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I recently had a 15A receptacle in my kitchen partially melt while my wife had our vacuum cleaner plugged into it.

We knew something was up when we smelled the telltale hot plastic smell. I felt the wall-plate and it was hot. I turned off the breaker, took everything apart, and found that the outlet was being used as a pass-through to power two of the counter-top outlets in our kitchen and our very large kitchen refrigerator/freezer.

Supposedly even a 15A outlet is rated for 20A pass-through, but something wasn't right if it melted, so I went and found a replacement duplex outlet, some #14 wire, and re-wired it using pigtails so the outlet wasn't passing through any current.

I wrongly assumed that the circuit was 15A. When I went back to turn on the breaker, I discovered that it was a 20A circuit. Uh oh. My pigtails were under-rated for the circuit.

I also used a take-out receptacle that I had lying around, and it was ivory rather than white.

I told my wife not to plug 2 heavy-duty loads into that outlet until the weekend, and today went out and bought a new white duplex receptacle, found some #12 Romex from which I could cut new pigtails, and rewired it properly, both with the correctly colored receptacle, and with the proper gauge of wire.

I got to thinking, though. Resistance is measured in ohms per foot. A 14 gauge wire carrying a 20 amp load introduces too much resistance over the length of the wire run, causing a significant voltage drop and heating up the wiring and risking a fire.

However, a pigtail is typically less than 4 inches long. I'm not advocating it, but couldn't you use 14-gauge wire for such a short piece of wire and not generate enough of a voltage drop to cause a problem, even if you were to plug 2 devices into the receptacle that drew a total of 20 amps? (Again, I understand that that would be a violation of code, and I'm not about to do it in my house. This is a theoretical question.)

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    It's not voltage drop your worried about in this case. To imagine the problem, let's pretend it's a 4 inch 14 gauge pigtail on a 100 amp circuit--the wire turns red hot burns. – Tyson Oct 2 '16 at 1:44
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Wire gauge requirements may be due to heat dissipation or voltage drop considerations. Standards can impose restrictions that are not really necessary for a particular situation but accommodating those situations would make the standard too complex.

Let us consider the actual heat generated by using a 4 inch #14 pigtail to carry 20 amps. The resistance of 1000 feet of #14 copper wire is 2.525 ohms. 4 inches would have a resistance of 2.525/1000/3 = 8.4e-4 ohms. The voltage drop is 0.0168 volts and the power is 0.34 watts. Double that for the other pigtail and you have 0.68 watts in your box from those pigtails.

By comparison, the resistance of 1000 feet of #12 copper wire is 1.588 ohms. 4 inches would have a resistance of 2.525/1000/3 = 5.29e-4 ohms. The voltage drop is 0.01 volts and the power is 0.21 watts or 0.42 watts total.

The increased heat load is thus 0.68 - 0.42 = 0.26 watts between #12 and #14.

  • So the difference in heat generated is insignificant. Now what if you over-load the circuit? How fast does a residential circuit breaker trip, and with how much overcurrent? With under-rated pigtails, could the wire of the pigtails overheat? My guess is no. If I draw 50% more power, 30 amps, I'd still only turn less than 1 watt of power to heat, and that only until the breaker trips. – Duncan C Mar 28 at 0:57
  • The time for a breaker to trip depends on how severe the overcurrent is. I can only assume the trip time curves are designed to protect the wiring and prevent fires. I don't want to do the calculations to determine how much current a pigtail can take before the insulation is damaged. It would require calculating the surface area per inch of the wire and the thermal conductivity of the insulation and then the heat carried by the air when there is nothing to move the air besides the tendency of warm air to rise. testguy.net/uploads/lsig-curve-explained.jpg – Philip Ngai Mar 28 at 1:25
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The problem isn't the excess I2R drop (voltage drop) in the wire, it's that the wire is loaded beyond its ampacity, which causes it to overheat and damage its insulation as well as the insulation of other nearby wires as most insulating materials used for household wiring cannot withstand being overheated (dry steam is too hot for them, if you want to get an idea) without deteriorating significantly.

There are rules in the Code for tap conductors that can be smaller than the circuit they are tapped off of, but they apply to limited situations (such as lights, hard-wired cooking appliances, and some other types of fixed loads), never to receptacles -- the risk of an oversized-for-the-tap load being attached to the receptacle is too high.

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