Lights "dim" when Central Air kicks on!


Old 07-20-05, 01:50 PM
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Lights "dim" when Central Air kicks on!

We bought an old farm house recently. One of the first upgrades we made to run 200 amp service to the house. Everything from the pole to the house in new. Everything inside the house is new, there's a new panel, new breakers, new wiring through out the entire house. Even a new 3 ton Central A/C unit. The problem is when the A/C unit kicks on the light flicker, or at least dim for a split second. It bugs me and I don't think it should be happening. Any suggestions as to why, or what I can do to get rid of the problem? Thanks.
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Old 07-20-05, 01:57 PM
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Very common in fact almost every house does it to some degree. The compressor draws so much current when it starts it takes a second for the supply to catch up. it causes incandesent lights to dim as long as it is only for a second and they dont brighten it is nothing to worry about.
Old 07-20-05, 02:53 PM
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This is to some extent normal. Slight dimming can be normal, severe dimming or flickering is not.

How far are you from the utility transformer? The longer the distance, the more severe the dimming will be. Some people report success moving the A/C DP breaker to the topmost slots in the panel (closest to the main breaker), although your results will probably not be significant.

It is possible that a connection on your service entrance wires is slightly loose. You mention that it's all new, so perhaps something at the transformer, weatherhead, meter base, or main panel isn't torqued as much as it could be. I would only have this checked out if the dimming is substantial or there is associated flickering.
Old 07-20-05, 07:21 PM
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Look at the nameplate of your AC unit. You will notice two categories labeled FLA and LRA. FLA is Full Load Amps, the amount of current your unit draws when it is up and running. The LRA is Locked Rotor Amps, the amount of current your unit would draw if it froze up and the rotor could not turn at all. You'll notice that the LRA is 4 to 6 times higher than your FLA.

When you first start the AC, the compressor is at rest (ie-not turning). At the instant of energization, it will pull LRA. This high current causes a voltage drop in the service entrance wires and the transformer. Since your main panel is connected to the service entrance, the entire panel sees this drop and anything connected to it (ie-all the lights in the house) flicker accordingly.

Other than being annoying, it is normal and should not be considered a problem unless the drop is significant enough to not allow the unit to start, or cause other problems (like re-setting your VCR and clocks).
Old 07-20-05, 07:53 PM
Join Date: Jul 2003
Location: Richmond,Va
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Check to see if inside blower and compressor start at the same time.Hvac people often put a timer in circuit to delay compressor start 10 sec.It makes a difference.

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