Bathrom GFI in Older House


Old 10-31-04, 05:09 AM
Thread Starter
Join Date: Oct 2004
Posts: 9
Question Bathrom GFI in Older House

I would like to put a GFI in my bathroom. Currently it has a non-GFI receptacle.
The house was built in the 1960's. The current receptacle has a black "hot" wire and a white neutral wire which is run in metal conduit. It does not have a green ground wire. Using a digital VOM I measured the resistance between the metal receptacle box and the white neutral wire. The resistance is zero. Does this mean that the receptacle and conduit are grounded at the circuit breaker box?

I measured the resistance between the green ground screw on the GFI and the metal plate that is used to attach the GFI to the receptacle box with metal screws. The resistance is zero. Can I therefore install the GFI using metal screws and assume that it is properly grounded since the green screw, the metal plate and the metal receptacle are at the same potential as the circuit breaker box (which I assume is grounded) and are at the same potential as the white neutral wire?
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Old 10-31-04, 05:15 AM
Join Date: Sep 2003
Location: Central New York State
Posts: 13,973
A GFCI receptacle does not need to be properly grounded. You can safely install a GFCI receptacle on a two wire circuit, with or without a ground wire.

You should not use digital voltmeters with household wiring.

The metal conduit is probably functioning as a ground, at least to some extent.
Old 10-31-04, 06:21 AM
Join Date: Jan 2004
Location: Oregon
Posts: 1,219
Your metal conduit and box is functioning as your ground conductor. Your neutral to box resistance check pretty well confirms this. When installed properly, metal conduit is considered an equipment ground that is acceptable by the NEC.

The yoke on a receptacle is connected to the ground terminal. However you cannot in general trust the screws that hold the receptacle to the metal box as a ground path. _Some_ receptacles are 'self grounding'. When a self grounding receptacle is installed so that the yoke is flush with the box ears, then this is an acceptable ground path. If your receptacle is not self grounding, then you will need to run a jumper wire from the box to the ground screw on the receptacle. Check the instructions that came with the receptacle.

racraft's comment about digital volt meters applies to measuring _voltage_ on open circuits. The high impedance of the meter means that you can read 'phantom voltage' from capacitive coupling to the wires. Digital meters are totally fine when measuring live circuits, and are totally fine when being used to measure resistance. But they can give _very_ confusing readings on open circuits.

Old 10-31-04, 04:48 PM
Join Date: Sep 2003
Posts: 344
Yep, the digital testers are capable of reading a voltage without pulling it down. So you could be reading voltage from inductive coupling.

An electronic continuity tester is capable of indicating the capacitive flow on a long run. Put the tester on two long open wires, and for a second it seems that two open wires are shorted together. Kind of freaky the first time or two.

The latest gee-whiz are those volt-stick thingies that detect hot (or cold) circuits simply by pointing at the wires. Pretty nifty.

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