• My wife has an iMac computer that is approximately 3 years old. When the computer goes into sleep mode the lights on the circuit the iMac is plugged into flicker.

  • My house was built in 1963 in Missouri. The house has 100 amp service all wiring is copper and grounded properly.

What I have done

  • Originally I didn't realize the iMac was causing the lights to flicker. So I looked at all the wiring on the circuit looking for a loose neutral. Nothing was loose anywhere.

  • I looked at the breaker in the panel to see if it was loose. It was not loose but I replaced the breaker anyway.

  • I tested the voltage on the panel and it appeared to be between 119 and 120 so I assumed the box was ok.

  • The flickering still persisted so I called an electrician. Unfortunately the flickering was not happening we he arrived (at this point I still didn't know it was the iMac). He looked for loose neutrals and checked the panel. He said the panel and circuit looked fine.

  • One morning my wife walked into the office and started using her computer. I was in the bathroom watching the lights flicker (and losing my mind) when the lights stopped flickering. Alas, I figured it out and could reproduce the issue. Whenever the iMac when into sleep mode the lights flickered. Typically my wife would be using the computer and let it sleep overnight. This is why I notice the circuits flickering only at certain times!

  • I plugged the iMac into other circuits, set it to sleep and the lights on the other circuits flickered as well! So it was definitely the iMac...

  • I take the iMac to the local Mac shop. They think i'm crazy as they have never heard of such a thing. The run it through all their tests and it passes w/ flying colors. They contact apple. apple declares it impossible and it must be something w/ my wiring. They said they could replace the power supply in the iMac (around 150 or so, not steep) but it is probably not the issue.

I posted this question on apples forums and didn't hear anything. I found someone on google complain of the same issue but didn't see a resolution.

Any thoughts from the electrical experts? I'm going to run the iMac over to the neighbors house and if his lights don't flicker i'm going to assume it is my panel, as it causes flickering on all my circuits in the house.

I do know that some of the circuits in my house have new copper wiring where another light fixture or two was added. Newer copper wiring shouldn't have any problems with older copper wiring should it?

  • All of the lights in the house flicker, or just the circuit where the computer is? If you move it, does it only affect that circuit? Are the lights incandescent or fluorescent? I would plug a multi-meter set to AC and the appropriate voltage into a nearby outlet and monitor the voltage as the computer goes to sleep/wakes from sleep.
    – Steven
    Commented May 3, 2012 at 0:09
  • Steven: Only the lights on the circuit where the computer is plugged in are affected. The lights are incandescent. I have plugged a multi-meter in the past but didn't notice a large range in the voltage. I will try it again tonight and comment. Thanks, Rob
    – RDotLee
    Commented May 3, 2012 at 0:22
  • I would plug it into a good quality surge strip or a battery backup. If the mac is sending a surge then the strip (or b/u) will pick it up.
    – lqlarry
    Commented May 3, 2012 at 0:43
  • Hi lqlarry: I'm sorry I didn't mention it in the original post, it is already plugged into a Belkin surge protector.
    – RDotLee
    Commented May 3, 2012 at 1:57
  • 1
    Is it possible for you to document this behavior on film? Or describe "flickering" better?
    – Tester101
    Commented May 3, 2012 at 2:34

2 Answers 2


A few things that can cause this:

  1. Faulty iMac supply.
  2. "Hot fault" in the wiring... A bad junction somewhere, or a physically damaged wire, or wires that are too small for the current draw (and the fuse or circuit breaker!).
  3. Exotic interference. Is there an electronic switch or timer, or a low-quality UPS, or one of those infernal "SpySmart Meters" somewhere?

Note that for items 1 and 2, you should see a voltage drop -- measured at the outlet that the iMac is plugged in to. A drop from 120V to 110V yields roughly a 16% drop in incandescent brightness. A drop from 120V to 100V, causes about a 31% drop in brightness.

What iMac model is it? What specs does it list for power? iMacs usually run at about 80W to 200W, but I read once that they can have short, peak draws of 1000W.

A surge protector will do nothing in this case, but a battery backup with brown-out protection will protect the computer and may lesson the flickering.

If it is a "hot fault" type problem, there is a very real danger of fire. A fuse or circuit breaker cannot guard against such faults either, but you should immediately verify that the circuit breakers are properly sized for the wire gauge(s) of each circuit (the smallest wire gauge, if more than one was used).

  • 1
    I hate Macs, I hate Macs, I hate Macs... Okay, now that I got that out of my system (for a reason), does your iMac have a switch in the back that allows for you to switch what the power-supply thinks the input should be at from 220v to 115v? I've seen a similar thing happen with a PC (I bought on eBay) that was still switched to 220v.
    – ShoeMaker
    Commented May 3, 2012 at 2:57
  • @ShoeMaker: Damn, you're right (about all of it). I should have remembered the input switch thing too. Okay if I add that to the answer? Commented May 3, 2012 at 3:01
  • LOL Go for it. "IF" you feel the need, you can explain how other countries use 220v for everything which is the reason for the existence of those switches. Although the iMac may have never been anywhere outside of the country, it is possible to inadvertently flip that little switch. :D
    – ShoeMaker
    Commented May 3, 2012 at 3:21
  • Thanks for everyones input. I did the following tests. Turned on our bathroom lights and bath fan (4 60 watt bulbs, 1 40w bulb and a standard bath fan w/ no bulb). I then watched the voltage on the same outlet as the imac while it was running (not in sleep mode). The voltage fluctuated from 116 to 118. I then put the imac in sleep mode and the voltage range was 115 to 118. I watched the voltage for 5 minutes for each test. During the test the only items on the circuit were the bathroom information I mentioned, the iMac and the a phone charger.
    – RDotLee
    Commented May 3, 2012 at 3:36
  • 1
    Digital multimeters measure RMS (root mean square) voltage, so it's not likely you'll see fluctuations that occur very quickly. An analog meter may show the fluctuation via needle bounce, depending on the sensitivity of the meter. You may want to try measuring current with a clamp on ammeter.
    – Tester101
    Commented May 3, 2012 at 12:11

I've experience something similar, the chandelier in my stairway was on a dimmer and it would flicker constantly. When I replaced the dimmer, everything appeared to be fine, but that was only because I moved my computer setup to a separate breaker while I had the power shutoff to the original circuit. I realized it was the computer setup, but before I went through the effort of running a new circuit, the power supply on my home office server bit the dust. I took the opportunity to do an upgrade, including a new power supply, and haven't had any flickering issues since.

Moral of the story: if you have dimmers on the lights, it doesn't have to be a power surge or bad wiring. All it takes is some noise on the circuit to cause a flicker because of how the dimmers work. I'd try replacing the power supply in the computer and see if that solves it, since it would likely be cheaper than an electrician and it never hurts to have a spare power supply.

  • +1. I should have mentioned dimmers by name, but this would be an "electronic switch" and "exotic interference", as mentioned before. Also, some surge protectors and some UPS's can reduce this kind of interference. Commented May 4, 2012 at 8:53
  • I am going to replace the macs power supply next week. I will post the results. I don't have dimmers on my lights.
    – RDotLee
    Commented May 4, 2012 at 16:40

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.