lights dimming

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xxrobertxx

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I have a dedicated line going to my FAU 20 amp everytime it goes on my lights in the dinning room dim slightly and then go back to normal power. Both the lights and the FAU are on a separate circuit. Does any one have an idea what could be causing this problem
 
I don't know what an FAU is but you are witnessing the affects of voltage drop. This is not uncommon for various reasons starting at the utility transformer all the way to the end of your branch circuits.

The dimming can be just annoying or an indication of bigger problems.

Did the dimming just start to occur or has it always been there? Do any of the lights come on brighter as another set dims? How old is you home? What size service do you have? Has it been properly installed and maintained? Lots of questions and the answers to them would change the solution to your problem.
 
At first I thought the FAU was a "20 amp" load. But then I decided its probably an amplifier.

It could be an indication of a problem, or it could just be the power supply caps charging up.

Steve
 
FAU is a Forced Air Unit (heater).

Most likely it is because the fan motor in your heater is drawing a lot of current at start-up and causing a voltage drop on your entire system. If you have a bad connection in the dining room light circuit somewhere, that voltage drop would be more noticeable there. I would check the dining room wring circuit, or better yet since it appears as though you are the homeowner, hire a professional electrician before it causes a fire.

And please don't cross post!
 
Jraef said:
FAU is a Forced Air Unit (heater).

Most likely it is because the fan motor in your heater is drawing a lot of current at start-up and causing a voltage drop on your entire system.
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Yes, it sounds like that if the lights dim slightly then go back to normal.
If the lights are incandescents their output is quite sensitive to even small changes in voltage and the human eye is quite sensitive to changes in light intensity.
 
I've had calls from people asking about a service upgrade because of dimming lights. As a job that pays enough, I usually offer a free visit to the house to price the work. Every service is different; to me, it's impossible to price a service upgrade without seeing the house and existing service.

After explaining the possible causes and possible cures, I tell them I will certainly price the work if it's necessary, but I'd like to see the dimming with my own eyes. More often than not, when we start the AC or heat, the dimming is so minimal, I tell them it's normal, and it would be a total waste of money.

A small amount of dimming is unavoidable. Sure, I like making money, but like an ethical doctor, I don't take advantage of people and tell them they need work they really don't need. I usually get more in trust, referrals, and future work than one unnecessary job would bring.
 
If you are using incandescent bilbs change them to CFL's.

If it is really annoying.... go for a softstarter for the Fan motor.

I had faced a similar problem in a office ( with irrigation pump on the same feeder)...sofstarter did the trick.

cheers.
 
From a past post.

Light flicker (dimming and restoration of light to near pre-flicker 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.
 
When this was happening in my own house it turned out to be a corroded neutral connection between the utility and my service conductors.
 
The OP said the problem was with his FAU (starting up) so I dont think he has a loose neutral or it would happen when any major load starts up (like the fridge) I would start with the capacitor for the blower motor, and then check wiring.
 
He also mentioned a couple of lights that brighten doring the motor start-up. That clearly points to a neutral issue.
 
Termostat?

Termostat?

This might be a stretch but what kind of stat is controlling the unit heaters? I've had 2 instances recently where the stat may have had something to do with it. One was a new home with Geo-thermal. It has a "new fangled" digital/programable stat and the lights would dim everytime the furnace OR A/C kicked on. Recently I upgraded to a "new fangled" digital/programable stat and guess what?!? My lights dim when the furnace OR A/C kicks on. Anyone else seen this?
 
Yes, it could be a neutral problem but the only mention of dimming was when the fau started. I would start there. Capacitors are used to minimalize the inrush on inductive loads like motors and go bad quite often.

Let us know what you find xxRobertxx
 
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