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I live in an old apartment where 2 electrical circuits have been installed, as was standard back then when people only had a few bulbs and maybe a radio set. Central heating works in all rooms except bathroom, the reason why I usually put a small, fan-forced electric heater on low setting to keep the place comfortable.

Since my kitchen appliances also require quite a bit of power, I ran a large gauge extension (the kind used for air conditioners, 14AWG) from an outlet that's not as heavily loaded to avoid tripping the breaker when using the coffee machine.

Now, low setting means the heater is pushing about 600W of heat in the room, on and off, following the thermostat setting. The actual voltage in my place is 114VAC (measured from Elgato's Eve home monitor).

I had to unplug it to clean it (routine maintenance), and noticed the wide (aKa neutral) blade was blackened and pitted, while the narrower blade is still pristine. The extension was a bit warm, but not the heater's own cord (HPN-type).

What would cause such a problem in the plug of an extension designed to pass 1800W when used with a medium, intermittent load with no large inrush current?

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    Could be that side of the receptacle as weaken enough and is not making tight contact anymore. The repair is to replace, but you need your landlord to get a license electrician in to do it.
    – crip659
    Commented Feb 29 at 17:00
  • Cord plugs (and the receptacles they go into) can sustain such damage simply from plugging the device in, and unplugging it, while it is turned on. Doing so will almost always draw an arc, and arcing means damage.
    – kreemoweet
    Commented Mar 8 at 0:10

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Typically this is a sign that the receptacles are old and worn out, so the contacts inside them are not making good contact to the blades of the plug. They last a long time, but not forever.

The pitting and blackening is from arcing, and the arcing is a sign that the connection (receptacle contact to plug blade) is loose.

Since this is an apartment, you should contact your landlord and request that the receptacle in question be changed by a licensed electrician. A good landlord would probably have all of them changed, rather than waiting for each to fail, but at minimum the failing one needs to be changed. In an apartment, or other Multiple Dwelling Unit occupancy, residents, as well as landlords and maintenance personnel who are not themselves licensed electricians are not permitted to perform electrical maintenance due to the greater risk to others, as compared to a single owner isolated home.

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I live in an old apartment where 2 electrical circuits have been installed, as was standard back then when people only had a few bulbs and maybe a radio set.

And the wiring isn't really built for modern loads either, nor were the legacy devices UL-tested in a world of widely used high power appliances. In my also-old cottage also with 2 circuits, the kitchen and bathroom circuits have been upgraded, but the rest of the house maybe not so much. We use a space heater on the kitchen circuit and just shut it off while cooking. Note that the layout is such that an extension cord is not needed.

The extension cord also suggests a problem with NEC Article 400 which forbids use of extension cords as a substitute for building wiring.

Our bathroom has no heater, but we leave the door wide open when not in use, and convection takes care of the rest. If you are habituated to keep the door closed, question that habit.

What would cause such a problem in the plug of an extension designed to pass 1800W when used with a medium, intermittent load with no large inrush current?

Could be a NEC 110.14(D) "torque terminals to spec with an actual torque screwdriver" problem if the socket was not torqued to spec and the real heat source is the screw terminal.

Could also be cherry-picked selective application of NEC 110.3(B) "Follow instructions and labeling". I have skepticism that the heater instructions say "OK to use with extension cords" and "OK to run unattended". I suspect it says the opposite, in boldface.

Cadet makes 750W baseboard heaters which hardwire in, which would eliminate the extension cord trouble, and are rated for unattended use. This is the way to go.

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  • No electric heater bears any warning as to letting it run unattended, or any warning about using extension cords like air conditioners do (they have a large inrush current + high continuous power requirement). If yours does, then it must be country-specific. BB heaters can't be used in my configuration (bathroom light circuit is the same as the kitchen's ovens + not enough wall width).
    – P. N.
    Commented Mar 7 at 21:33
  • @P.N. Because you are ignoring the instructions which come with the heater. My heater doesn't say it on the labeling either, but it sure does in the instructions. And "wider neutral" suggests NEMA 1-15; so the instructions absolutely apply there. But you do you; we're just here to help you be safe.
    – Harper - Reinstate Monica
    Commented Mar 7 at 22:09
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In fact, the receptacle (from the extension cord I'm using) doesn't "feel right" anymore. The two flat holes aren't exactly parallel, perhaps from continuous, abnormal heating inside.

On the same circuit, a previous smart plug I was using heated up so much that the case cracked, which would point towards a hardwired receptacle issue.

Our bathroom has no heater, but we leave the door wide open when not in use, and convection takes care of the rest. If you are habituated to keep the door closed, question that habit.

I'd gladly do that, but the layout is such that the bathroom is a dead end and no significant heat gets in. In fact, quite the opposite when I need to open the kitchen window. thanks for suggesting, though.

In short: Check hardwired receptacle for loose contact, replace as needed If OK, check for screws not tight enough. Tighten or replace as needed.

EDIT: actual picture of extension cord.Heavy-duty extension cord

Picture of oven plug (same circuit as the heater, but never on at the same time) Oven plug (replaced)

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