Last week I had an electrician in to put in some new wiring; I also had an HVAC guy come do the annual maintenance on our system. Since then, the lights in our living room flicker whenever the A/C comes on.

I would usually suspect the start-up capacitor in the compressor, but the compressor is on a separate panel from our living room lights. On the other hand the 1st floor lights (1) and the attic air handler (4-6) are on the same panel, as is the new wiring, which was added to the Bath GFCI circuit (2) as shown here:


The new wiring is supporting our basement de-humidifier and a desktop computer. I confess to knowing next to nothing about home electrical -- is it possible that the additional current being drawn by circuit 2 could be causing the flickering of lights in circuit 1 whenever the HVAC kicks on?

  • 2
    What size is the electrical service? If the service conductors or panel feeders are slightly undersized for the new load, the A/C could be causing a larger than normal voltage drop during startup.
    – Tester101
    Commented Jul 18, 2016 at 20:29
  • But the A/C load and service conductors have not changed from before the work, so adding new loads should not change the voltage crop associate with them turning on pre-existing loads.
    – AndyW
    Commented Jul 18, 2016 at 22:08
  • 2
    Don't hold back from suspecting the A/C capacitor merely because it's on a different panel. Panels are literally not even wire, and have no surge/noise/power-factor/voltage correction whatsoever. You're thinking of a transformer, they do most of that. Commented Jul 19, 2016 at 0:16

1 Answer 1


Find out what the A/C guy did. I would guess he did something which improved the performance of the A/C system, so now, instead of the motor nearly freewheeling against little load, it's having to hunker down and really work. The increased amperage is "bringing to the surface" a pre-existing problem with wiring, probably between the sub-panel and the supply.

I would now measure to see how serious the problem really is. It may not be worth worrying about, it may just be over-sensitive bulbs.

You can mask the symptom by buying modern lamps. Any multi-voltage rated LED (typically 100-240V) or fluorescent tube ballast (100-277V) will ride through voltage sags. Avoid the cheapies (Feit, Utilitech, Lights of Amerchina, and CFLs of any make; you will not be satisfied with their long-term performance, and they will give LED a bad name.)

  • Thanks, this is an interesting suggestion. I think the A/C guy only inspected the system to see if any maintenance was required, rather than actually doing anything of substance, but I can't be 100% sure. I was with him when he was in the attic looking at the air handler, but I stayed inside when he went out to look at the compressor. Funny that you should make the suggestion about the light bulbs. That was precisely my intention, but I had been thinking that maybe I should sort out the electrical issue before putting expensive lamps in, lest the voltage sags damage the lamps.
    – D. Savitt
    Commented Jul 19, 2016 at 2:30
  • The A/C tech's notes (from my receipt): "Checked air filter, cleared drain. Checked high and low voltage and amp draws on blower, fan and compressor. Also checked low voltage safeties and checked refrig charge. Also made sure all electrical connections were tight. All is well at this time."
    – D. Savitt
    Commented Jul 19, 2016 at 2:47

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