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#41806 - 09/09/04 06:46 PM Lights dim for a milli-second when A/C kicks on  
Pat@Amber  Offline
Joined: Aug 2004
Posts: 106
Gaithersburg, MD USA
I have a customer that is convinced that something is horribly wrong because her lights flicker for a millisecond when the A/C kicks on. It happens if one light, or all the lights are turned on. She claims it never happened at her old house. Keep the replies clean- I am going to print out your posts unedited and give them to her.

Thanks in advance,


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#41807 - 09/09/04 07:34 PM Re: Lights dim for a milli-second when A/C kicks on  
cpalm1  Offline
Joined: Jan 2004
Posts: 66
lights dimming when the A/C comes on is normal. she may not have had this problem at her old house because it had a smaller A/C unit, a larger power main, or was closer to the power company transformer. one way to reduce dimming is to install a hard start kit on the compressor. it is basically an additional start capacitor. it kick starts the unit and get is running faster. since the compressor gets up the speed faster, the the duration that the voltage is low is decreased. therefore dimming of the lights is less noticable

#41808 - 09/09/04 07:49 PM Re: Lights dim for a milli-second when A/C kicks on  
twh  Offline
Joined: Mar 2004
Posts: 919
Regina, Sask.
I agree with cpalm1. It happens with many types of motors - air conditioners, furnaces, deep freezes, etc. I don't like it either, but I wouldn't use a hard-start unless it was severe. My lights dim, too.

#41809 - 09/09/04 08:45 PM Re: Lights dim for a milli-second when A/C kicks on  
Bjarney  Offline
Joined: Apr 2002
Posts: 2,527
West-Southern Inner-Northeast ...
It may be possible to have the serving utility check at the building service entrance.

Lamp flicker during motor starting can be very difficult to cure completely, and as to human perception—very subjective.

#41810 - 09/09/04 09:17 PM Re: Lights dim for a milli-second when A/C kicks on  
CTwireman  Offline
Joined: Feb 2002
Posts: 840
Connecticut, USA
The same thing happens at my home when the condensers come on. We have variable speed air handlers that "ramp up" which help a little in the start up current.

I've never thought anything about the lights dimming and I assume it's because we are about 150 feet back from the road and maybe 75 feet more to the transformer. There are 3 or 4 other homes on the same transformer.



#41811 - 09/10/04 05:44 AM Re: Lights dim for a milli-second when A/C kicks on  
maintenanceguy  Offline
Joined: Dec 2001
Posts: 300
Southern NJ, USA
If her a/c compressor is starting while her a/c system is still under pressure, it will draw a large amount of current for a longer than normal time while it tries to start. The a/c should be off for 4 or 5 minutes before the thermostat calls for cooling again so the pressure has time to equalize in the a/c for easier compressor starting.

A delay timer could be added to her a/c pretty cheap if that's the problem.

If that's not the problem it's nothing to worry about. But the poco would probably put in a bigger transformer and bigger cable and it would only cost the home owner a few thousand $.

[This message has been edited by maintenanceguy (edited 09-10-2004).]

#41812 - 09/10/04 07:59 AM Re: Lights dim for a milli-second when A/C kicks on  
winnie  Offline
Joined: Sep 2003
Posts: 649
boston, ma
Here is my stab at explaining this issue someone untrained. Of course, I am saying things that most of the readers of this forum already know [Linked Image]

The flickering that you are noticing is to be expected, and is _always_ present to a greater or lesser extent in an electrical system. Depending upon the characteristics of the particular electrical system, the flicker may not be perceptible. This sort of flicker can be diagnostic of problems, or it may simply be part of the normal operating aspect of your electrical system. In cases where this flicker is normal for your electrical system, 'fixing' it can be quite expensive, essentially involving replacing the entire electrical supply to your home...and this fix will _not_ eliminate the flicker, but will only reduce the magnitude of the flicker.

All electrical components used in your home will _resist_ the flow of electricity. This resistance reveals itself as a reduction in the supply voltage available to your home when the electrical load increases. In essence, when you use more electricity, a bit gets wasted in the wires coming to your home, and the voltage available to run your lamps decreases. Resistance is present in the normal wires to your house, properly connected. The transformer supplying your house also has its own resistance. If the transformer is undersized for the loads, then the resistance might be excessive. (I wouldn't bother trying to explain the difference between resistance and impedance; for the purpose of this discussion they are the same.) Finally, any bad connections will introduce resistance. It is because of the danger of a bad connection that I will analyze your electrical system to determine if the flicker that you are seeing is consistent with safe and normal operation.

These same rules of resistance apply to individual circuits within your home, however the effects much more pronounced on the circuits involved. You may notice this if you have a high power vacuum cleaner or window air conditioner.

Motors, such as are used in the compressors of air conditioners, consume a tremendous amount of power on startup. This power consumption will cause the voltage drop described above, but only for the instant of startup. This is why motor loads will often cause lights to flicker.

As mentioned above, the voltage drop associated with a motor starting, and the light flicker associated with that voltage drop, can either be totally normal and acceptable from a safety point of view, or indicative of a problem. The only way to know for certain is for me to measure the voltage drop and the current flow, and to calculate the resistance of the supply circuit. I can then calculate the expected voltage drop, and tell you if what you are seeing is normal for your particular electrical system.

If I find excessive unexpected voltage drop, then I can locate the damaged part of the circuit and make repairs. If I find that the transformer is undersized, then I can call the power company and inform them of this fact, although they may not do anything if the transformer seems to work. If the 'offending' air conditioner is on a circuit that is shared with the flickering lights, then I can install a new circuit for the air conditioner. Finally, if the air conditioner seems to be consuming too much power on startup, I can recommend that you call a qualified HVAC technician to evaluate your unit. Finally, if I determine that the voltage drop that you are seeing is within the expected value for your particular electrical system, I can discuss with you upgrades that will reduce the voltage drop and light flicker.

Should you even notice a sudden change in the magnitude of the flicker, or if any of your lights flicker _brighter_ for a moment when a motor starts, this almost certainly indicates a problem that will need to be fixed. In this case, contact me immediately.


#41813 - 09/10/04 01:16 PM Re: Lights dim for a milli-second when A/C kicks on  
drillman  Offline
Joined: Mar 2003
Posts: 97
Somewhere in Texas
My parents had central AC installed and a service upgrade done in a 50 year old house. When the AC kicked on the lights dimmed a LOT. The AC company was useless so what my dad did was call the power company and they found a loose screw in the meter. On a new service no less. They just flicker a little now.

Some dimming is normal. But always check the obvious.

#41814 - 09/10/04 11:11 PM Re: Lights dim for a milli-second when A/C kicks on  
Jps1006  Offline
Joined: Jan 2004
Posts: 615
Northern IL
I used to think I had loose connections throughout my house because of periodic flickering and/or dimming. Then one night while out in the garage I saw my lights flicker when the neighbor's air conditioner kicked on.

I scratched "rewire the house" off my to do list.

#41815 - 09/11/04 09:21 AM Re: Lights dim for a milli-second when A/C kicks on  
CharlieE  Offline
Joined: Apr 2004
Posts: 200
Most of the electric utilities are required to follow the old GE Flicker Curve and that does not include the customer's own equipment. If this is just her own AC, I am afraid she is out of luck. However, if the electric utility is called and they chart the flicker, they can do some things that will help the problem. Just move the transformer closer to the home, install larger secondary conductors, or replace the transformer are the most common remedies. [Linked Image]

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