Dimming lights and unbalanced voltages on single phase residential service

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jaylectricity

Senior Member
Location
Massachusetts
Occupation
licensed journeyman electrician
Friend of mine complained of flickering and dimming lights at his house. Often when the heat kicked in, but also during other times.

Found many issues in his panel including a conductor on a breaker that was loose, had heated up and melted the insulation. Corrected that. Also noticed that just flipping some of the AFCI breakers on and off caused lights to flicker.

One such AFCI breaker was for the forced-air furnace. After investigation also noticed the furnace was on GFCI protection through the personnel GFCI receptacle at the unit.

Through various tests I noticed that phase A was giving 114V and phase B was 124V. When the fan motor of the furnace kicked in, there was a 10V drop that dimmed the lights for a second. The heat was initially on phase A. When I moved it to phase B, the lights didn't dim as much and the fan sounded much more efficient. But on both phases the voltage dropped by 10 at startup.

Other than the unbalance, the voltage held very steady for each phase, moving less than 3/10 of a volt in either direction for a full minute of testing.

Is this a power company issue or should I be looking for loose connections or improperly bonded neutrals? I advised him to turn off one breaker at a time for half an hour and take note of any improvement.
 

jaylectricity

Senior Member
Location
Massachusetts
Occupation
licensed journeyman electrician
And yes, I realize it's late. Just haven't had time to check out the internet. I appreciate any responses at any time over the following days.
 

Canton

Senior Member
Location
Virginia
Occupation
Electrician
Friend of mine complained of flickering and dimming lights at his house. Often when the heat kicked in, but also during other times.

Found many issues in his panel including a conductor on a breaker that was loose, had heated up and melted the insulation. Corrected that. Also noticed that just flipping some of the AFCI breakers on and off caused lights to flicker.

One such AFCI breaker was for the forced-air furnace. After investigation also noticed the furnace was on GFCI protection through the personnel GFCI receptacle at the unit.

Through various tests I noticed that phase A was giving 114V and phase B was 124V. When the fan motor of the furnace kicked in, there was a 10V drop that dimmed the lights for a second. The heat was initially on phase A. When I moved it to phase B, the lights didn't dim as much and the fan sounded much more efficient. But on both phases the voltage dropped by 10 at startup.

Other than the unbalance, the voltage held very steady for each phase, moving less than 3/10 of a volt in either direction for a full minute of testing.

Is this a power company issue or should I be looking for loose connections or improperly bonded neutrals? I advised him to turn off one breaker at a time for half an hour and take note of any improvement.

First thing I would do is look for loose connections at all points (looks like you have already found one on a breaker)..neutrals also.

Couple of Questions:
Why is the furnace on a AFCI?

You are way up north, I am assuming this is a Natural Gas Furnace (or other) and not a Heat Pump, correct?....so...

You only have one 120 volt circuit for the Furnace Fan...20amp...?....on a AFCI.

Are the lights that flicker on the Furnace blower circuit or separate circuit?..Are they on the same Phase?

Do the lights in question that flicker only in a certain area...or any that are on in the house...?

Before looking at the Utility...are the lugs tight in the panel...especially the neutral connection..?

These are the first things I would look at, I had a similar situation in the past but it was the utility in the end....their neutral connection at the transformer. The neighbors had the same issues.
 

jaylectricity

Senior Member
Location
Massachusetts
Occupation
licensed journeyman electrician
It was strange that the furnace was on an AFCI. The circuit only served the furnace and the GFCI receptacle on the furnace. The condensation pump was plugged in to that.

It's all lights in the house. And it happens during times the heat isn't on as well. Fridge causes same kind of problems.

I didn't think to tell him to ask the neighbors if they were having similar problems. And that's a good idea about opening the main breaker and testing for voltage then.

Main lugs were tight.

Thanks so far.
 
Location
MA
Call the power company. Sounds like you have a broken neutral. If you shut off the main and the voltage goes to normal at the main, then you know you have a broken neutral. You won't be able to see the voltage fluctuating without load. Also, the voltage is probably going up by ten volts on the high side that is not feeding the furnace.
 

Canton

Senior Member
Location
Virginia
Occupation
Electrician
It was strange that the furnace was on an AFCI. The circuit only served the furnace and the GFCI receptacle on the furnace. The condensation pump was plugged in to that.

It's all lights in the house. And it happens during times the heat isn't on as well. Fridge causes same kind of problems.

I didn't think to tell him to ask the neighbors if they were having similar problems. And that's a good idea about opening the main breaker and testing for voltage then.

Main lugs were tight.

Thanks so far.

I agree...take a voltage reading on the line side of the Main with it open.

Since it is not just isolated to when the furnace kicks on and its the lights throughout the house that flicker you can possibly rule out branch circuits.

Is it an overhead or underground service? It is starting to sound more and more like a utility neutral issue. Let us know the voltage readings on the Main...and then take them again after you reclose the main with the house loads on it.
 
Location
MA
Two most common places that I see for bad neutrals besides tree contact is weather head connections and meter trough connections. Barrel type house connections(the ones with the two allen screws) are the worst connections ever made so if those are up there, they're most likely the culprit. Also, you should check the meter trough as well. Either way the POCO should be able to narrow it down really easily with the readings that you have pretty quickly.
 
In the past,I have called the local supply authority and they tested the meter socket for voltage fluctuation and found none with the meter pulled. They quickly reinstalled the meter and walked away claiming the problem to be in the house or at the panel. After 2 attempts to no avail I met them at the meter and had them check the voltages with the load connected. The problem was the neutral connection at the meter can even though there were no physical signs of problem at the lugs. This can also cause a poor resistance connection at the service ground connection. Check the connection for corrosion and clean or repair as needed.
 

Little Bill

Moderator
Staff member
Location
Tennessee NEC:2017
Occupation
Semi-Retired Electrician
In the past,I have called the local supply authority and they tested the meter socket for voltage fluctuation and found none with the meter pulled. They quickly reinstalled the meter and walked away claiming the problem to be in the house or at the panel. After 2 attempts to no avail I met them at the meter and had them check the voltages with the load connected. The problem was the neutral connection at the meter can even though there were no physical signs of problem at the lugs. This can also cause a poor resistance connection at the service ground connection. Check the connection for corrosion and clean or repair as needed.

Had a call a while back for things burning up when plugged in to certain receptacles. HO said he had it checked two or three times before. I believe POCO was involved on two occasions.

I quickly discovered that there was a neutral problem ahead of the main panel in the house. It was a mobile home and of course there was a disconnect/panel outside. I found that the problem was even ahead of the outside main.

I called the POCO and they checked (reluctantly) the weatherhead connections. They said there was no need in checking at the transformer (pole mounted) because they had just replaced the transformer to try and fix the problem and made sure of their connections at the time.

I asked them to pull the meter. When they did, I saw corrosion around the neutral lugs. So I removed the conductors, cut off the ends and re-stripped them. But I also took a wire brush and cleaned out the lugs.

Put everything back and the problem went away.
Oh, I was getting around 170V on one leg and 70V on the other before any repairs were made.
 

jaylectricity

Senior Member
Location
Massachusetts
Occupation
licensed journeyman electrician
So I'm glad that it sounds like it's going to be a simple issue that either the POCO will solve or maybe I'll have to tighten some lugs or connections at the meter or the drop.

But now I'm wondering how flipping a breaker on and off would effect the lights in the whole house. I'm positive one of you knows why. A lot of smart people on this board.
 

GoldDigger

Moderator
Staff member
Location
Placerville, CA, USA
Occupation
Retired PV System Designer
Because any large load one one line will offset the voltage on the neutral. And that will affect the voltage applied to every other load in the house.
Lights on the same line will get dimmer. Lights on the other side will get brighter.

Sent from my Droid Maxx
 

cowboyjwc

Moderator
Staff member
Location
Simi Valley, CA
I agree...take a voltage reading on the line side of the Main with it open.

Since it is not just isolated to when the furnace kicks on and its the lights throughout the house that flicker you can possibly rule out branch circuits.

Is it an overhead or underground service? It is starting to sound more and more like a utility neutral issue. Let us know the voltage readings on the Main...and then take them again after you reclose the main with the house loads on it.
I agree also.
 
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