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I have an old house with a lot of knob & tube (and some sort of slightly newer but still ungrounded) wiring.

The previous owner replaced most outlets with 3-prong ungrounded. To improve safety I am replacing most of those with no equipment ground GFCI outlets. Which is pretty pricy but less than a house rewire. :)

I know with modern NM grounded wiring you can place a GFCI at the beginning of a circuit which will then provide GFCI protection to all downstream load recepticals. But I am not sure if there is still true if there isn't a common ground connecting all the receptacles?

It would be nice if true, because I can reduce the number of GFCI outlets I use around my house. And not that it matters but I am using these GFCI outlets.

Thanks!

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  • Do I interpret that answer right from 406.4(D)(2)(b), it seems to imply that receptacles will be placed in series on the "load side" of the GFCI, and that will be fine as long as you leave the ground(s) unconnected?
    – Samuel Davidson
    Commented Jan 14, 2022 at 20:36

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Yes, GFCIs don't care about safety ground. They do not wire to it in any way.

Even in the K&T age, they understood the importance of having neutral wires be monogamous to their partner hot wires. Note there is one legal and safe configuration called an "Edison circuit" or "MWBC" (Multi-Wire Branch Circuit), where 2 hot wires share 1 neutral wire. MWBCs won't play well with GFCI downline protection, so they will be indistinguishable from "stolen/borrowed neutral" situations. The difference is, that an MWBC can be successfully GFCI protected at the source (2-pole GFCI circuit breaker).

But other than an MWBC or a mis-wired circuit, yes -- GFCI downline protection works fine on 2-wire circuits.

I would advise installing the GFCI receptacle at the "closest to the panel" breaker position as far as you can discern it, and once it's installed, see what receptacles are knocked out when you trip it. Of course in doing so, you will place loads in those receptacles, which will test/validate whether the neutral is monogamous to that hot wire.

Always use the 2-step procedure for installation -- hook up LINE only, power back up and test the GFCI fully, and do not move forward until it works properly. Only then, power down again and hook up LOAD. That validates the GFCI so you can focus troubleshooting on the downline.)

We have a Q&A for the situation of having over-installed GFCIs.

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