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Lights dimming on a/c load?? #189840 10/27/09 01:36 PM
Joined: Oct 2002
Posts: 830
sparkync Offline OP
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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Re: Lights dimming on a/c load?? [Re: sparkync] #189846 10/27/09 04:22 PM
Joined: Jul 2004
Posts: 9,569
gfretwell Offline
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
Re: Lights dimming on a/c load?? [Re: gfretwell] #189850 10/27/09 11:48 PM
Joined: Feb 2004
Posts: 1,437
Lostazhell Offline
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.

Re: Lights dimming on a/c load?? [Re: Lostazhell] #189859 10/28/09 11:17 AM
Joined: Jul 2004
Posts: 9,569
gfretwell Offline
I have never seen a service drop larger than #2al for 200a or less.

Greg Fretwell
Re: Lights dimming on a/c load?? [Re: gfretwell] #189865 10/28/09 12:43 PM
Joined: Mar 2005
Posts: 1,213
SteveFehr Offline
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.

Re: Lights dimming on a/c load?? [Re: SteveFehr] #189939 10/30/09 01:41 AM
Joined: Feb 2004
Posts: 1,437
Lostazhell Offline
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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