Q&A / 

Lights Dim When AC Comes On

At 11:28 PM on 8/21/2004, Ron Elliot asked me:

DEAR TIM: I built a new home about two years ago. I have the electrical pole next to my barn, not next to the house. The pole has a transformer, meter and a 200 amp service. I had the contractor dig a trench (200 ft) from this pole to the house. This is how the electrical service comes to the house. Each time the air conditioning unit comes on, I notice a slight dip in the lights within the house. The electric company that supplies power says its because the pole is not adjacent to the house. I do not believe this, can you shed any ideas as to why my lights dip when the air conditioning unit kicks in. Thanks.

Ron Elliott

DEAR RON: Air conditioners, refrigerators, freezers, furnaces, etc. that have large motors require enormous amounts of power when the motor first starts to spin up to speed. I am talking huge amounts of energy. Once the motor is spinning, the energy requirement drops way down.

This instantaneous need for motor shaft speed is what causes that momentary dimming of your lights. AC units come equipped with start-up capacitors. These give the motor an electric boost when it first starts up. But over time, these capacitors can get weak and become ineffective. Eventually the power draw can become so enormous, the actual circuit breakers protecting the wiring to the units can pop from the current.

Your air conditioning system may also be overcharged with too much refrigerant. When this happens it puts lots of stress on the compressor and in turn the motor when it tries to get the compressor engine moving.

Relocating the pole would do little to solve the problem. I would ask your AC service man to look to see if your AC unit is overcharged or if it needs new start-up capacitors.


I received this email on 12/05/05:

DEAR TIM: The answers to this problem can also be that the conductors pulled in by the homeowner are too small to prevent the voltage drop being created over 200 feet. The problem could also be with additional homes having been added to the high voltage lines. That will also cause the dips. If surges get serious enough it can affect modern day electronics.

Eric David
Forensic Electrical Specialist


One Response to Lights Dim When AC Comes On

  1. I recently had an airconditioner installed in my cabin. When it kicks in the lights flicker and the thermostat make a klick. I checked with the power company and they said acording to their meters there is plenty of power coming into the building and there have been no power surges regestered. The contractor that installed it said there was no problem he could find and charged me a servic call to tell me so. do you have any idea what the problem could be?


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