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house wiring voltage drop

Discussion in 'Electronic Repair' started by bilfre, May 16, 2013.

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  1. bilfre

    bilfre Guest

    Suspected a wiring problem at a friends house where her computer and
    monitor fed via a UPS is connected. The UPS sounds it's alarm
    sometimes when there is no apparent reason. The outlet in question has
    120v no load and 110v with a 1500 watt load. At my house voltage only
    drops from 120v to 115 volt under the same 1500 watt load. Just
    wondering if this much voltage drop (10v @ 1500 watts) is excessive and
    does it warrant further investigation of the friends wiring? I only
    tested the one outlet at the computer so plan to go back and check some
  2. Guest

    That voltage drop is excessive. The National Electric Code specifies
    a drop of no more than 5% from the breaker box to the furthest outlet
    at full load (Note that a 1500 watt load is about 85% of full load).

    Checking the drop at various points from the breaker box to the outlet
    supplying the computer is a good idea. The faq at also has suggestions.

  3. bilfre

    bilfre Guest

    Hi PlainBill,

    Thanks for the response and the link. I will go back to my friends
    and look for the problem. I suspect there is a loose connection at one
    end or the other since it seems to be intermittent. Checked my own
    wiring with same load and only had a 4 volt drop.

  4. Guest

    Your own experience is more typical. 10 V is excessive. Aluminum
    wiring??? I would be careful not to keep the load on continuously
    just in case there is some heating somewhere in the walls. P = IV =
    20 x 10 = 200Watts, probably not enuf to start a fire but just in
  5. amdx

    amdx Guest

    I could have started a fire in my home a few years ago.
    I plugged a couple of chest freezers into an outlet with out trouble
    for a couple of years. One evening I smelled a burned plastic smell in
    our living room. I could not isolate it before I didn't smell it. A week
    or three passed and the smell recurred, this time I moved a TV away
    from the wall and found the plug was hot and slightly deformed. The
    outlet was also hot.
    I removed the paneling and found the wiring looped through this outlet
    on it way to the one I plugged the freezers into. The connection to the
    outlet was poor and caused the heating with the extra current going to
    the freezers. The heat over time had caused the 35 year old outlet to
    pretty much crumble during removal.
    So, I guess I would say, make sure that most of that voltage drop is
    NOT at one outlet.
  6. Guest

    With standard wiring practices there are several outlets in this run
    of wire. The most likely point for a problem is at the point an
    outlet or switch is connected to the wire. IF you can determine
    intermediate points along the wire run, the best approach is to
    determine if the majority of the drop occurs between a pair of

    Someone else gave a wattage - they miscalulated. The TOTAL power
    dissipation for the drop is about 125 watts - still a sizeable amount.
    The EXTRA dissipation is about 75 watts. One approach would be to
    hook up the 1500 watt load, then carefully check outlets to see if any
    feel warm. If no suspects are identified, wait 15 minutes or so (with
    the load still present), then repeat.

  7. Billfre-

    Also look for evidence of a poor neutral connection. When that happens,
    voltage will divide according to the load on each line. If that were
    the case, ten volts drop at a high current load could cause a ten volt
    rise across the other line-to-neutral circuit.

    Five or ten volts doesn't sound like a serious problem, but a poor
    neutral connection could get worse with age.

  8. bilfre

    bilfre Guest


    Thanks for that tip, I think it may be the problem here. I went back
    and tightened the screws on both leads at the outlet and at the hot
    side on the breaker. One screw at the outlet tightened maybe a quarter
    turn. This brought the voltage drop down from 10v to 7v. Still over
    the 5% specified by code. I also put a voltmeter on another outlet in
    the same room that was on a different breaker. When I loaded the
    outlet that had the 7v drop, the voltage at the other outlet went up a
    couple of volts from 120v to 122v and I wondered, what could cause that?

    Then you posted about the poor neutral and I thought about the 2v rise
    I had measured at an adjacent circuit. Then remembered that I did not
    check tightness of the neutral wire connection in the panel. Think I
    will go back again and make sure the neutral wire is good and tight at
    the panelboard. Maybe that is why it is still over the 5% drop spec.

    Does this make sense?
  9. tuinkabouter

    tuinkabouter Guest

    Measure the drop at the breaker too.
  10. bilfre

    bilfre Guest

    The problem is only at one wall outlet inside the house. Please
    explain how it could be caused by a problem outside the house. .
  11. Bilfre-

    This problem exists somewhere between a power company transformer and
    where the neutral connects in your breaker box.

    When I had the problem, I called the power company. Their man checked
    the transformer, but said I would have to hire an electrician to fix my
    end of the circuit. While he had the meter removed, he went ahead and
    tightened the connection there, which solved my problem.

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