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Thread: Voltage drop in underground cable

  1. #1
    DIY Junior Member silverfox52009's Avatar
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    Default Voltage drop in underground cable

    I have a problem with the underground wire feeding my cabana I run a 110 volt pump to water my lawn as I am on a river ,I have done this for the past twenty years . This year I plugged in the pump and nothing so I removed the gfi receptical and put my tester on the wires comming out of the ground and got a reading of 62 volts. I then went to the panel and tested from the breker to neutral and got 120 volts.I am wondering if maybe something has bitten through the neutral wire ? Is this possible I would appreciate any advice you can give thanx in advance Fred

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    Retired Defense Industry Engineer jadnashua's Avatar
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    A loose or corroded connection anywhere on the run might show up as a low voltage at the end. Does the cable have a ground wire? What's the voltage between hot and ground?
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

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    Electrician ActionDave's Avatar
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    My bet would be a break in the wire underground somewhere.

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    DIY Junior Member silverfox52009's Avatar
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    Quote Originally Posted by jadnashua View Post
    A loose or corroded connection anywhere on the run might show up as a low voltage at the end. Does the cable have a ground wire? What's the voltage between hot and ground?
    yes it does have a ground wire and the voltage between hot and ground is the same 62 volts

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    DIYer, not in the trades LLigetfa's Avatar
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    A high resistance on the feed would result in a voltage drop that is commensurate with the current draw. A high impedance voltmeter testing a circuit that is not under load usually does not give a true picture since it in and of itself, provides no current draw.

    My guess is that along with the high resistance, there is also a short to ground.

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    Moderator & Master Plumber hj's Avatar
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    quote; yes it does have a ground wire and the voltage between hot and ground is the same 62 volts

    That would imply a problem with the "hot' wire, not the neutral.
    Licensed residential and commercial plumber

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    Jack of all trades DonL's Avatar
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    Quote Originally Posted by LLigetfa View Post
    My guess is that along with the high resistance, there is also a short to ground.
    I don't think that is correct.

    hj is correct.

    A short reading from Ground to Neutral is normal, at the source.

    The open is on the Hot leg, if there is a open.

    Or if the pump is trying to run, it may be locked up and not running. that could explain the voltage drop.

    You can use a light bulb for better testing under a small load.
    Theory only works perfect in a vacuum.

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    DIYer, not in the trades LLigetfa's Avatar
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    Quote Originally Posted by silverfox52009 View Post
    ...I removed the gfi receptical and put my tester on the wires comming out of the ground and got a reading of 62 volts.
    Quote Originally Posted by DonL View Post
    I don't think that is correct...
    If he removed the receptacle I very much doubt there is any load on the circuit beyond that of the tester alone.

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    Jack of all trades DonL's Avatar
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    Quote Originally Posted by LLigetfa View Post
    If he removed the receptacle I very much doubt there is any load on the circuit beyond that of the tester alone.
    Very True.

    But if their was a short then the breaker feeding it should trip.
    Theory only works perfect in a vacuum.

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    DIYer, not in the trades LLigetfa's Avatar
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    Quote Originally Posted by DonL View Post
    Very True.

    But if their was a short then the breaker feeding it should trip.
    Depends on your definition of a short. There is a good chance you could tie the hot to a ground rod and it won't draw enough current to trip the breaker. I've seen where a bad heat shrink splice on a pump would leak current to the earth through the water. I've also seen bad elements in a water heater leak current through the water.

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    General Engineering Contractor ballvalve's Avatar
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    If its not in conduit, its time for a new run.

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    Electrical Contractor/Instructor jwelectric's Avatar
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    Quote Originally Posted by DonL View Post
    You can use a light bulb for better testing under a small load.
    This is a very bad idea and very dangerous.
    Don Be warned that another post suggesting the use of a bulb for testing will be deleted

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    Jack of all trades DonL's Avatar
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    Quote Originally Posted by jwelectric View Post
    This is a very bad idea and very dangerous.
    Don Be warned that another post suggesting the use of a bulb for testing will be deleted
    Why is that ?

    It is safer than using the Pump that is in water.
    Theory only works perfect in a vacuum.

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    Electrical Contractor/Instructor jwelectric's Avatar
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    Ground fault is a connection between the neutral point of a power supply and one of the phase conductors.
    Short is a connection between two phases or a phase and the neutral point without load other than the conducting path.
    Open circuit is a break in the path from the source back to the source be it in the phase conductor or the neutral conductor.

    In order for current to flow there must be a source and a path from one potential of the source to the other potential of the source.
    Current CAN NOT leak out of this path and must always return to its source.

    The power companies connect the neutral point of their systems to earth and we also connect the neutral point of our systems to earth. The idea that current is trying to get to ground or earth as ground is defined by the NEC, or that current is “leaking” to earth is only founded in the amount of resistance there is of the earth in that area.

    Let’s be fair and say that the resistance to earth is as low as 100 ohms there could only be 1.2 amps of current flowing through earth. The true resistance of earth would be better than 500 ohms just about anywhere you would want to test. Now the current is less than ¼ amp. The resistance in my neck of the woods between my service and the transformer that supplies me would be more like 1000 ohms and now only a little more than a tenth of an amp will flow through earth back to its source.

    We would hope that there is an equipment grounding conductor connected to the exposed metal of the pump motor and now we have only the resistance of the equipment grounding conducting path back to the neutral point of the power supply and at least 125 amps will flow back to the source causing the overcurrent device to open.

    Knowing this, should a well pump or water heater for that matter is leaking current and it is installed properly then this leaking current will be carried back to its source through the equipment grounding conductor. It will not be leaking through the water to earth or to be correct very little will ever see the earth path back to its source.

    A little test is to connect one side of your car battery to a driven ground rod and see how dead the battery is after a couple of hours, days, weeks, or however long you want to test it. See how much current leaks out of it to earth.
    Last edited by jwelectric; 05-18-2012 at 01:38 PM.

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    Electrical Contractor/Instructor jwelectric's Avatar
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    Quote Originally Posted by DonL View Post
    Why is that ?

    It is safer than using the Pump that is in water.
    Don, Telling untrained people to use a light bulb for testing purposes around electricity is dangerous. If you don’t see the danger there then you need a safety class yourself.

    I have explained the dangers to you before and will not let this thread to be high jacked into a safety course.
    If you don’t truly understand the dangers then start a new thread with any question you feel needs addressing.

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