I have a receptacle that reads 120v when tested, but shows 75v when the dehumidifier (4.4 amp -- no other hogs on that circuit) is plugged in.

There is no problem on this line until AFTER a junction box. BEFORE the junction box, everything measures fine.

I disconnected the hot leads at the j-box, and unwound and re-wound the twist (there are 3 wires connected).

There is nothing else in-between the receptacles and this j-box.

Why am I getting low voltage when the dehumidifier is on?

PLug something other than the dehumidifier in the receptacle and report the voltage... a lamp, fan, anything. Does the dehumidifier seem to run correct? Plug it in another receptacle and see what voltage you read.

No, there's not enough juice for the dehumidifier to do anything but run its fan.

I plugged it into other receptacles -- all good. No voltage drop.

If there is nothing else on the circuit... you have either a loose connection or a bad breaker. I have seen outlets and breakers show full voltage until a load is put on them. Are you savy with your electrical knowledge to get into the breaker box? If not, call an electrician. If you are comfortable with the breaker box, switch the wire with another breaker on the same phase and try your dehumidifier. If that solves it, it's the breaker.

It shouldn't be the breaker because everything on the line BEFORE the j-box measures fine, load or no load.

Everything on the line? Like what? Is there another receptacle on that breaker? Lights... what is on that circuit?

Several outlets are on the line behind the j-box. The greatest load is a coffee maker that sees occasional use.

If the problem is at that outlet, change the outlet and see if the problem goes away. If it doesn't it's a connection problem. Check all connections. If that doesn't work, it's a wiring problem. It's all about trouble shooting, start at the outlet and work towards the panel until you isolate the problem.

Measure between the Hot and Ground, If the voltage doesn't drop under Load, the Neutral is the problem and should be checked as well, in the junction box first.

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Originally Posted by Stratmando

Measure between the Hot and Ground, If the voltage doesn't drop under Load, the Neutral is the problem and should be checked as well, in the junction box first.

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Yep, the voltage doesn't drop. I assume it's the neutral.

Now, I have replaced the outlets on one end, examined, re-stripped, and tightened the junction on the other end.

Still, there is a problem. Do I start ripping open walls to examine the wire?

Wire seldom breaks inside the walls. I would check all recepticles/junctions between last know good receptacle and first bad receptacle.
Including the last good and first bad one.

What kind of connections are being used at this and any receptacle on this circuit?

I understand the problem is between a JB and the last receptacle, so this should only involve the splices that you did rework in the JB and the problem outlet.

If is is as simple as that, and the connections at this outlet are wrap around the terminal screw, seems the cable is in question, thous cables rarely do fail, it can happen, and the cable may need to be replaced.

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Originally Posted by tkrussell

What kind of connections are being used at this and any receptacle on this circuit?

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Are you asking how I joined things? The connection in question was three 12AWG wires nutted together (after twisting.) I went so far as to cut and re-strip the wires.

And yes, I wrapped around the terminal screws.

It MIGHT be the wire: there is a section in the run where the wire is buried in an older wall (not my work). I have no idea what shape it's in. I DO know that it is made from 2x3s, and maybe those aren't deep enough to protect the wire running through it from nails?

All I know is that small loads work on this line, and heavy loads do not.

If the wire is nicked, why does it read 125v with no load, but 70v with a heavy load? Shouldn't it read 70v all the time?

Before you rip the walls open can you test the voltage at the JB while you have a load on it? Maybe wire in another outlet temporarily?

Where does the feed for the JB come from? Have you checked those connections?

Nicking the wire doesn't drop voltage much.
It does create a weak point and if you find nicked wires, it may be all over in which case, a wire may have broke at a wirenut, but appears solid.

I can imagine that a nail or screw could pierce a cable, break the conductor, and perhaps even act as a conductor in between a broken wire.

Or some event similar.

Any broken or loose connection can conduct voltage and give a normal reading. Once a decent load, such as a dehumidifier, which is a refrigeration compressor, and these always have high starting currents, will cause a bad connection to heat up, and increase the resistance at that point. This resistance becomes just another load in series with the actual working load, the dehumidifier, and drop the voltage, just like what is happening.

Very feasible.

Kiss, another expert here, can actually show the formulas to calculate this voltage drop using real values and show exactly how it works mathematically using Ohms Law.

I don't have the patience to do it.

Why you want to get this fixed besides not having use of the dehumidifier is that a bad connection under these conditions can be what is called a "glowing connection", where there is heat like this there can be fire.

So discontinue any use of this circuit until it can get repaired.

Can you Identify the Cable in the Junction Box That goes to the Box with Low Voltage?
Then Double Check that Neutral.

You might have to go do things very unconventional.

Locate the drop non-destructively.

You can use a breaker finder or other AC detection system and trace the circuit in the wall. Might cost you $40 or so.

SO once you have the best map, put a load on the circuit and start measuring voltages between outlets. The neutral of the outlet before and another. Voltages measures will be small until you find the bad link and you'll find 30V.

I'd also make sure the breaker is not warm to the touch. Measuring between the main bus and the output of the breaker is also not out of the question. This may be a potentially difficult method, but it works

I didn't say to do this, but as long as the outlets are of the correct polarity and I'd check, you can re-route sections externally or for that matter you can make a smal cord with two male ends. A kill me quick for sure. You can use it with an extension cord an bypass certain sections until the problem goes away.

It's essential to know where you think the wires are heading and a circuit breaker finder may be the best way.

http://www.circuitdetective.com/


And a bit more spiffy: http://www.drillspot.com/products/92...Breaker_Finder

Hopefuly if will work through the walls. An AC voltage detction pen will, but it can't tell which circuit.
An inline fuse would be great.

There was a nice two part article in Circuit Cellar the past two months for designing a circuit tracer. A bit high power and only works on denergised lines.

It can locate stuff through walls and stuff buried 6' underground. Pretty cool.

Guy though commercial ones were to expensive to locate a break, so he built his own.

Quote:

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Originally Posted by medic-dan

Before you rip the walls open can you test the voltage at the JB while you have a load on it? Maybe wire in another outlet temporarily?

Where does the feed for the JB come from? Have you checked those connections?

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Connections checked, and I checked the voltage: the feed is 120 with a load.