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I've 'tested' three listed extension cords, one at a time, by plugging a 4 year old 1500W fanned heater ($10 at Home Depot) into a 20 amp house circuit (no other appreciable load). The cords:

  1. orange 20+ year old 9 foot 14/3 round
  2. grey 20+ year old 8 foot 14/3 flat
  3. yellow unknown-age (but modern) 25 ft 12/3 flat

All cords have molded plugs and are unmodified. None are damaged or look worn. All are single-taps, that is, none are triple-taps. Likely three different manufactures, none known.

I suspect code probably requires such a heater be plugged directly into the wall a outlet. I don't know. Regardless, this is how I tested them.

Anyway, used one at a time, all three female ends get warm. The middle of the cords stay 'room temperature' 60F. The female ends get warm to the touch, noticeable, but no where near 'hot', just pleasantly mildly warm. The 12/3 flat cord has very thick insulation, and the warmth travels from the plug further up the cord - say 12" - than the either of the two 14/3 cords.

The plugs on the 14/3 cords feel warmer than the adjacent cord. The plug on the 12/3 cord feels the same, or possibly a bit cooler. I attribute the differences to differences in heat-conduction of 12 ga vs 14 ga wire.

I imagine all 9 female terminals (molded within rubber plug) are soldered to the conductors with no increase in resistance at the connection.

I suspect this means that there is a huge 'contact' resistance between female terminals and the male terminals in the heater's end.

How can I further investigate and hopefully eliminate the 'problem' if there is one. Maybe cord amperage/gauge/length tables are for peak amps, not sustained amps.

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    There is nothing wrong with the female ends getting warm to the touch while carrying 13 A. This does not mean there is a huge contact resistance, there is in fact very low electrical resistance. If there was a high resistance then the heater would not work and the plug would get so hot it would melt and then catch fire. The warmth of the female plug is due to the resistance of the plastic to heat flow. The same thing would occur to any section of the cord that you would wrap three to five times with an ace bandage.
    – Jim Stewart
    Dec 20, 2017 at 20:52
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    I tested @JimStewart claim with a five foot length of 3/4" thick pipe insulation. The yellow 12/3 cord itself did NOT get hot, but near the plug the cord warmed up a few more inches. Sorry Jim.
    – user
    Dec 21, 2017 at 2:20
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    What's in common? The heater's male plug. Look there.
    – Harper - Reinstate Monica
    Dec 21, 2017 at 8:07
  • I plugged in my 1500 W space heater into a very heavy extension cord. The heater has a 6' long 16 AWG flat polarized ungrounded power cord. The male plug got warmer than the cord, but did not get hot. How hard can it be to make a male plug which has zero resistance? Can it really be that these plugs are so poorly made that the crimp connection is heating? I still think it is likely that the thick plastic of the male plug is the cause of the temperature rise of the male plug.
    – Jim Stewart
    Dec 21, 2017 at 15:25

3 Answers 3

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What you're learning is that wire gauge is only part of the equation. As you noted, plugs introduce resistance, and that makes heat.

You're right that heaters should not be run on extension cords. If you must, use high-grade cords with good plugs, and expect them to get warm. Monitor them for excessive heat which can melt the plastic and cause a failure of the connection.

FYI, those connections are likely crimped, not soldered.

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  • According to this page wattstoamps.net/1500-watts-to-amps the 1500W heater is drawing 27.3 amps. If so the 20 amp circuit breaker should have shut off, no?
    – user
    Dec 20, 2017 at 20:16
  • You used the calculator incorrectly. I get 12.5A. 1500W is a common size for heaters and hair dryers so that they function on most 15A residential circuits.
    – isherwood
    Dec 20, 2017 at 20:17
  • Yeah, common size for residential heaters. The 27.3 is from the text on that page. Also, 1500 / (0.5 * 110) = 27.3
    – user
    Dec 20, 2017 at 20:19
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    @user, that's using a made-up power factor of 0.5. The power factor of a purely resistive load (like a heater) is 1. 1500 / 120v = 12.5A...
    – JPhi1618
    Dec 20, 2017 at 20:22
  • @user That page is spammy clickbait. They don't care if their page is accurate, it's just random text to keyword-stuff Google and get you to click the ads. Everyone else is getting it correct at 12.5A.
    – Harper - Reinstate Monica
    Dec 21, 2017 at 8:14
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I've noticed the same phenomenon at two out of three plugs in a system with two heavy duty extension cords connected in series to a 7 amp load off from a normal 230 VAC 10A NZ outlet. I carefully 'lab' tested the voltage drop across a typical socket/plug interface and found the contact voltage drop to be minimal, 22 mV at 5 amps, the most I could do on the bench. I actually think the issue is not connector resistance but instead the 'end effect' of heat dissipation of the copper wires. The only oddity is that the location where the two extensions connect together does not exhibit this issue. Perhaps that's because either side of the interface is copper inside an 'exposed' extension cord. This is probably completely consistent with the thermal mathematical model of this system.

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At Jim Stewart, I have to take issue, guy. Heat and current carrying conductors is never a good state. Although we may have to accept a certain amount of heat being generated in our wires as they carry the load to the end use, we are never happy about it. To tell the OP that warm cord ends is all cool, I think sends the wrong massage. Every winter folks young and old die in house fires because of warm cord ends that deliver power to portable space heaters. To say, "hey! here is another cool way to make heat with an extension cord is misguided at best." Those folks are looking to us to keep them out of trouble not to show them how to make more.

Here is what I would say to 'user'; First, any cord that is 20+ years old is suspect and probably should be retired. Second; these types of cords are mass produced and the manufacturer uses molded cord because it is cost effective and they are competing for shelf space in hardware stores around the world. And if the truth be known, they are not the best cord ends available today. I would go on to say that if I needed to support a load of 1500-watts at 120-volts for any extended period of time I would be reluctant to do it with this quality of cord at all. If I had to make arrangements for the above situation I would insist on a 20 Amp dedicated circuit with a 20-amp spec grade receptacle into which I could plug the cord. I would buy or use a fairly new cord that is 12-gauge wire, no longer than necessary. I would cut the existing cord ends off, throw them away, buy and install Hubbell heavy duty replacement cord ends. Now the customer has a genuinely safe application. Just because it is all over the shelves at Home Depot doesn't mean it is a good and safe set-up for a given situation. If the Hubbell cord ends are installed properly any heat generated with this load would be truly negligible. With the molded cord ends we don't know what is going on in there. But if it gets warm quickly, we know something is wrong.

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  • I think the recommendation against using extension cords with 1500 W space heaters must be that the connection between the cord of the heater and the extension cord could well be hidden under a table or under and in contact with flammable drapes and lying on a flammable rug. If the connection pulls most of the way out and so gets to be a high resistance, then it will get very hot.
    – Jim Stewart
    Dec 21, 2017 at 1:36
  • Paul, @JimStewarts claim didn't pan out. See my comment that follows his comment. The 15 amp receptacle (no t-slot) is on a 20 amp circuit breaker. Of course I am posting about the female ext cord end. I will have to look at the it's male end for any heat build up. Fwiw, can't remember the brand or retailer for the ext cords. The heater was from HD.
    – user
    Dec 21, 2017 at 2:28
  • Paul, you wrote 'any cord that is 20+ years old is suspect and probably should be retired'. Lots of forum members disagree with you: diy.stackexchange.com/questions/128716/…
    – user
    Dec 21, 2017 at 2:31

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