Hi all. I don't know much about electricity and I'm hoping someone can answer my question. I recently moved into a new house. In the garage there is a 220 volt outlet that was used by the previous owner for woodworking tools. I am planning to hook a table saw up to it as well. While I was setting up the saw the other day I touched the metal conduit that runs along the wall to the outlet and I was shocked. It was not a bad shock, but was definitely noticable.
I decided to check the conduit with my multimeter (which I'm not very good at using) and it looks like there is anywhere from 9-15 volts in the conduit itself. When I shut off the breaker to this outlet it of course goes away. I opened up the outlet to take a look at stuff and everything looks fine. What is strange it that if I pull the plug itself away from the metal box as far as the wires will allow, about 2 inches, turn the breaker back on and check the conduit again the voltage is gone and I can touch the conduit with no problems. Does anyone know what would be causing this and what needs to be done to fix it?
Josh, It seems that you have an exposed wire touching the inside of the box causing the problem (leak). When you pull the receptacles out of the box you are pulling the wires away from the box and the leak stops.
Another concern seems to be that your box and conduit are not grounded properly because I don't think you should get a shock---that's the purpose of grounding---and you should correct this problem for your safety.
Do you rent, let the lanlord get an electrician, do you own get an electrician. You could have other problems even loosing a good bit of money every day to ground. New purchase ? did you have an inspection ?
It is hard to say exactly what is happening, but it does appear that the conduit is not properly grounded. If I recall correctly if the garage is not attached to the house, it must have its own additional ground.
I would not take this lightly. If you are not comfortable with your abilities, I suggest getting a pro in. If you are renting, you are not allowed to do the work yourself and the owner must hire a professional.
A quick guess would be a damaged outlet. It's a cheap and easy test, new outlets are so cheap they are practically free at the big box store. But you should probably spend an extra buck or two and get the industrial grade.
The conduit should be bonded. Turm off the power and tighten every locknut, screw and other joint along the conduit run. See if that fixes the problem. It is generally safer to have a separate grounding wire in conduit but it is not required in the code.
I have a problem with what you are saying. You received a shock from the conduit, yet when you checked with a multimeter you only got 9 to 15 volts. This is not enough voltage to give you a shock. It sounds like something else is going wrong, and you don't know enough about electrical wiring to be able to diagnose nor fix it. It is best to have an electrician come out and throughly check it out. We cannot do that over the internet.
I was waiting for someone to say that. Besides, what kind of short could leak 9v? I think the OP should get someone in quickly who knows how to use a voltmeter.
For a 220 circuit, you need at least three and more probably four wires. One of these wires is a BARE or GREEN wire - house ground. There must be a GREEN pigtail wire with a screw on one end screwed into the box that the outlet is installed into. This, along with a six to ten inch additional BARE or GREEN wire is twisted together with the house ground. The other end of the 6-10 inch pigtail goes to the GROUND connector of the outlet. NEVER EVER count on the conduit to act as a ground.
If it's a three wire plus ground circuit (4 wires total), WHITE is the house neutral, RED is HOT and BLACK is hot. Since the BOX is currently ungrounded, DO NOT count on this being the case. Trace everything back to the panel and confirm that it's installed correctly. Instead of a BLACK wire, it may be BLUE. For two wire plus ground, you will not have a WHITE wire. This is a 220 only circuit, probably for a large motor.
If there's no bare or GREEN wire, rip it out and rewire it from the panel.
As others have said, mysterious problems like this will probably require an electrician's help.
Here's some hints of what he should look for:
The conduit is not properly grounded... that's the starting point. Check all the mechanical connections like pipe couplings and the connectors at the boxes. You've gotta get that thing grounded before going any further.
Of course, this could be a symptom of a more deeply rooted problem elsewhere, like the conduit is becoming energized because the box is energized. A bad ground at your box could cause this, or an improperly wired outlet elsewhere in the house could be energizing the ground because a neutral is open. I'd suspect this isn't the case if you turn the suspected breaker off and the voltage goes away.
Once the grounding is taken care of, then you need to find out where that voltage was coming from to begin with. If pulling the outlet away from the box is causing the voltage to go away, it's probably a nicked wire near the outlet box conduit connector. I've seen many DIY jobs where incorrect fittings were used or bushings omitted... this can easily nick the wiring insulation during installation.
A good electrician will see any of these problems fairly quickly....
Thanks Jake. I figured out how to use the multi-meter I have correctly and it is now reading 125 volts. This probably makes a little more sense. I also figured out that when I pull the receptacle away from conduit and the conduit loses the charge I can still read 125 volts from the metal parts on the 220 volt receptacle that connect to the box. Would this indicate an incorrectly wired receptacle?
Where are you getting the readings from? Obviously you have one meter lead on the mountings for the receptacle, but where are you putting the other lead?
If you're measuring from the receptacle mounts to the conduit, then yes, that indicates either a bad receptacle or improper wiring.
So, to help you further, answer these questions:.
Where are you placing the meter leads when you get 125 volts?
How many wires are attached to the receptacle and what are their colors?
Is there a green or bare copper wire anywhere?
Josh, you may have found part of your problem but there is still a very dangerous situation in that it appears the conduit is not properly grounded. That will need to be looked at before you put this thing back together.
1) Two scenarios for the 125 volt readings: a) 125v from conduit: I am placing the red lead on the conduit and the black lead on the conduit for my furnace which is right next to the outlet. The furnace was jusr installed last weekend so I'm assuming everything is grounded correctly. b) 125v from receptacle: With the receptacle pulled away from the metal box I am touching the red lead to the metal piece that comes out of the top of the receptacle (the piece that gets screwed to the box) and the black lead on the same conduit of the furnace.
2) There are 3 wires attached to the receptacle. They are black and red (which are attached to the gold colored screws) and a bare copper wire that is attached to the green screw.
3) Yeah there is a bare copper wire that is attached to the green screw.
I also checked the breaker panel for where these wires are connected. The sheathing on the wire in the outlet is yellow and there is only one yellow sheathed wire going into the panel so I'm assuming it is the same one. The red and black wires each connect to differnt poles on a two pole 20 amp breaker and the bare copper wire attaches to the ground bar (I believe that is what it's called).
From what you've described, you have a defective receptacle which has an internal ground fault between one of the ungrounded (hot) conductors and the EGC (equipment grounding conductor).
Replacing the receptacle is easy and will correct the immediate problem, but the more serious situation is that this ground fault is persistent. In order for the circuit breaker to open, there has to be a solid "low impedance" ground fault path from where the fault is occurring back to the main service where the ground bus is connected to the grounded service conductor. In a properly bonded system, enough current would flow to open the circuit breaker. That this is not happening indicates that the ground fault current path back to the main service is effectively broken somewhere. This has nothing to do with any grounding electrodes which may be connected to the garage sub-panel (if it is a separate structure the sub-panel should have a grounding electrode conductor run to a ground rod or equivalent.) These electrodes are there to dissipate high voltage surges into the earth. They can never pass enough current back to the transformer to open the circuit breaker. That requires an EGC (which can be the feeder conduit) back to the service panel.
It's possible the sub-panel was not properly installed and is improperly grounded (actually "bonded"), or it's possible that the problem is just in the 220V saw receptacle branch circuit. I suspect the panel, because you described the equipment grounding conductor as properly connected at the panel and at the receptacle. It's hard to image a ground faulted receptacle that would only energize the conduit and not the EGC as well. On the other hand, the furnace circuit conduit appears to be grounded. I assume that originates at the sub-panel as well? If that's the case, the situation is unclear.
You said the wires were in a yellow sheath. Is this Romex (type NM cable) that has been run through conduit to protect it? Does the conduit go back to the sub-panel, or just end somewhere with the uncovered Romex continuing back to the panel? If so, then that could be the problem IF the ground fault is somehow not energizing the EGC, just the conduit.
I hope you realize that you have a highly dangerous situation. The "touch potential" of the conduit, and everything connected to it is possibly lethal. If you were grounded (for instance by holding on to the furnace conduit) while you grabbed the faulted conduit with your other hands enough current could flow across your chest to kill you.
It's good that you're calling an electrician to repair this. Hopefully, they'll correct the problem. There are better and worse electricians, of course. Do not let them just replace the faulty receptacle. The ground path problem must be remedied as well. Best of luck.
Thanks for all the great info! I believe it is romex wire. The conduit runs up into the ceiling, so I got up in the attic to take a look at the conduit. It just runs up into the attic a few inches and then the wire runs the rest of the way to the panel without conduit.
At this point, Kenneth is absolutely correct. It would be irresponsible for me to go any further with this because I'm also afraid there is a problem in that panel.
I did pick up on the fact that a new furnace was installed very recently, and I do wonder if that might be part of the problem here. Some contractors do not take the time to fully check out the wiring to circuits for newly installed equipment, and it's possible they may have confused a ground with a neutral... I've seen it happen.
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