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Old 08-17-2009, 09:02 AM   #1
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AC kicks on, lights dim slightly...


I know it can be alot of different things.. but I'm in the process of swapping out my main panels, and I have a sub running off a 50A breaker. Kind of a moot point because the circuits in question aren't on that trunk.

Anyway, kinda curious.. I have a window sized AC unit in the bedroom off the bathroom. When I'm in the bathroom I can see the lights dim slightly when the AC compressor kicks on. The lights aren't on the same circuit as the bedroom outlet, but it is possible they are on the same leg. What could cause this? Is it normal? I would imagine having a 15A surge to kick the compressor out of the 140A service shouldn't impact circuits?

Something that will go away when I finish swapping out my service to 200A w/new panel, wiring, grounds..

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Old 08-17-2009, 09:15 AM   #2
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AC kicks on, lights dim slightly...


Changing your service will not solve this issue in most cases.

An a/c compressor requires approx 6 times of its running amps to start. This results in a short duration sag of available voltage on the utility transformer, connecting wires, etc.

Unless the lights are dimming significantly, this is something you'll probably have to live with.

If significant dimming is occuring, then an inspection of your electrical connections might be warranted. A loose connection can result in significant dimming of lights during starting of large loads, such as an air conditioner.

FWIW, I recently had a new heat pump installed -- an 18 SEER unit. My old a/c unit was a 10 SEER.

Whenever the compressor starts, my UPS box on my computer bleeps, indicating the voltage sagged enough to make it think power was lost. This never happened on the old unit. The newer, energy efficient units require much more starting current than the older less efficient units.

So, even though you might have a more efficient unit, it will require more juice to start, resulting in more noticable dimming of your lights....

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Old 08-17-2009, 09:23 AM   #3
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AC kicks on, lights dim slightly...


Ya, it's a pretty mild dip. I will put a multimeter on the circuit to try and measure the voltage drop. I'm rewiring each room as I go along, just ran out of overhead in the subpanel (will be my new main). It's amazing how piss poor the wiring is in my 60yr old house. No thought as to design, just wires all over.
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Old 08-17-2009, 11:21 AM   #4
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AC kicks on, lights dim slightly...


It's nothing to be concerned about. Just forget it.
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