How do I connect old 2-conductor circuits into modern 3-conductor wiring (with third wire ground)?

Question

I'm moving an electrical junction box that was installed by an electrician. Generally I assume an electrician does things right, but I'm suspicious about something he did in this box I'm moving.

The box has modern 3-conductor (NMB 12-2 Romex, which is 2-wire plus a third wire bare dedicated ground) run to it from the circuit breaker. The box junctions this with an old circuit that is two conductor with no ground.

The way the electrician did this was to tie the white (neutral) wire in the old 2-conductor cable to both the white (neutral) and bare (ground) wires in the new 3-conductor. Obviously, black (hot) was tied to black.

Now, I would have thought to dead-end the third wire (bare ground) to the (metal) box, and to just connect black to black and white to white. I thought you weren't supposed to connect dedicated third-wire ground with  the neutral return wire, even though they're both grounded eventually. What should be done in this situation?

John himself · Accepted Answer

You need to take the ground wire off the neutral wire connection in the junction box. The big problem is, you now have parallel wires carrying the neutral current back to the panel. Anyone disconnecting that ground wire in the panel could get electrocuted, not knowing it is carrying current. The ground wire should never carry current except to clear a ground fault (short circuit). You are correct to dead end the ground wire at the junction box. The rest of the circuit will simply be a two wire circuit with no ground, as it is now. Good catch.

analize2much · Answer

You are exactly right. JUNCTION BOX guys.

The only time you put grounded (white)and grounding(green) wires together is in a disconnect or panel. And only once unless you run it to a seperate building. And at that point a ground rod should be there too. 

This causes transient voltages which is not safe. The neutral or grounded wire has become a loop.

DaveUSA1 · Answer

You are exactly right the neutral (grounded conductor) and the ground (grounding conductor) are only connected at the first means of disconnect. That's in the panel.  What really needs to be done if you want it done right is to replace the old two wire with new 12/2 NMSC. used to we could buy 12/2 with-out a ground now it only comes with a ground(bare wire).  But I wouldn't freak out over it, your still safe.  But if you payed for it to be done according to code, its not.

Irv S · Answer

You are right. That ground should not be tied 
to the neutral conductor. If the box is metallic, 
the ground should be bonded to that.
If your 'old 2-conductor circuits` are run in 
armored cable, ('AC` or 'BX`), this will ground 
the cable jacket and thus the rest of the wiring 
on that circuit.

scott n · Answer

You are right about the bare ground wire,it should be attached to the metal box only and not to the neutral.

How do I connect old 2-conductor circuits into modern 3-conductor wiring (with third wire ground)?

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