with a quality industrial end. I believe after years of use the molded ends get loose and start arcing damaging both the male and female connectors. I have found it more often on cords that are overloaded but have found the same problems on 12 gauge cords that really could not be overloaded because they had standard 15 amp connectors. And we're in use with only 1 tool connected. The female end failures are more common in my experiance. Since I really can't see the damage on the male end it may be possible to continue using the male if it is intact with no missing metal. The female loosens and causes the arcs where the males are usually 2 solid layers folded together so if the metal is ok clean it up put a new female end on the cord and get back to mowing.

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