Dimming lights

Does anyone know what would cause a light to dim about 50% and stay dim when another light is turned on on a different circuit. Did the same when the AC was turned on and same effect on different circuits throughout the house. Of course I did call it out and recommended further inspection and repair by a licensed electrician of the entire electrical system.

Mismatched dimmer switch and lighting fixture, i.e., taking a dimmer switch that is rated for, say, one 60-watt light bulb and making it dim a chandelier of forty 20-watt light bulbs, or any other combination of mismatched wattage between the switch and the fixture.


What causes lights to dim?
If your lights dim when an appliance (fridge, air cond, etc.) comes on, check to see if the lights are on the same circuit with the appliance. Most fixed appliances (fridge, air cond, etc.) should be on a dedicated circuit back to the main panel. These devices draw quite a bit of current in the first second or so when they first start up.
If all the lights in your house are dim all the time or periodically, the problem could be at the local utilities substation, transformer or their service feed coming into your house. This is one of the first thing you might want to have checked.
Your lights could dim if the wiring in your house is not large enough, it will have too much resistance. When a large current is present in the wire, there will be a significant voltage drop, leaving less voltage available to your lights.
If the light in your house dim and also get brighter it could indicate a lot more serious problem, a loose, broken or corroded neutral wire. Electricity is usually delivered to your home via three wires, two hot and one neutral. Each 120 volt circuit taps one hot wire and the neutral. A 240 volt circuit taps both hot wires. If the neutral wire comes loose, there is a risk that the 120 volt circuits will, at least momentarily, be supplied with 240 volts. The problem could be with the utility company connections, the main panel, or the branch circuits.
For the three-wire cable, two of the wires will insulated. They are called the “hot” wire (black) and the return wire (white). The third wire is typically a bare or green covered copper wire. The 120 VAC potential will be found on the hot wire, while the return wire should be close to zero potential. Current will flow from the hot wire to the device and return along the return wire. No current flows without a return path. There should always be the same current flowing in the return as there is in the hot wire.
But the return is not always at zero potential relative to your local ground. For safety, there should always be a local ground. This is the purpose of the bare copper wire. It should be connected at one end to a conductor that is buried into the ground. All metal electrical casings and electrical outlets should be connected to this wire.
If you are burning bulbs to frequently you also might try using lower wattage bulbs. A 40 W bulb has a higher resistance than a 100 W bulb. Both bulbs will have the same voltage, the 100 W bulb must have more current. And that means the 100 W bulb must have a lower resistance. So the filament for the higher resistance 40 W filament must be heavier or have a smaller cross section. There is also a rough service buld that is made with a heavier filament. The best bulb on the market today is the CFL (Compact Fluorescent Lights) (Philips, Sylvania). These bulbs may cost 10 to 15 dollars, but will last about 10,000 hours.
Newer light fixtures require that the temperature rating of the wire feeding these fixtures be at least 90 degrees C. This is the temperature rating for these light fixtures. It’s important that the temperature rating for the wiring feeding these fixtures match or exceed the rating for the fixture. If the temperature rating of the wiring is lower than the 90 degrees C. required, the insulation around it becomes brittle and may break away. This allows arcing between bare wires, which causes heat that melts the fixture and could be a fire hazard.
Look for the letters NMB on the jacket of the wire. The NM means nonmetallic sheath cable (Romex) and the B suffix means that the cable’s conductors are rated for a maximum operating temperature of 90°C 194°F. We can assume that wiring made prior to 1984, without the B suffix, is rated at 60°C 140°F. This is the type of wiring found in most older homes that were built before 1982.


As long as the lighting only dimmed, and no other lights actually got brighter (a sign of a very serious hazard), my first guess would be a loose hot leg, bad main breaker, burned up main lug, something along those lines. In any event, it needs called out, which I’m sure you did.

By the way, if you ever observe that some lights get dim while others get brighter, I’d highly recommend that you stop your electrical evaluation right there. This is a sign of a loose neutral, and is one of the very few genuine electrical emergencies.

Amperage to low at the panel is the most common cause.


You actually as clueless as you sound?

CMI = I take tests well.

One can only hope that you weren’t being serious. . .

Thanks guys, I was just curious. All lights dimmed and stayed dim…no dimmer switches…all different circuits…yest I called it out.

After some stormy weather, I had a bad connection out at the utility pole once, on one of the hot legs. First thing I noticed was that some 240 circuits were not coming on, I think might have been the well pump wouldn’t run. I checked voltages with DMM, only had 80 VAC on one of the legs.

Another time, out at my shop, when I flipped the switch on my welder, 240 V, it killed the lights. Corroded connection on aluminum “triplex” feeder cable.

John Kogel


You may have 1/2 power in the home. A bad main where one phase is not working or a bad main fuse on a older home. What happens is one phase is hot and when you turn on a 240 Volt appliance such as A/C or Dryer the hot side goes through the windings of the motor and back feeds the other phase and the lights will come on dim on the back feed phase because of the voltage drop through the motor. That’s what it sounds like to me. Did any of the 240 appliances work?