used leg voltage drops under load/unused leg voltage rises

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Electron_Sam78

Electron_Sam78

Senior Member
Location
Palm Bay, FL
I came across a couple of hand dryers (120V, 20a) that when turned on immediately dim the lights. I tested the voltage when the dryers turn on and the leg which they are attached to drops to 85 volts from 125 and the other leg on the system goes up to 155 the whole time the unit is running. They still put out heat and blow air. I checked the connections in the panel and everything is tight. I haven't gotten into the dryer yet. We have other dryers (the same ones)on our property that don't do this. I suspect something inside the dryer, perhaps a coil going bad. Any thoughts or past experiences that could help?
 
Electron_Sam78

Electron_Sam78

Senior Member
Location
Palm Bay, FL
they aren't fed from a mwbc and everything in the panel is tight. It is a feeder supplied building though and I haven't checked the supply panel yet. There might be a loose connection there. Could it also be a loose connection at the dryer?
 
iwire

iwire

Moderator
Staff member
Location
Massachusetts
No not at dryer.

If they are not multiwire branch circuits then the problem has to be in a feeder or service neutral.
 
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btharmy

Senior Member
Location
Indiana
Put both circuits on the same phase in the panel (different breakers of course). If problem goes away then the neutral up-stream from the panel is to blame. By the way, how long are the circuits, and what size wire?
 
H

hardworkingstiff

Senior Member
Location
Wilmington, NC
Electron_Sam78 said:
nope, separate neutrals and they're tight at the panel.
Click to expand...

Did you check the voltage at the panel with the one dryer operating?

As others have said, it appears to be a neutral and yes, you do have a MWBC with your service conductors. My money is on a neutral problem on the service side of the panel (based on what you've posted). If this is correct, it's a problem that should be fixed today, or the risk of damaged equipment increases.
 
growler

growler

Senior Member
Location
Atlanta,GA
K8MHZ said:
Or two neutrals under one lug in the panel that became loose, but the two wires are still connected to each other, maybe even fused together. (Thus creating a MWBC with an open neutral)
Click to expand...

That's a good point. The multi-wire branch circuit wouldn't have to originate in the panel. The problem could be in a juntion box down the line where the neutrals tie togather.

It is possible for the MWBC to start in a juntion box.Maybe they ran 3 conductor MC down to feed two different dryers with two circuits and one neutral from a junction box.

At times you have to think about what could happen and not what is normally done.
 
Electron_Sam78

Electron_Sam78

Senior Member
Location
Palm Bay, FL
btharmy said:
Put both circuits on the same phase in the panel (different breakers of course). If problem goes away then the neutral up-stream from the panel is to blame. By the way, how long are the circuits, and what size wire?
Click to expand...

The two dryer circuits are short- about a foot of #12 copper. The feeder to the building is about 600 feet from service/distribution panel. Not sure of the size feeders. I got called off the job before I could do any real digging. Will get back to it Tuesday
 
Electron_Sam78

Electron_Sam78

Senior Member
Location
Palm Bay, FL
hardworkingstiff said:
Did you check the voltage at the panel with the one dryer operating?

As others have said, it appears to be a neutral and yes, you do have a MWBC with your service conductors. My money is on a neutral problem on the service side of the panel (based on what you've posted). If this is correct, it's a problem that should be fixed today, or the risk of damaged equipment increases.
Click to expand...

I did check it at the panel while one was operating. That's how I found that the voltage was rising on the other leg since these are 120 volt dryers. One other thing I also noticed was that the phase voltage only dropped about 10 volts from around 250 to 240 V
 
kwired

kwired

Electron manager
Location
NE Nebraska
Electron_Sam78 said:
The two dryer circuits are short- about a foot of #12 copper. The feeder to the building is about 600 feet from service/distribution panel. Not sure of the size feeders. I got called off the job before I could do any real digging. Will get back to it Tuesday
Click to expand...

600 feet is long enough that it could easily be voltage drop problem on the feeder. A long undersized neutral can have similar symptoms of an open neutral because it does have an impedance, the more unbalanced the load the worse the neutral problem will be. With that long of feeder you need to know what your load is and what size conductors you have before you can rule out this possibility.
 
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kwired

kwired

Electron manager
Location
NE Nebraska
btharmy said:
Put both circuits on the same phase in the panel (different breakers of course). If problem goes away then the neutral up-stream from the panel is to blame. By the way, how long are the circuits, and what size wire?
Click to expand...

If you do that the problem should get worse because you are creating even more unbalanced current for the neutral to carry.

best way to test this is to turn off the main breaker disconnect the incoming neutral so it is isolated from other potential current paths and apply only a fixed resistance load such as a heater as a test load (connect it from ungrounded conductor to your disconnected incoming neutral). Test line a to neutral, line b to neutral and watch the voltage with each test.

If you do have an open neutral your test load likely will not operate because by lifting the neutral off the main neutral lug you removed the grounding electrode and equipment grounding systems from the circuit and they were carrying your neutral current before.

If your test load does operate your voltage will likely still drop in a similar way it did with all equipment connected - you have undersized conductors for the length of the circuit if the drop is too much.
 
Electron_Sam78

Electron_Sam78

Senior Member
Location
Palm Bay, FL
found it!

found it!

Ok I checked all the connections from the building to the service equipment and no problems. Then I figured it was underground and after some quality time spent with a greenlee buried line locator, an ideal underground fault locator, a backhoe, and a plain ole shovel I found this:

View attachment 4843

I spliced in a new section of wire and no more problem! :cool:
 
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