Lights dim when the ac kicks on...

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lengesjohn

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I have a customer that is complaining about his lights dimming when his ac turns on or when ever he uses some type of tool that has a great pull at start up. Is there any way to prevent this from hapening?? I use to be in the car audio field and we use to use capacitors to prevent the headlights from doing the same type thing... Do they make a capactior to be installed inide a panel to fix this issue or would a possible service ungrade work as well? Thanks
 
Depends on some variables, check for VD at panel.

Light flicker (dimming and restoration of light to near preflicker level) of lights from motor inrush) is a byproduct of several factors and generally is difficult to over come.

AC motor loads (in particular compressors) but all motor loads have inrush currents. These inrush currents result in voltage drop in the branch circuit, panel bus, service laterals, utility transformer and possibility the utility HV feeders.

Flicker is in addition user dependent; some people are more susceptible to flicker. For some reason flicker seems to bother women more that men (based upon personal experience).

A Voltage Drop (VD) as little as 3 volts on a nominal 120 VAC system is noticeable (by me), 2.5% VD.

We have had cases were the VD was all a byproduct of primarily single phase 120 VAC loads, in these cases we were able to put the lighting on L1 and all the motor loads on L2, the HVAC compressors were something the homeowner had to live with. This minimizes the flicker.


To test for this VD and resulting flicker to determine the source of the inrush, use a min max amp clamp and min max multimeter at the main service watch the lights(it may take two workers) and correlate to the meters.
 
This a common event. The A/C pull 6 time its running current when starting and you get excessive VD. The largest drop can be in the utility transformer and the secondary conductors. You may have a bad connection any where in the circuit from the transformer to the inside panel. Check the drop at the panel when starting. If you have the right meter, you can record the starting drop. The transformer may be too small and to far away.
 
one thing i have done before was put all my largest breakers on the top of the panel. dont really know why or if i ever been told to do that, i just always have.
 
kjless said:
one thing i have done before was put all my largest breakers on the top of the panel. dont really know why or if i ever been told to do that, i just always have.
Click to expand...

Because of the heat larger amperage breakers produce, some people put them at the top of the panel to keep the heat from affecting any breakers above them. Now you know why you were told to do it that way.
 
I wasn't sure about why the 2-poles were at the top either. It would seem sensible that it was related to heat... except most of the overloaded and burned out breaker situations I've come across have been with single poles. It's way easier to overload receptacles than it is to overload a calculated and dedicated 220V circuit ;)
 
In my expierence in checking VD and light dimming the location of the CBs on the bus had little affect. I have also generally installed larger loads at the top of the panel due to heat dissipation.

Most residential flicker I have investigated had issues with the utility lateral conductor size distance and transformer sizing. There is little you can do short of putting the lights on a UPS, or as noted with single phase loads you can put them on one leg and the lights on the other. Though this minimizes the issue and may not totally resolve it.
 
090517-1949 EST

lengesjohn:

Brian has outlined all the components leading to your main panel that contribute to the voltgae drop. Even if there was zero impedance from the transformer to your main panel you would still have flicker. My neighbor and I share only about 4 ft of wire from our transformer. His air conditioner causes flicker in my lights, and about a 1 V drop for steady state while his A/C is on.

If you have one or two lights that are the major problem, then replace these with GE dimmable CFLs. In particular the GE dimmable has very stable light output down to slightly over 100 V with sine wave excitation. It drops out about 100 V. May not be true with other dimmables.

The following is a judgement. If your main panel voltage drops below 105 V during inrush, maybe 8 to 16 MS, then contact the power company. If nominal is 120, then 105 would be a 12.5% drop.

.
 
In my old house, we observed the same problem. The dryer line was connected at the end of the string, and half the lights in the house were upstream of it. When I was upgrading the box from 60A to add A/C, I had the electrician run a dedicated line from the dryer to the breaker box. Problem solved.
 
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