Build my own cabin - DIY projects

Why do my lights dim when my AC kicks on?

One question many homeowners ask is why do my home lights flicker or brown when the air conditioner cycles on?  Do I have something wrong with the wiring in my house?  Some of the major contributors are the heat-pump compressor.  When the compressor is started, there is a huge current inrush to the motor.  A 4 or 5 ton AC unit can draw over 200 amps when it it started.  Depending on the length and sizing of the service entrance conductors between the utility company transformer and your house, this startup current is often enough to drop the voltage to your main panel a few volts. 


Your eyes are actually quite sensitive to lights flickering and just a few volts can be enough for you to take notice.  In some cases, the instantaneous voltage at your lights could be dropping from 124V to 115V but others may only have voltage dropping from 123 to 120V.  In either case, you will see the flicker even though the voltage is still fully within specification for lights or other appliances you have plugged in.  In many cases there are no code violations that you should be concerned about. To know for sure, you test with something like a recording Fluke 43 meter.  When voltage dips by 5% the light output will go down by 10-20% with a standard incandescent bulb.

If for some reason you are seeing voltages drop more than 5% in your home you probably do have a biggest issue to deal with.  You should consider having your utility company look into the electrical service to your house (since they typically size and provide the conductors from the transformer to the meter on your house).  There are cases where the lugs in the main electrical panel have become lose  which creates more excess resistance and higher voltage drop.

If the voltage drop you are seeing is less than the 5% but the dimming lights still really bug you another option is to change out the incandescent bulbs for a different type of bulb.  I had a dimming problem in my own home related to the AC kicking on and a few years ago ended up switching all of our bulbs to compact fluorescents.  This fixed the problem and I no longer notice dimming lights.



Wiring a House