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#189840 - 10/27/09 02:36 PM Lights dimming on a/c load??  
sparkync  Offline
Joined: Oct 2002
Posts: 826
I've seen this happen over the years and have not really found a problem but maybe one time that I can think of. When an a/c comes on, the lights will dim momentarily
(just for a split second). Is this something that can be called normal or is there actually a problem hid in the circuit somewhere? I know if other appliances such as range, dryer etc... all are doing it, that there is a possibility of a loose neutral somewhere or even possibility of the power company loosing a phase or neutral somewhere, but I'm talking about just a momemtarily and slight dimming. The connections in the electrical panel have already been checked. Thanks for the input..

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#189846 - 10/27/09 05:22 PM Re: Lights dimming on a/c load?? [Re: sparkync]  
gfretwell  Offline

Joined: Jul 2004
Posts: 9,103
It is not unusual. There are a lot of things that can contribute to that. The size of the LRA on your compressor, the voltage drop in the service conductors and the transformer are a the main things. Those are really all out of your control. I don't really notice the lights dimming here except the ones on a dimmer but I do see the dip on the meter I have on the power.

Greg Fretwell

#189850 - 10/28/09 12:48 AM Re: Lights dimming on a/c load?? [Re: gfretwell]  
Lostazhell  Offline
Joined: Feb 2004
Posts: 1,431
Bakersfield, CA (Originally Or...
My house was built in 2005, and my lights don't dim when the A/C kicks in. PG&E's transformer is very close though (on the other side of my neighbor's yard) and I believe it's a 25kva

I've performed service upgrades in older neighborhoods and had customers call saying their lights still dim, etc... and usually it's the poco trying to feed a whole block off of a 70 year old 15 kva transformer with 1/0 (or even #2) secondaries. (Anaheim and LADWP are notorious for this)

Only in SDG&E land did I ever see a poco upgrade their system after the city turned in final on the panel upsize.

#189859 - 10/28/09 12:17 PM Re: Lights dimming on a/c load?? [Re: Lostazhell]  
gfretwell  Offline

Joined: Jul 2004
Posts: 9,103
I have never seen a service drop larger than #2al for 200a or less.

Greg Fretwell

#189865 - 10/28/09 01:43 PM Re: Lights dimming on a/c load?? [Re: gfretwell]  
SteveFehr  Offline
Joined: Mar 2005
Posts: 1,208
Chesapeake, VA
Dimming of incandescent bulbs goes by the square of the voltage, so even a small difference is voltage is amplified, and the human eye is very good at noticing even a tiny difference. It's normal and expected to have some voltage drop in the main feeders whenever a large motor load like a compressor starts up. The poco lines from the transformer to your house are the main culprit, I think. They're simply very small.

#189939 - 10/30/09 02:41 AM Re: Lights dimming on a/c load?? [Re: SteveFehr]  
Lostazhell  Offline
Joined: Feb 2004
Posts: 1,431
Bakersfield, CA (Originally Or...
Greg, I was referring to the "hardline" secondaries from the transformer feeding multiple drops going from pole to pole... a typical new construction 200A service here will get a 1/0 duplex and #4 or 6 for 100A here

San Diego actually upgrades the "pole to pole" duplex up to 4/0 from what I've seen

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