Two Circuits, Neutrals and Bare Wire Grounds Gathered, Problem?

In a post to another news group about wiring a 3-way switch leg with a GFCI someone mentioned that the reason the GFCI was tripping when the light was lit via the 3-ways is because the return current for the lighting circuit returned to the panel via the neutral of another circuit. The fix was to ungather the neutral for the lighting circuit and direct it back through the appropriate neutral. OK. Fair enough.
But there was also some discussion of having to unbundle the bare wire ground for the lighting circuit for the lighting circuit from the second circuit also. I surely don't understand why.
As that thread has died out and I'm not getting any more information about this and as I still need to complete that repair I thought I'd ask here about the unbundling or ungathering of the bare neutral wires. Why would it be necessary to have the ground wire grounded to it's own circuit rather than through a second circuit. I've seen bundled grounds all over the house even when there is more than one circuit in the box.
Thanks
David
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Yes, unbundle the bare grounding wire from the neutral (white) wire! While the neutral is grounded, it should be grounded (connected to the same bus bar) ONLY at the main service panel for the building per the National Electric Code. After that point they should always be seperate (even there is (virtually) no voltage differance between them). The neutral is a curent carrying conductor, the bare or green grounding wire normally is not -- only when a fault occurs does it carry current, and on a circuit w/ a GFCI the GFCI will see that the neutral is not carrying the current it should and trip. In any case it is desirted (required to keep then seperated except for the one point at the service enterance.
Further questions or clarifications on grounding, neutrals, 3 ways, GFCI's, other?

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I think he's talking about 2 different circuits run in the same raceway sharing a ground wire. I don't see anything wrong with it -- especially if they are already sharing a neutral wire.
120v GFCI's are oblivous to the ground wire (I *think* 240v GFCI's are oblivious to it also.) They only measure the difference between the hot and neutral wire. They should be grounded, like any other device with a ground connection, but that's a totally separate issue.
-Bob
The Masked Marvel wrote:

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Two *differant* circuits with a common neutral (hot, neutral, hot) assuming the two hots are opposite phases (240V between them) are fine -- in fact superior to two seperate pairs of hot + neutral, as the neutral carries only the *differance* in the current in the two hot legs, whereas two seperate hot neutral pairs the neutral carries all the current in its hot pair. Also, for a hot, neutral, hot set, as long as they are together, a single bare/green ground should be fine.
The GFCI can work even w/ no ground, in fact is required for old work whare no ground is available, the fault current can flow through *you*, through a grounding wire, through spilled water, anything, and the GFCI will see that the current coming to it does not equal the current leaving it and it will trip.
If you had, hypothethicly, two GFCI circuits in a single square box, fed w/ three wires (+ ground) thone one one hot and the neutral, the other on the other hot and the neutral each GFCI is unaware the other tis there or that there is another phase anywhere in the world, its own j-box included, and will trip as required or not, if not required. However if current is drawn by both, th ereturn currents in thr neutral will be 180 degrees out of phase and they will cancel each other, reducing the current in the neutral. The GFCI doesn't see this or care though. In the case of a balanced load, say two 150W, 120V light bulbs, one on each circuit you could cut the neutral wire and nothing would happen as the current to the first bulb cancels the current from the second bulb in the neutral wire, all th ecurrent comes in one hot and out the other. The bulbs are effectively in series, each using 1/2 the 240V. HOWEVER if one burns out, or if one is a 15W and the other still 150W then there is trouble, either no light at all, or the 15W bulb is way too bright and the 150W way too dim. Now you NEED the neutral wire to carry the differance in current and keep the midpoint between the two bulbs at 120V with respect to the two hots.
***** I'm not sure how the 3 way circuits come into this, though sonetimes there are two (potentially) hot and a neutral wire (+ground) in a three way circuit, but it is one circuit, not a pair of circuits as in the example above, and only one of the two hot wires is live at a time --and only if *both* switches have that one live hot wire connected to the load/lamp does it light, if th eother switch is switched to the dead "hot" wire the lmp is off. Only by switching it to the live one (or by switching the other switch to change which is live) does the light light. Again there is only one neutral (and it may not even be run to the switch's junction box), and it is never switched, and there is only one grounding wire.
To further complicate life though, when the neutral is not run to a switch's j-box the white wire in the cable is used instead as a part time live hot lead. In this case it should be permanantly recolored black, red, blue, etc. not white or light gray and not green, since it is NOT neutral.

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99.99% of the time there is nothing wrong with it. However, there can be a situation where you open the breaker, verify the circuit is dead, and then find out there is voltage on the ground from the other circuit. Unlikely, but possible.
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Not at all unlikely. I found that out the hard way but getting shocked off the white wire. Feed to outside yard light runs underground and has common white with another underground feed to a shed. Circuit is non fixable without replacing the underground feeds. At leas now, every box the two circuits run through are prominently marked warning anyone working in them to pull -both- breakers.
Harry K
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Yes, it is unlikely; just not impossible as you found. Since the white it grounded, it ought to be a great deal better ground than anyone touching it; unless there is a problem, there should never be any voltage on it..
However, we are talking about the bare wire here, the grounding conductor. Unless there is a problem there should never be any current on it, and unless there is a second problem, there should never be any voltage on it.
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Giving it a bit more thought; grounds are crossed all the time. If you have a metal box with two outlets on different circuits, the grounds are crossed through the grounding straps. However there are then TWO grounded conductors, so it is actually safer than if they were not crossed. If there was only one, then they would be more dangerous, though only nomially.
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