Is it normal for my kitchen lights to lower their intensity for a second when I'm starting my microwave oven? Is it a symptom of a bad/particularly unstable power environment?

The reason I am asking is because I have had episodic problems with my computer turning off at random times, and I have already checked the power/reset button connections to my motherboard, ensured that my power supply unit is running probably, and made sure that my computer is not overheating.

Should I invest in a UPS or run further tests on my apartment's electric stability?

Would an outlet tester help in any way?

  • 1
    For the several decades I've been using computers, a UPS is a good investment. It can put the computer into hibernate, saving on file system corruption from unplanned shutdowns and the attendant recovery misery. As long as long as the power problem isn't a bad connection somewhere that ends up burning the place down, at least you'll have one less headache in your life from a computer being switched off randomly from bad power. – Fiasco Labs Nov 4 '12 at 18:35
  • Are the lights (or computer) on the same circuit as the microwave? If you don't know, find the breaker that disables the microwave and turn it off. Still... It shouldn't happen if you have good wiring and an adequate supply to the building. – Brian White Nov 4 '12 at 20:42
  • This answer might be helpful. Understanding inrush current might help explain what you're seeing. – Tester101 Nov 4 '12 at 21:21

In the best case, this may be normal if the dimming is minor. That said, you should still consider changing your wiring so that lights are not on the same circuit as outlets. This is a common wiring practice to prevent lights from going out when an overloaded breaker trips.

The second best case is that your wiring is under capacity for your microwave and you should consider getting one or more dedicated circuits for your kitchen appliances. To determine this, you'd want to check the wattage on the microwave and other devices on the same breaker and see if they are using less than 80% of that breaker's capacity.

The worst case is that you have a failed (or failing) neutral. If that happens, you'll see voltage lower on the same circuit as the microwave, and increase on circuits that are on the other hot leg. To detect this, you'd want a voltage tester that's designed to measure household AC voltage and check the voltage at multiple outlets when the microwave is running vs off. If you see the voltage increase anywhere when the microwave is running, this is a sign of a bad neutral connection between the breaker panel and the electrical service and will likely require a visit from the power company to repair.


It is certainly not normal. The power system is unable to adapt to sudden increased demand. A typical hardware store outlet tester will probably not help. They only identify grossly bad wiring, such as reversed wires, open grounds, etc. The only wiring that should be in common with the lights and micro is a sub or main panel. If they are on the same circuit, they shouldn't be.

All your panels, as well as devices on the affected circuits, should be inspected for any poor wiring installation. I had a similar problem. I went over my circuits, panel, and service entrance carefully, even redoing some connections though they appeared fine. Never found anything wrong. Eventually, the power company replaced the cable supplying our neighborhood and the problem went away. I cannot imagine how a home appliance could have any effect on a 14.4kv supply, but the problem may not be you.

If your wiring checks out, sounds like a UPS would be a good idea. Also ensure any other important electronics are on good quality surge suppressors.


An outlet tester or even the low cost type outlet voltage/power meters such as the "Kill-A-Watt" meter are unlikely to be able to show you if short duration surges are occurring on your power mains. The reason is that such devices sample the line at rates slower than the time that a surge event can take to happen.

If there is a question about whether you have a sagging voltage problem (brown out) that lasts for seconds then a "Kill-A-Watt" type meter in voltage reading mode could help you to assess the situation.

If you have situations where your computer is experiencing hang up problems it is unlikely that this is caused by momentary dips in the power line voltage. The reason is that virtually all computer gear these days use power supplies that work over a wide range of AC input voltages. Many even work over a range wide enough to cover the range of both the common 120VAC and 220VAC systems used throughout the world. The computer is much more likely to be responding to very high voltage spikes on your apartment power mains. Such spikes are caused by the switching on and off of motors, fans, pumps and heater systems. You can help to prevent problems with the computer through the use of a suitable power line filter. You can purchase power line filter devices that can help with this problem. Common medium cost UPS systems often include reasonable quality spike filters that can deal with these EMI and RFI interference issues. The UPS feature to keep your computer going for a short time due to a power outage would not likely be applicable to your current situation but may be a feature that you want to invest in for the future.


If the outlet that microwave is plugged into shares the branch circuit with the lights, then it's perfectly normal for the light to dim when the appliance comes on. If not, then theres definitely a problem.

If your house is pretty old, I would invest some time into checking all outlets/wiring for signs of arcing/burning.

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