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If I place a GFCI as the first outlet in a series of ungrounded 2-prong outlets, and connect the wires going to the other outlets to the load terminals of the GFCI, will it "protect" the entire series of outlets if a grounding event occurs?

If it would protect it, would a GFCI breaker on an ungrounded circuit accomplish the same thing?

I may have read some misinformation that said that the GFCI measures differences between the hot/neutral and hot/ground, so I'm not sure what's true.

I found a similar question (Are non-GFCI outlets that are downstream of a GFCI outlet ground fault protected if they are fed from the GFCI outlet's load side?) but it didn't say if it would work on an ungrounded system.

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2 Answers 2

Ground-fault circuit interrupters (GFCI) work by looking for imbalances between the ungrounded (hot), and grounded (neutral) conductors. A grounding conductor is not required for a GFCI device to detect ground-faults. Any devices connected to the Load side of the GFCI device, are GFCI protected. This blog post might help you understand how GFCI devices work.

If you take a look at the code quoted in this answer, you'll see that not only are the "No Equipment Ground", and "GFCI Protected" labels required. You should also not connect any grounding conductors to any 3-prong receptacles that are downstream. If there is no grounding conductor in the circuit, no grounding conductors should be connected anywhere in the circuit.

Also of note, is that this only applies to existing circuits that were installed before the code change. All new circuits must meet or exceed current codes.

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Thanks for the blog post, I understood it immediately after a 3 second trip back to my old EE classes :) –  Alex Moore Dec 7 '13 at 1:36

Found my own answer after further research, with this nice video by Mike Holt.

I can use a GFCI as the first receptical on the source side, and then use 3-prong outlets on the remainder load side recepticals as long as they have the "No Equipment Ground" and "GFCI Protected" stickers on them.

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