I've recently started having electrical issues whenever the compressor for my central air conditioning is running: lights flicker all throughout the house, the UPS for the computer triggers, and something in the main breaker panel buzzes/hums for a second or two when the compressor first turns on. I measured the voltage at a wall outlet at 97 volts, when it should be closer to 120. I have 200 amp service btw. I'm thinking either the compressor is malfunctioning and drawing way too much current, something is wrong with my house wiring, or there's just too much collective power draw in my neighborhood for the infrastructure to handle. It is hot here, and probably most houses are running A/C all at the same time, although I didn't have this problem last summer when it was hot. I'd appreciate any insight into what could be happening before I start calling an electrician.

Edit: Voltage at the wall outlet when the compressor is off is 114 V. I will try to measure the other leg today as well.

  • Can you measure the current (ideally oscilloscope to see the transient as the compressor turns on)?
    – bobuhito
    Sep 4 at 4:12
  • How's the voltage when the compressor is inactive?
    – Martin
    Sep 4 at 11:16
  • When you measured the 97 volts at one outlet, you measured one leg of your 240/120 service. What voltage was on the other leg (also 120V nominal) and between the legs (240V nominal)? It will probably be easiest to measure these at the panel or at the plug for a dryer or oven. Alternately, you likely would be able to find an outlet that's connected to the other leg and which shows a voltage that's different from the 97 volts (anywhere from slightly different to significantly different). BTW: It's quite unlikely to be a general issue with large power use in the neighborhood.
    – Makyen
    Sep 4 at 15:32

2 Answers 2


Buzzing from the main panel is usually a sign of a loose connection either where the AC breaker attaches to the bus bar or where the wires connect to the breaker or a failing breaker. This could cause low voltage at the outlets and flickering of lights. If the problem occurred when other 240volt appliances were operating, water heater, oven, dryer, then it could be traced to a connection at the main breaker but this doesn't appear to be the case.

A compressor that is malfunctioning normally would not operate.

  • I had this happening in my house a number of years ago. The connectors bringing power into the panel had been arcing and one of them lost so much material to arc vaporizing that lost connection. Once I started hearing the noise, the connection was almost gone. Needed a new panel.
    – Yorik
    Sep 5 at 16:08

Something in the main breaker panel buzzes/hums for a second or two when the compressor first turns on.

This is a normal function of HACR rated circuit breakers.

Brownout caused by A/C run is definitely not normal. The compressor should have no effect on other circuits after startup.

Consider measuring the circuit amps as suggested above. I would also check all of the connections from the compressor all the way to the panelboard. I would even run some other appliances like a clothes dryer to see if there is any other abnormality of line voltage.

Also consider that the UPS only measures voltage on one phase. Check voltage on the other phase to gather more info.

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