I would like to install a GFI outlet in my bathroom that would replace a standard plug; the old wire is 2-wire (no bare ground). Will this work? Also this same circuit is wired into a splice box that has two circuits in it;( the one from bathroom plus another) it is fed by a three wire cable from the breaker box that uses the neutral wire for both circuits; will this configuration affect the operation of the GIF.

With no bare wire(ground wire) you must have old bx cable. If this is true I believe the metal bx cable is your ground and you must have a metal box? If so what I have done is to make a ground wire that goes from the gfi outlet and screw it to the metal box to conduct your ground. Don't know if this is code I am no electrician but it has worked for me. Black wire should be hot, white wire should be neutral, and you use bare or green wire to make a jumper from the green ground screw on the outlet and screw it to the metal box tightly. Be careful old bx wire may have insulation that is dry rotted and it may just fall off when working with it. If you have any experience try to losen the bx clamp and pull some slack into the box were you can resplice back remove some of the bx cable to get to some good insulation.

with no bare wire(ground wire) you must have old bx cable. If this is true I believe the metal bx cable is your ground and you must have a metal box? If so what i have done is to make a ground wire that goes from the gfi outlet and screw it to the metal box to conduct your ground. don't know if this is code I am no electrician but it has worked for me. black wire should be hot, white wire should be neutral, and you use bare or green wire to make a jumper from the green ground screw on the outlet and screw it to the metal box tightly. Be careful old bx wire may have insulation that is dry rotted and it may just fall off when working with it. If you have any experience try to losen the bx clamp and pull some slack into the box were you can resplice back remove some of the bx cable to get to some good insulation.

No I don’t have a metal box and the cable is not metal sheath. My question is will a gfi work OK without the bare ground wire? Or should I run new wire to splice box? Also I still have the concern with the two circuits in the splice box using the neutral wire in common. Thanks.

[QUOTE=daniel38]No I don’t have a metal box and the cable is not metal sheath. My question is will a gfi work OK without the bare ground wire? Or should I run new wire to splice box? Also I still have the concern with the two circuits in the splice box using the neutral wire in common.

The outlet will work with a hot & neutral but it won't be a ground fault circuit interrupter (GFCI) without a ground wire. So your answer is no and yes you would have to run a ground from your ground-neutral bar in your service box to the gfci. Better to just run a new line altogether. I am NOT an electrician but sharing a neutral I believe is OK as long as the two neutrals are tied together with a wire nut and has a pig tail going to the outlet or switch.

Are you sure they are 2 different circuits? If they are, they need to be on opposite phase,
Or neutral could carry up to 40 amps. Measure between breakers of the 2 circuits, needs to read 240 volts, NOT zero. Zero means, it is on same phase.
3 wires would be best.
You could have 4 or more whites and blacks on same circuit. Don't know if when you say 2circuits, do you mean 2 recepticles?

I will try to summarize what I think I understand from what you both have told me; if I want the gfi to work as a gfi I will need a separate wire ground back to the breaker box ground bar is this correct? Also the two circuits I am talking about are to separate circuits that are in a splice box that go to the breaker box via a 3 wire cable the red wire in the three wire cable goes to the bathroom circuit that I want to replace with a gif the black wire goes to another room; both of these circuits use the same neutral from the splice box back to the main breaker box; will this use of the common neutral affect the operation of the gfi. I understand the gfi detects the flow of current by comparing the flow of current between the hot wire and neutral. This is the way I understand it; Please correct me if I am wrong. Thanks for all your help.

The GFCI only cares about the differences in current in N and L1, so a GFCI should trip with only two wires. The difficulty arises when you try to use the test button. That likely puts a resistor to to ground on L1. I'd like to know myself.

So, if the 2 prong outlet is at the end of the chain, your OK. Remember the GFCI can be wired such that it protects anything wired after it.

You can test it by taking a resistor R~=120/10E-3 or 12 K ohms and connect it from L1 of the 2 prong outlet and a solid GROUND and the GFCI should interrupt. The ground might be a plumbing fixture or a nearby outlet. P ~= 1.2 Watts. Use two 1/2 watt reistors in parallel. If they are the same values in parallel, the reistance will be 1/2. So two 2.4K 1/2 watt resistors in parallel will be OK for the test. With the resistor your diverting 10 mA to ground.

The other choice is to put in a GFCI breaker in the fuse box.

Everyone, let's get something straight... GFI devices WILL WORK WITHOUT a ground wire!

The info provided by Thomas is incorrect.

Replace the two wire outlet with a GFI in the bathroom, and apply the label " No Equipment Ground" to the outlet. These labels are provided with the GFI outlet.

A shared neutral can feed a GFI outlet, and the GFI outlet can protect a 2 wire circuit from that point.

A shared neutral cannot be used with a GFI breaker.

Thanks everyone for all of your help, I think I understnd what I need to do.
This is a great site. Thanks again. daniel38