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Old 10-26-2009, 10:14 AM   #1
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voltage regulation problem - big voltage drop when load applied


Just moved into a 1960's home. In the bedroom a there is a single pole switch that didn't do anything. I found that it switched a cable that was at one receptacle box, but with no receptacle - it was blanked out. I installed a receptacle - thinking I could put two room lamps on the switch. The lamps have energy savings bulbs. Now the voltage at the switch is good, and the switch resistance is good. The switch gives the receptacle 120V. But under one lamp load, it drops to 100V, and with both lamps on it drops to even less, to the point the bulbs won't work. It would seem the previous owner ran into the same problem, and instead of fixing it, blanked over the receptacle and left the switch going to nothing.
What the heck is going on?
Why is the voltage regulation so bad?

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Old 10-26-2009, 11:29 AM   #2
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voltage regulation problem - big voltage drop when load applied


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What the heck is going on?
Why is the voltage regulation so bad?
You have a high resistance upstream, possibly 25 ohms, due to a bad connection. It could be a wire nut. The bad connection should get pretty hot when current is passing through it.

With a 10A hair dryer loading it a 3v drop is reasonable.

I guess there is some remote chance that this outlet is in series with another outlet, so the 120v is being shared between your bulb and some other load or appliance in your house.

Measure the voltage with the hair dryer loading it; it's resistance is constant at 12 ohms or so, unlike that of bulbs. If the dryer fan doesn't run turn it off immediately.
Note that this test may cause the bad connection to open completely.

If some voltages in your house go up while this goes down you have a bad neutral.


Last edited by Yoyizit; 10-26-2009 at 11:44 AM.
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Old 10-26-2009, 11:32 AM   #3
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voltage regulation problem - big voltage drop when load applied


Give a detailed description of all wires in the box and the switch.
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Old 10-26-2009, 12:25 PM   #4
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voltage regulation problem - big voltage drop when load applied


Sounds like this has been wired in series with other loads. You will need to trace the wiring... do as jb asks, tell us what wires are in each box.
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Old 10-26-2009, 12:53 PM   #5
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voltage regulation problem - big voltage drop when load applied


I don't think it's in series. The switch only goes to that one box, and the ears aren't broken on the duplex receptacle, and both lamps were plugged into that receptacle.
I didn't think about a high resistance connection upstream towards the box. That would make some sense, as the more current pulled to the lamps would drop more volts.

Thanks for all replys. I'll get some details tonight in both the switch box and the receptacle box. It looks like 14 AWG, and the receptacle has a ground, neutral and hot. There are other goofy wiring mistakes, like copper 'hooks' that face the wrong way for the screw to pull it in, so it makes me wonder what's behind the wall...
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Old 10-26-2009, 03:42 PM   #6
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voltage regulation problem - big voltage drop when load applied


You could have a loose neutral problem, possibly some distance away, in the house. Are there any other lights that get brigher when you turn on the lights in question? Disconnect all electronic equipment from any circuit whose voltage varies widely up or down and keep all of it unplugged until the circuit is fixed. Do troubleshooting with incandescent lights on different circuits.
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Last edited by AllanJ; 10-26-2009 at 03:47 PM.
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Old 12-13-2009, 08:22 PM   #7
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voltage regulation problem - big voltage drop when load applied


Ok fellow problems solvers, here is the skinny -

In the bedroom, with the circuit on:
53 V Line to Ground
120 V line to neutral


Ok, lets check the kids room switch that doesn't work:
53 V with switch off and no load
0 V with switch off load (lamp) plugged in
120 V with switch on and no load
Ground Fault Interrupt trip with switch on and load

Where is the 53 volts coming from?
I guess I have a ground current for the GFI to trip - I guess maybe he tied neutral and ground together somewhere?
Why is the voltage different in the bedroom between ground and neutral - means that neutral and ground are at different potentials - maybe ground is at a higher voltage than neutral...

Someone please help.
By the way - I think my house is old split phase where 240 is cut in half with neutral in the middle - but if the two sides are imbalanced it will create a natural ground current. ...

Jeez.

Thanks in advanced.
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Old 12-13-2009, 08:28 PM   #8
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voltage regulation problem - big voltage drop when load applied


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Where is the 53 volts coming from?
Hopefully it's a
http://en.wikipedia.org/wiki/Phantom_voltage
load down your meter with any incand. bulb and read again.
If you measure hot to ground with a GFCI upstream it will trip with this method.

Last edited by Yoyizit; 12-13-2009 at 08:32 PM.
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Old 12-13-2009, 08:43 PM   #9
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voltage regulation problem - big voltage drop when load applied


Digital multimeters will give a "phantom voltage" if there are wires which are not connected but running next to other wires. Ignore the 53 volts.

What you can do is run a long grounded extention cord from the outlet/switch you are testing to a good working outlet in another room.

Then you will have a known good hot, neutral, and ground at the end of that extention cord!

Then you can use those to check the others in the outlet or the switch.

1st in the extention cord end, measure hot to neutral, then hot to ground. You should get 120 volts with each.

Then measure from...
extension neutral to hot at the outlet.
extension hot to neutral at the outlet.
extension hot to ground at the outlet.

What do you get?

This way you can see which wire is bad.

And you can check from extention neutral to hot at the switch.

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