Lights dim on separate circuits?


Old 07-07-06, 11:50 PM
Thread Starter
Join Date: Aug 2005
Location: Indianapolis, IN
Posts: 278
Lights dim on separate circuits?

I'm wondering what causes this: I have noticed that when certain appliances kick on, like my washing machine, refrigerator, and window A/C (120V), it will cause the lights to dim on other circuits.

My house was built in `48; it was upgraded to a 100 amp breaker box quite some time ago (I'm guessing at least 15-20 years). It is actually a duplex--I'm describing the side I live in. Some of you may remember that I rewired the other side a few months ago; I haven't rewired my side yet. Each side has its own 100 amp box, the power comes in from the pole on one line and feeds both meters. My 100 amp box has five 120 circuits (three 20 amp & two 15 amp), and one 240 (30 amp) for the dryer. They ran 12-2 Romex from the five 120 breakers to the old fuse box, which they turned into a junction box and connected the Romex to the old BX cable that runs throughout the house.

I'm wondering if this is anything I should be concerned about. I will eventually be rewiring everything like I did next door. Over there, I replaced the 5 old breakers with 10 new ones, eliminated all the old BX and ran new 12-2 Romex throughout the house with new switches, receptacles, and fixtures everywhere, following all the advice from here as to code and had it inspected before and after by an electrician (quite a job for a DIY'er, but it's all done thanks to all of your help!)
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Old 07-08-06, 04:49 AM
Join Date: Sep 2003
Location: Central New York State
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Some dimming is normal and nothing to worry about.
Old 07-08-06, 04:55 AM
Join Date: Jun 2006
Posts: 85

Tininindy: Don't think you have anything for undue concern. I've been in many homes which were built in the 60s and which have mere 100a services. For example, in one house, the lights flicker when they turn on the microwave oven. Your 100a service these days is considered minimal. My house built in 1974 has a 150a service and even I notice dimming at times. Since then I think 200a minimum has become the norm.
Old 07-08-06, 06:02 AM
Join Date: Jan 2004
Location: Oregon
Posts: 1,219

Loads on one complete circuit _cannot_ dim loads on an entirely _separate_ circuit. But when we talk about separate circuits in a home, we are talking about separate _branches_ off of the main panel. All of these share the same 'feeder' or 'service' conductors going into the panel, and all of these share the same transformer supplying the service conductors.

If a load on one branch circuit causes dimming on another branch circuit, this indicates resistance in the shared portion of the circuit, 'upstream' of the panel. Since all wire used in homes has some resistance, and since the supply transformer itself has some resistance, there will _always_ be some dimming when loads are started. Bad connections also cause resistance, as does any sort of partial break in the wire.

So the existence of some dimming or flicker when large appliances start up should not concern you, but if you find _excessive_ dimming, or if you notice that the dimming gets _worse_ over time, then this indicates a problem that you should investigate.

Old 07-08-06, 07:39 PM
Thread Starter
Join Date: Aug 2005
Location: Indianapolis, IN
Posts: 278
Originally Posted by winnie
but if you find _excessive_ dimming, or if you notice that the dimming gets _worse_ over time, then this indicates a problem that you should investigate.

That's how I would describe it--excessive and worsening. I'll start out by checking some connections on those branches I haven't worked on yet.

I also noticed today, when working next door, that the lights dimmed over there when my washing machine kicked in on my side. Again, they have separate breaker boxes and meters, but it comes in on the same line to the building and splits where the meters are. Is this something the PoCo should look at on their side?
Old 07-09-06, 01:05 AM
Join Date: Mar 2006
Location: Wet side of Washington state.
Posts: 18,388
This does sound like a Power Company problem if it is affecting multiple houses. It is possible that the transformer serving your and your neighbor's homes is being overloaded.

Call the Power Company and explain your concerns, at the least they will check out the transformer and they may install a recording voltmeter or wattmeter for a period of time to determine the problem.
Old 07-09-06, 04:47 AM
Join Date: Jun 2006
Posts: 85
line xfmr overload

At another location where I resided years ago, a line transformer was in fact the culprit for the problem where several houses were concerned. This particular xfmr had a red lamp that lit indicating the failure. I reported this to the company and it was soon upgraded.
Old 07-09-06, 04:48 AM
Thread Starter
Join Date: Aug 2005
Location: Indianapolis, IN
Posts: 278
It's all one building. I own a duplex and live alone on one side. The other side is vacant--I've been renovating it. I don't think I use that much power.

I'm not sure how they normally set this stuff up, but it looks like 5 other houses are fed off of the transformer behind my property. I know when a squirrel got zapped and the transformer blew, these other houses were out of power too. Mine is the only one that's a duplex. I'll ask the neighbors if they see dimming as well.

Old 07-09-06, 11:41 AM
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Call the power co.. This is not uncommon for their connections to fail over time.

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