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I have a house with old 2-prong wiring in most of my bedrooms.

My toaster oven is plugged in on the kitchen countertop. Whenever I turn it on, the brightness of the lights in 3 (out of 4) of my bedrooms starts pulsing. I measured the voltage from the outlets in those rooms. It is about 119v when the toaster is off vs 117v when the toaster is on. The 4th bedroom that is probably on a separate power line does not pulse at all.

I've been in my attic and saw that some of the wires have been chewed bare by mice. Although it looks like just that the oven draws too much power that drops the voltage, I'm a bit worried that the pulsing might be related to those exposed wires.

Is it time for me to find an electrician to rewire my house?

EDIT

To clarify, the lights are LED and they do not flicker. Just the brightness of the lights oscillates a bit.

  • LED lights... on a dimmer? – JACK Jan 7 at 12:47
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    LEDs are dimmable. – LarryBud Jan 7 at 13:16
  • The lights actually pulse and not just dim? If the exposed wires were causing the pulsing then it would be happening constantly instead of just when the toaster is turned on. You should get those wires replaced and probably placed in steel conduit to prevent future issues. Exposed wires aren't inherently an issue; that's what knob & tube used to be. The issue arises when exposed wires touch eachother or a person. – MonkeyZeus Jan 7 at 14:56
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    @breezymri are these LEDs "dimmable"? If so, that's likely why they noticeably dim when the voltage drops a little -- they think a dimmer is trying to control them. See Harper's answer here for the gory details of how that works: diy.stackexchange.com/a/177536/91556 – Nate S. Jan 7 at 17:46
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    Lights and sockets should be on a totally different ring main and therefor be unable to effect one another. If you flick off a fuse on your fuse board (you should have many) and both the toaster and the bedroom lights don't work then you need re-wiring, not just repairs to the chewed wires – Anthony Jan 8 at 2:32

11 Answers 11

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I've been in my attic and saw that some of the wires have been chewed bare by mice.

Whether this is the cause of the dimming or not, you need to immediately get an exterminator to rid your home of the mice problem and an electrician to assess and correct the damage. Bare/exposed conductors is an exceedingly dangerous condition that cannot wait.

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    The problem of bare wire is that pat of the metal conductor could also be damaged, this means that part heats up more during use, and can become a fire hazard (And there are more problems) – Ferrybig Jan 7 at 19:52
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    Not to mention that exposed live wires, if something causes them to move, could short and cause sparks, landing on the likely very dry wood that your walls and ceilings are made of.. – X-27 wants to Reinstate Monica Jan 9 at 2:00
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There are more issues to cope with:

  • Damaged wiring, as jwh20 suggested. Yes, call electrician in and inspect all wires for damage. Enclose them in rodent-resistant tubes or something. (I am used to stone or brick walls so this is not an issue for me.)
  • Excessive power consumption.

In the ancient times, all the house wiring was powering were lightbulbs. Then more and more equipment was designed to be electric-powered and the power of each equipment increased as well.

The dimming and the voltage drop may be caused by some current limitter, may it be a protective device or malfunction. In case the wire is transmitting more power than it is rated for it heats up significantly which leads to increased resistance and thus the voltage drop.

Do some math. Find/draw the wiring diagram of your house. Estimate the power consumption of all your devices and calculate the current it is drawing. Compare it with the ratings of the wires used and breakers.

You can try to look for something heating up or even burning. Check the boxes and outlets, every single screwed joint. This is no fun, when overloaded it may start a fire or destroy some more expensive equipment.

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  • In principle, the wires shouldn't be transmitting more power than they are rated for, because if they were, the breaker would trip. Of course, if someone "fixed" a tripping 5A breaker by replacing it with a 20A breaker ... – Martin Bonner supports Monica Jan 7 at 17:30
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    @MartinBonnersupportsMonica, the ratings of the wiring are almost certainly not accurate once they have been chewed by mice – Nate S. Jan 7 at 21:31
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The damaged wires (and the underlying pests that could cause even more damage) are definitely your #1 concern.

Once's that's addressed...

The pulsing could be the thermostat in the toaster oven. To maintain a constant temperature, the heating coil is turned on when the interior is below the set temperature and off when it's above. There's usually some middle ground (hysteresis) where it won't switch, to keep it from oscillating unnecessarily quickly. However your toaster oven may lack this feature, causing it to turn on and off more rapidly.

When the coil is on, there would be more voltage drop in the wiring due to increased current, resulting in a dimmed light. When off, you get full voltage, and it's bright.

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Obviously, other posters are correct to note that the bare wires are the real issue here. However, as explanation for the dimming itself:

Many LED lights, especially those designed to be backwards-compatible with incandescent dimmers, are unusually sensitive to voltage fluctuations, moreso than incandescents. As Nate S points out in a comment on the original question, if these bulbs are even just designed to be dimmable, they may be interpreting the voltage fluctuation as a signal from an actual dimmer switch to get dimmer, when in reality it's just a mildly overloaded circuit. Either way, the effect is the same: the LEDs dim with voltage drop, not because they have to, but because they think they're supposed to. LED dimming is a black art.

As for pulsing, you've got me. Maybe it's cheap bulbs, maybe the toaster has an uneven current draw, maybe the voltage is right between two dimming settings on the bulb? Either way, it's not inherently dangerous unless the breaker is blowing, but if it's annoying it could be worth grabbing a non-dimmable LED bulb and seeing if that helps things at all. That said, this happens in my house whenever my furnace's electric starter turns on, and we have good dimmable LEDs and up-to-code wiring. So, worry about the bare wires, not the LEDs.

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    Surprised how far I had to scroll for "led is misinterpreting voltage fluctuations for dimmer behavior" – trognanders Jan 9 at 6:33
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    @trognanders well, if your house is burning your LED lights are going to be seriously flickering, in much more important ways. – Wayne Werner Jan 9 at 16:38
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    maybe the toaster has an uneven current draw that is almost certainly the case - the toaster oven has a cheap thermostat that turns it's heating element on and off, and it only draws current when on. Sometimes you can even hear the contacts in the unit click on and off as it cycles. – dwizum Jan 9 at 20:50
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A lot of older homes are not up to code especially in regards to wiring & circuits for the kitchen. I suspect you have lights sharing a kitchen outlet, which is not in compliance with current code requirements (though it may have been OK when the house was built).

My daughter's kitchen has similar problem. 1960's era ranch house. Mostly original wiring and main panel. An over the range microwave was added (before she bought the house) and I suspect for convenience, it was wired to the light circuit for the dining room. Anyway, every time she uses the microwave, the dining room light dims, a lot.

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Old wiring can develop high resistance at various junctions throughout the house which can cause noticeable voltage drop under load, as you are experiencing.

Since this seems to affect multiple outlets, I would start at the breaker box. If you're minimally technically inclined, carefully remove the cover to expose the screws that clamp the wires to the breakers, and tighten all of them. Often they work themselves loose over years of heat/cold cycles and the poor contact causes problems like you describe.

Next I would find the path of the wires from the breaker to the rest of the house (might have to crawl around in the attic for a bit) and look for junction boxes. Turn off the breakers, open up the junction boxes, and make sure the connections in them are tight and not corroded.

Finally, if those don't help, its quite possible that the connection at the outlet is loose and/or corroded. Turn off the circuit breakers and pull the outlet covers and outlets out off the wall, and tighten all of the screws.

Signs of poor contact are discolored/burnt wires at connection points, and personally I wouldn't say its a huge risk but you probably want to take care of this sooner because these bad high resistance contacts generate lots of heat and/or sparks and can eventually cause fires.

Also as another answer posted, the chewed wires should be taken care of ASAP.

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It sounds like there are multiple issues with your house wiring. If your house still has the original electrical service and fuse box, the amount of amperage the service provides is probably not adequate to power all of the devices in your home.

The wires also sound like they are in poor condition. Bare wires can short out and create an arc which is a fire hazard. It most likely has not been upgraded for a very long time since you still have ungrounded outlets. The plugs may not even be polarized, which allows plugs to be put in upside down. Old electronics such as radios that have tubes in them have high voltage, that can put out enough voltage to be fatal just by touching the case if the plug is not put in at the correct orientation. Modern plugs and outlets only allow you to plug them in one way, and are much safer.

An appliance should not cause your lights to dim. There could just be a loose wire, or there may be serious issues. It is best to bring in a licensed electrician to sort everything out. It may require rewiring the majority of your house to make it safe.

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  • "Many modern electronics also have a ground plug and can be damaged if they are not grounded properly." Can you explain how that works please? If there is any current in the ground conductor, there is a fault condition. (I understand how a missing ground can be dangerous, but not how it can damage equipement.) – Martin Bonner supports Monica Jan 7 at 17:34
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    Surge protectors may not work properly in an ungrounded outlet. Modern electronics are more susceptible to damage from power surges. – Jason Hutchinson Jan 7 at 18:57
  • But those are not normal operating conditions. What you're describing is a two-fault scenario - one fault being the broken or damaged ground return (green wire), and the second being a surge or overload. – SteveSh Jan 8 at 12:50
  • SteveSH- The bedrooms have 2 prong outlets, so no ground. Surge protectors work by shunting overvoltage to ground. Zener diodes 300-600v between hot/ground and neutral/ground. – VWFeature Jan 9 at 20:38
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To an extent, this is unavoidable. If you're far away from the nearest transformer, or the high-voltage lines are near capacity, etc., you will get some voltage drop when you switch on a device that draws a lot of current, like an oven. A 2V difference is well within the specification (120 V +/-5%).

But it could also be a symptom of a problem in your house wiring (excessive resistance somewhere), so it's worth having the wiring checked.

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Ah, the bare wires in the attic... you're not the first old house owner to see that. I found nearly sixteen feet with absolutely no insulation at all, next to a couple of mouse skeletons. Others have found the problem when tracking down the smell of a dead squirrel. It seems wire insulation is irresistibly tasty, and low occupancy areas like attics are where rodents can feast on it at their leisure.

Unfortunately, it's extremely difficult to completely exclude mice from an old house. The rule of thumb is "if a dime can fit through it, a mouse can fit through it". It can take many years to rodent-proof a house, and in the meantime, it might burn to the ground!

So I strongly recommend you replace all the horizontal wire runs in the attic with metallic sheathed cable. It's commonly called "greenfield" around here, but typically sold as type AC, MC, BX, or armored cable; I don't know why it has so many names. The steel jacketed type is literally mouse proof, and even the softer aluminum coated stuff seems to be enough to discourage them.. they could chew through it if they really wanted to, but I've never seen it happen.

(My local electrical code actually requires use of metallic sheathed cable anywhere that rodents are expected to be encountered, although this is usually interpreted by inspectors as meaning agricultural buildings and sheds.)

I also recommend you have the current carrying capacity of the circuit you're using for your toaster oven checked. It's not terribly uncommon for people to stupidly replace fuses or circuit breakers with larger ones, which can also lead to your house burning down. When the voltage is dropping like that, it can be an indication that the wiring is literally melting - it's almost certainly overheating, although it might be just at a junction point rather than over the entire wire run.

Whenever I add, repair or replace wiring in an old house I always use greenfield. The additional cost is not high compared to labor costs - and never having to replace it due to mouse or squirrel damage is much cheaper in the long run.

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(Insert obligatory "fix those wires" comment here.)

What you're seeing is almost certainly the result of inrush current causing voltage drop on your house circuits, which in turn is confusing the LED lamps into trying to respond to the changes in input voltage.

Long story short, it's probably the LED lamps being sensitive to voltage fluctuations.

Any resistive element - including the elements in incandescent lamps - will draw a higher current when cold than it does when it reaches operational temperature. This is referred to as inrush current and is responsible for a lot of the power fluctuations you see when turning on heaters and such. Elements used in cooking appliances tend to have lower temperature coefficients specificially to minimize the effects of inrush current over their long heating period, so inrush current will typically be less than 2x operating current. For a cheap 1.2kW toaster oven on 120V supply the inrush current should not exceed 20A and is more likely to be around 15A, dropping to 10A over a minute or so.

By itself this isn't a problem. When you combine inrush current with old wiring - especially old supply wiring - the current spike can cause the supply voltage to dip due to power loss across the cables and connections between you and the supply transformer. I've seen a 15A load drop the supply voltage by 5V in an older house and heard of worse drops.

If you're getting larger drops than this then you've got some serious line resistance and you need to get your wiring professionally checked. If you have any incandescent bulbs left in your house and you notice a dip in their brightness when you turn on the toaster oven then that's a good indication that you have a serious wiring issue.

LED lamps however are very different. Incandescent fillaments take time to cool down while LEDs change brightness faster than you can generally perceive. Any irregularity in the supply can cause the brightness to change quickly and the control circuits may oscillate while the supply voltage is ramping during the inrush period. Generally the more expensive the lamp the better it will handle these situations, but even some of the high-end lamps with dimmer support will get themselves mildly confused by small changes in supply voltage.

If you have the equipment to check the voltage drop then do it. If you don't then grab a cheap 25W or 40W incandescent bulb and see if you can spot the drop when you turn on your toaster oven. If you can then don't even bother looking for the problem yourself, go direct to an electrician. Preferably one with thermal imaging gear to find hot spots before they become ignition sources. Big drops for light loads is an indication of severe fire risk.

And for the sake of whatever you hold dear, get those damaged cables fixed.

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Find the breaker and see if the 3 lights and 1 counter-top outlet are on the same breaker. Replace with an identical Amperage and compatible breaker. Don't do this yourself if you have never worked in a breaker box, since the poles are ALWAYS LIVE. An almost bad breaker had caused this once for me.

If that checks out use a Electrical outlet tester with GFCI (like this one https://www.homedepot.com/p/Commercial-Electric-Outlet-Tester-with-GFCI-OTG-102R/206029151) to make sure that the outlets are good, but you will have to use the meter on the light socket.

If you find the junction box either behind the lights or behind the socket from where the wires are pulled and joined it maybe possible that either you are getting a short or grounding. I would guess behind the toaster plug. By the way the counter outlets should be connected to a GFCI (newer code).

Honestly if you cannot find the issue with this troubleshooting, time to get the electricians and get a quote for fixing/rerunning new cables.

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  • A house that old could very well have fuses instead – ThreePhaseEel Jan 8 at 2:23

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