I have some flex conduit with two hots, one neutral, and no ground. Is it OK for two circuits to share the same common? Can I rob the ground from a nearby circuit and have several circuits share the same ground wire?
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"Is it OK for two circuits to share the same common?" - Yes, it is called a multi-wire circuit. The two hots have to be on different legs of the 120/240 so that the current in the neutral cancels, rather than adds. If a single device has both hots on it then you need to use a two pole 240 breaker. Many people suggest the 240 breaker in all cases to help reduce the possibility of working on a hot circuit when only one leg is turned off. Also the neutral has to be pigtailed and not feed through a receptacle. Because of these complications and possible confusition many people recommend against using them, but they are legal if done right.
"I have some flex conduit with two hots, one neutral, and no ground."
Exactly what do you have armored cable, flexible metal tubing, flexible metal conduit, metal clad cable. They all have different rules.
For common "BX" the cable is the ground.
"Can I rob the ground from a nearby circuit and have several circuits share the same ground wire?"
Yes. That is one of the ways to add grounds to ungrounded circuits. To connec to a ground in a circuit that goes back to the pannel ground (or the ground electrode system).
Also in any box, with multiple circuits, all grounds get connected together.
Thanks Bill--that was very helpful. I think I have the common BX maybe [same as flexible conduit?], the stuff with ribs that you can unravel if you pull hard on the end. If a light or appliance is on in the circuit not being worked on, can enough electricity travel through the shared common to give a person a shock, or would that happen only with a malfunction in that circuit? Is there anything special one needs to do in the main breaker box to connect the conduit to the grounding bar?
"If a light or appliance is on in the circuit not being worked on, can enough electricity travel through the shared common to give a person a shock, or would that happen only with a malfunction in that circuit?"
Only with a "malfuction". However, it is every easy to have such a malfuction. For example the connections are not well made and you move the receptacle on the "dead side" and the pigtail to the neutral connections come loose. Then the neutral on the load side will be hot.
ONLY WORK ON SUCH A CIRCUIT WITH BOTH HOT LEGS DEAD!
"Is there anything special one needs to do in the main breaker box to connect the conduit to the grounding bar?"
The grounding is done via the BX connector. That is the device that clamps on the end of the BX and then goes into the metal box.
Also make sure that there is a bushing or sleave that goes into the end of the BX to keep the cut end from cutting the wire.