lights dimming AC

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gilly

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I have a small problem and am hoping you guys can set me straight. The lights dim when the AC comes on.
I have a 100 amp panel to this house. Line to Neutral reads 123 v on both phases. With all lights on and no AC load. When measureing L1 to neutral when the AC kicks on the volts drop to 104v, on L2 102v. Then within 5 to 10 sec (aprox) the volts returns to 119 with the AC on. Does this mean that the problem is with the utility company or is the service to small. I have found some other problems that we are fixing. The house is around 12 yrs old.

thanks everyone for your time
gilly
 
Re: lights dimming AC

Just a guess is that the problem is a utility one.

You could try running the smallest allowed conductors to your AC and routing them the long way around to get there. :(
 
Re: lights dimming AC

Where did you measure the voltages -- at the HVAC Disconnect or at the main panel? If these are main panel voltages, I think you have a problem with transformer size and/or service drop size. If these are voltages at the HVAC disconnect, then the branch circuit wiring could be too small or the same problems listed for the service panel.

Regardless of the size of this unit, I would hope that the voltage drop at your main service panel would be less than 1V once it is running. What is the MCA of this unit, and what is the wire size and length from the main panel to the HVAC disconnect?
 
Re: lights dimming AC

The measurements were taken at the main panel. The MCA is 21.4 on the nameplate, they have run a #10 to the disconnect. The overall length is 30 ft (aprox) one way. I will have to look at the SE conductors in the morning for that size. I do know that the conductors are ALUM.

If the home owner descides to change to a 200 amp service what is the average size transformer the Utility would install. This house is in the country and the transformer is for this house only.

thanks for the help
gilly
 
Re: lights dimming AC

A larger service will not require a larger transformer or a larger service drop. However, a voltage complaint may require the electric utility to replace both even with the service you have now. Even if the transformer were right next to the service and the drop was sized for 100% of the service size, you will still have some flicker. The key question is how much is too much. Call in a voltage complaint first and leave the service size alone. :D
 
Re: lights dimming AC

also as a reminder, the new high efficiency a/c units incorporating "scroll" compressors require a very high starting torque and therefore draw a very large starting current. most homeowners aren't used to this dimming and think something is wrong? i have had problems with undersized transformers and loose connections on power company services cause problems of this nature!
 
Re: lights dimming AC

On commercial jobs when the lights dim, the utility company normally has bad connections. On residential, I have found loose and burned connection all over the place. I'd call and have the power company check to the meter & check your side while they are there. If it's an underground service, the cable may be about gone. We get a lot of those around here. The POCO say's they are just old and they do quite a few.
 
Re: lights dimming AC

I gave the utility campany a call and they are willing to come out and check everything over.

Just wanted to say thanks for all the information,
will let you know what we find out.

thank you
gilly
 
Re: lights dimming AC

I have been involved in several cases where the homeowners were not happy with the lights dimming.

In most cases the utility had the load calculations correct (supplied by the builder) the wire and transformer were sized per the utility specifications, and the light still dimmed.
A 3-volt dip at motor start up was enough to cause a noticeable flicker.

In our area the utility was happy to change the transformer and lateral at the customers expense.
 
Re: lights dimming AC

Get your a/c man to put a "hard start" kit on the condenser. Then you problem should be solved.
 
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