finding ground in an old building

[Galen Ulrich Elfert]

Some friends of mine and myself have just taken over a room in an old building downtown (Vancouver) to use as a studio. It's a great situation in many ways, but it is also a decrepit and sketchy old building in many ways, and wiring is no exception.

There are two pairs of outlets in the room. One is two-pronged. The other is three-pronged, but the third prong is not connected to anything.

The first outlet appears to have been professionally installed at some point.

The second was simply connected to a piece of romex running up the wall and along the ceiling to a former light fixture, which is also some sort of junction with a bunch of different wires coming and going from three different conduits. None of them appears to be a safety ground.

After some testing I've identified at least one neutral, and what appear to be two separate live circuits 180 degrees out of phase. I get 120v between either of them and neutral, but 240v between the two of them.

First question: what's up with that?

Second, while I've I'm almost certainly going to replace this outlet with a gfci outlet, I would still like to have a proper safety ground, if at all possible. All the wiring is run through rigid metal pipe conduit. Can I use that conduit as a safety ground? Is there a way to tell? I believe I measured a very small resistance between the neutral and the conduit. Is there a better option? Some people have mentioned water pipes but that sounds fishy to me.

Thanks in advance,

Galen

[A.David.MacKinnon]

Using the cold water pipe is pretty common. Calling an electrician is the best idea.

[sound for sandwiches]

After some testing I've identified at least one neutral, and what appear to be two separate live circuits 180 degrees out of phase. I get 120v between either of them and neutral, but 240v between the two of them.

First question: what's up with that?

Second, Can I use that conduit as a safety ground? Is there a way to tell? I believe I measured a very small resistance between the neutral and the conduit. Is there a better option? Some people have mentioned water pipes but that sounds fishy to me.

Thanks in advance,

Galen

Disclaimer: I am not an electrician. You should hire one to do stuff especially in a building where you don't have control over the whole electrical system. I have been upgrading the electrical in my house (with the help of a friend who's in the trade, for the tricky parts) so I have researched these questions above with respect to my local building code, which is probably different from yours.

For the first one: this is a normal situation in your typical 3 wire, single phase household electrical system. It is acceptable for two circuits on opposite legs to share a neutral like you describe, but you have to pay attention so as not to inadvertently supply any one piece of equipment with 240v, which could happen if there is a fault in the neutral and the equipment is somehow wired to both legs. better to have someone who knows, evaluate the situation.

For the second: metal conduit is commonly used as the safety ground. Your typical duplex outlet will connect the ground pin to the conduit through the screws that attach the outlet to the box cover- no need for a green wire. There are so many rules and exceptions about implementing the grounding scheme, At least in Chicago, it's really important that someone who knows the ins and outs of the whole building system see what's up. Typically the water pipe comes into play only once in the system, where the electrical system ground is bonded to earth ground. this usually happens at the main service panel. Especially for audio, I would think you would not want to be grounding things willy nilly to pipes or structural steel, even if it were allowable by code (which it isn't where I live). You'd be asking for ground loops and other headaches.

Good luck.

David

[Galen Ulrich Elfert]

Calling an electrician is the best idea.

Agreed. My assumption was that an electrician would tell me there is no safe way, which seems more and more to be the case.
My hesitation about tying to cold water pipes, radiators, etc. is from not knowing the building's age, or what codes have or haven't been adhered to. If there is a dielectric joint somewhere between the radiator and ground (unlikely, but not unheard of) then a fault might simply electrify the pipes and endanger other people in the building.
The main service panel is just down the hall. I'll investigate the conduit grounding. But my best option might be to run a separate ground line directly from there.
Or maybe I will get a professional to check it out. My real hesitation comes from being cheap and cocky, a combination more dangerous than any faulty grounding scheme.

[sound for sandwiches]

My assumption was that an electrician would tell me there is no safe way, which seems more and more to be the case.

why? likely all that has to happen is to replace the romex with EMT (conduit), and verify that the circuits available have the capacity to handle the loads you will plug in to them. That and swap out your ungrounded with grounded receptacles (assuming there's a ground connection in the box, no additional wiring should be necessary). Even if everything is buried in the wall/ceiling this shouldn't be too difficult.

Assuming the circuits can handle the load, an electrician could also probably add some outlets to the room while they're at it, to save you from having to run extension cords everywhere.

[goose134]

System grounding is done at the panel. The panel neutral is connected to the water main (usually) and then bonded to the enclosure and ground bus. Open up the panel and verify that the neutral is in fact bonded (usually a screw through the neutral bus to the enclosure). Then make sure that there is a GEC (grounding electrode conductor) that goes to an acceptable grounding source. Again, this is usually a water main but can vary.

Your fears of a dielectric joint are well founded, which is why systems are required to be attached to the water main within 5 feet of the main's entrance into the structure ( not sure if Canada has same requirement). The romex will have a grounding conductor in it which should be bonded to the boxes ( if they are metal) and at least to the green terminal on the outlet. In this case, your pipe will be sufficient for a grounding conductor so you should not worry about that.
As for the two hots and one neutral, as has been mentioned, it is common and referred to as a multi-wire branch circuit. Neutral will carry imbalance current. Kind of like balanced cable. Different theory but similar.

I am an electrician by trade. I think you have good questions that can easily be answered with a bit of digging. Hope this helps

[goose134]

Here is a link that shows how a main bonding jumper works and what to look for
http://images.google.com/imgres?imgurl= ... s%3Disch:1

Don't go running grounds because you feel unsure about something. Multiple grounding is a bad situation for people and equipment.
If you have more specific questions feel free to PM me or ask away here. Maybe some images would help too.

[daveg62]

The 2 hots and a neutral are a 120/240v circuit. You could just cap off the one hot leg and use one hot and a neutral for a 120v circuit. Bond your ground lug on your receptacle to the box if it is metal, the pipe can be used as an equipment ground. Best would be to pull another green ground through the pipe to the panel, but if you trace the xtra wire you are capping off and find it goes directly to the panel, there's no reason why you couldn't use it for a ground wire by taping it green, taking it off the breaker and bonding it to ground bus. The other option if you do not have a decent ground would be to install a GFCI receptacle. I would not bother with the water pipe idea.