Lights flicker and Voltage Drop Question

Discussion in 'Electrical Forum discussion & Blog' started by Pip, Dec 16, 2008.

  1. Pip

    Pip New Member

    Messages:
    3
    Hello,

    I stumbled across this forum while researching an issue with a house I just purchased and was hoping to get some advice. The problem I'm having is that the lights dim, seemingly randomly, all over the house. I connected a voltmeter to an outlet and witnessed a drop from 119/120 Volts to 114/115 Volts during these dims.

    My house is 100 years old in good old Altoona PA, and has had half of its knob and tube wiring replaced. The house used to have an old fuse box, but the seller had an electrician come in and install a 100 amp service and breaker box before we purchased it. The city of Altoona took their sweet time inspecting the work, so we had already moved in by the time that happened. I had already paid a home inspector to look everything over and he hadn't found any issues with it. When the city inspector came, I told him of the issues we were having. He doubled checked the tightness of the neutral lines and couldn't find anything loose. He passed the install and put his sticker on the breaker box door.

    The line from the breaker box to the meter is new, the meter is new, and the line from the meter to the feed in from the street is new.

    I've had everything turned off in the house (including the gas furnace and refrigerator) and just left a light on. The voltage drop still occurs. I'd really like to believe that this problem is a result of the breaker box being installed and that the seller didn't hide a pre-existing problem, but I just don't know.

    Should I try and get the power company come out and examine their part outside the house? Should I try and track down the electrician that did this install and see if he made a mistake? Or should I try and find a new electrician and employ him to investigate this mystery. It's Christmas and I just bought a house, so I'm trying not to go crazy spending money, but I want to be safe and not have the house burn down.

    Any advice will be greatly appreciated. Thanks!
  2. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    22,348
    Location:
    New England
    My unprofessional opinion, if everything but one light was off and you have fluctuations, and everything from the meter to the panel is tight, it could be the transformer feeding your house, the drop from the transformer to the meter, or you're at the end of a long country road and the power to the house fluctuates. There may be nothing you can do about it but complain. I think I'd call the power company, explain what's happening, and see what they say.

    Depending on where you live, they may not have too many crews available, as those from a huge number or states have sent crews to New England to try to restore power to people that have been wtihout since last week's ice storm. So, you may not get anyone to come look for awhile.

    That voltage dip is visible, but probably not dangerous (it is annoying, though). I'd put good quality surge suppressors with line filtering on any expensive electronics and maybe consider a UPS if it is the incoming power.
  3. jwelectric

    jwelectric Electrical Contractor/Instructor

    Messages:
    2,569
    Location:
    North Carolina
    I agree with this statement
  4. Thatguy

    Thatguy Homeowner

    Messages:
    1,459
    Location:
    MD
    Put a 10A load (hair dryer, toaster) on any outlet. 50' (from the panel) of #14 AWG Romex (100' conductor length) should give a 2.5v drop in the measured voltage.

    Another guy had twice this drop with his hair dryer load so he tightened all the wirenuts between the outlet and the panel, and it fixed it.
    If the romex is dissipating 25w @ 10A, his half-dozen wirenuts were dissipating another 25w.
    No risque jokes, please. . .

    In MD, if the voltage at the panel is not between 114v and 126v (228v to 252v) PoCo is supposed to do something. You can measure panel voltage without removing the panel cover at an elec. dryer outlet (with the dryer off) or any 220v outlet. In this case the connecting wires are just used as voltage sensing leads.
    Last edited: Dec 16, 2008
  5. Pip

    Pip New Member

    Messages:
    3
    I forgot to mention that this seeming happens randomly. Some evenings when I'm home it will happen 20 times an hour. Then it's gone days without happening. The drop is in knob and tube lines as well as the romex lines. I replaced the dryer outlet a week ago, and had tested it before turning the power off. It was at 240 between the two hots. Also, I live in the middle of town, so it's probably not a rural thing.

    I figured I probably should call the power company and try and talk them into coming out. This should be a pleasant call.
  6. Thatguy

    Thatguy Homeowner

    Messages:
    1,459
    Location:
    MD
    There is a high resistance upstream of the flickering lights, but downstream of the panel, in your house.
    I doubt that PoCo is interested in this.

    The power that would go into lighting the lamps is going into heating the bad, high resistance connection.

    Do you have aluminum wiring?
  7. Nate R

    Nate R New Member

    Messages:
    472
    Location:
    Milwaukee, WI
    How do we know that it's downstream of the panel!?
  8. Thatguy

    Thatguy Homeowner

    Messages:
    1,459
    Location:
    MD
    "all over the house"
    My bad!

    Now I'm thinking it's the neutral.

    If you have a elec. dryer outlet you can use this to monitor voltages at the panel without removing the panel cover.
    Turn off everything in the house. Put a 10A load into any outlet. If the voltage on the other side of the neutral increases at all as measured at the dryer outlet when the 10A load is switched on, the neutral connection is bad.

    I can't post a spreadsheet, but here is what you can expect with a 10A load on one side and a very small load on the other side and a neutral resistance of 0.1Ω (normal is probably 1 microohm). The voltage on one side of the neutral will be down 1v and the other side will be up 1v. With 10w being dissipated in the neutral connection, it will probably be hot or warm.

    Ia, Ib are the currents circulated due to V1, V2, respectively
    They both go through Rn, the current through Rn being Ia - Ib.
    V1, V2 are voltages at the panel and do not take Romex drop
    into account.

    120.0 ent V1, volts
    120.0 ent V2, volts
    0.1 ent Rn, ohms
    12.0 ent R1, ohms
    3,000.0 ent R2, ohms

    3,000.1 calc'd A= Rn+R2=
    12.1 calc'd B=R1+Rn=
    1,464.0 calc'd Num1= V1*Rn+B*V2
    36,301.2 calc'd Denom1= (A*B)-(Rn^2)

    0.0 calc'd Ib= Num1/Denom1, amps

    120.0 calc'd Num2= C*Rn + V1
    12.1 calc'd Denom2= B

    9.9 calc'd Ia= Num2/Denom2, amps


    1,195.0 calc'd total power in, (V1*Ia) + (V2*Ib), watts
    9.8 calc'd power dissipated in Rn, [(Ia-Ib)^2]*Rn, watts
    1,180.3 calc'd power dissipated in R1, [Ia^2]*R1, watts
    4.9 calc'd power dissipated in R2, [Ib^2]*R2, watts
    1,195.0 calc'd total power dissipated, watts

    1.0 calc'd voltage across Rn, volts
    119.0 calc'd voltage across R1, volts
    121.0 calc'd voltage across R2, volts

    (2.0) calc'd voltage difference, R1-R2, volts

    If it's not the neutral, 100A service loaded with 10A at the dryer outlet (two identical 120v hair dryers in series?) should give no more than an 8v drop, measured at the 240v outlet. A 16v drop would be cause for concern.
    Last edited: Dec 18, 2008
  9. Pip

    Pip New Member

    Messages:
    3
    Thanks! I'll try it this weekend. I've a Kill-A-Watt so I should be able to measure the load to see if I can manage to draw 10 Amps.
  10. Thatguy

    Thatguy Homeowner

    Messages:
    1,459
    Location:
    MD
    The nameplate amps are close enough. A hair dryer, toaster, whatever should be fine. Look for the change in voltage as you switch off the load.
    If it's a 12A load instead of a 10A load, then it's 12/10ths of the voltage changes I posted.

    Hopefully, you'll either see normal readings or way-off-normal readings.
    Last edited: Dec 18, 2008
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