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Thread: Lights Dimming when AC comes on

  1. #1

    Default Lights Dimming when AC comes on

    I have had the HVAC guys out here for three days to work on my AC system because the lights dim when the AC comes on. So far, what they have done is replaced the piston(I assume in the compresson), chased what they thought was a kinked line(busted up my brick for this) and complained that the pressures were to low and to high on the high and low sides. All that and all they have accomplished is to get the shorten the length the lights dim(it's more like a flash now).

    There current thought is that the panel in the garage needs to be re-wired. They said it might be that because the 60 amp breakers are on the top, they are pulling too many amps and the ones below it are the lights dimming. So they think the breakers need to be moved to the bottom. Having a electical/electronic degree, I know this is not going to help. They have that question out to a electrician themselves.

    We had this house built and have been in it for a month. Are there other things I can have them check on? Can this really be an electrical problem.

    Thanks in advance!

  2. #2


    Unless you have a very large service (400A) and a dedicated transformer some dimming is unnavoidable. It's not bad, just annoying. The larger the AC the bigger/longer the dimming. Changing the location of the breaker won't make any difference. Since this is a new house it probably has at least 200A service, so perhaps the utility's transformer has too much load (i.e. too many houses).

  3. #3


    I did not see if this was a new home or older home. However answer wouild be the same.

    If it were me I would go to the following link and perform a demand load calculation to find the minimum service size you are required per NEC rules to have serving your home. If you service size is more than minimum calculated by the link below the follow the instructions after the link.


    If you service size is adequate then take off your panel cover and read to ground the 120 volt rated voltage of both line one and line two of the main breaker service connections from the meter at the main breaker. Turn on the a/c unit while reading voltage on both lines separatly. If the voltage drops during strating of a/c but returns then your voltage problem is before your main service panel. Then ask the power company to open your meter base. Again test the two incoming lines if this voltage drops again inside the meter the problem is with the power company lines or transformer. You may or may not get the power company to correct if on their lines.

    If you find no voltage drop showing on your voltage tester at teh main breaker when the a/c starts then suspect an undersized branch circuit conductor going to your a/c unit.

    Good Luck


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