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I learned about wiring multiple GFCIs with 12-3 wire from this post: How do I install a GFCI receptacle with two hot wires and common neutral?

And a I have a related question as to why this circuit (shown below) doesn't continually trip the GFCIs.

from the Complete Guide to Home Wiring

My Question: Here's an example--a toaster is plugged into the 2nd receptacle from the left, drawing 5A. I assume that 5A will then flow through the neutral. Therefore, 5A is on ALL the neutral wire in the diagram, because they're all pigtailed together.

If that's true, now the 1st GFCI from the left will sense a difference of 5A and should trip because no current is flowing through its ungrounded (red) wire, but 5A is flowing through its neutral.

So, why doesn't a load on any one of the GFCIs trip all the other GFCIs? Why does this work? I know this is correct because its published, but I don't understand why.

And yes, I've read this post: https://diy.blogoverflow.com/2013/07/demystifying-the-mystifying-gfci/.

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A GFCI can only sense a difference in currents which flow though that GFCI.
In your example, the current for your toaster flows through GFCI #2, but completely bypasses GFCI #1 (and #3 & #4).

The other GFCIs are not 'aware' of this 5A toaster current, because current flowing in the neutral wire doesn't flow through these GFCIs.

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    This way will work, but costs more because each outlet is on a gfci, where we have problems with a MWBC is if a 2 pole breaker is used.+ but brhans at 5 amps that toast is going to take forever LOL. – Ed Beal Aug 13 '18 at 20:38
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    @EdBeal - well - 5A is kinda ball-park for a 220-240V Brit/Euro/Ausie/South-African toaster, but afaik none of them call the device a GFCI - it would be an ELCB, RCD, or whatever ... – brhans Aug 13 '18 at 21:06
  • Brhans, see the LOL and the plus, don't get your panties in a bunch. GFCI'S on a multi wire branch circuit in the U.S. are 120v and most OP's have no idea that a toaster draws 2x that ammount, look at the outlet that is a U.S. standard 15 amp 120v outlet last time I was on your side of the pond your outlets were not this style. – Ed Beal Aug 14 '18 at 2:09
  • Britain does have 110v on construction sites. It's canter-earthed 55V per leg. – Harper - Reinstate Monica Aug 14 '18 at 16:44
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You have hooked up these GFCIs exactly correctly. This the way to do it.

The notable thong about your install is that you did not remove any of the tape from the LOAD terminals, and did not connect those terminals. That is what made this installation so safe and sure.

Now, why doesn't it trip on the shared neutral? Because the neutral isn't shared to the GFCI. Every GFCI is providing a "zone of protection", so you have four zones of protection. For each GFCI+receptacle combo device, the only thing inside its zone of protection is its own sockets. That's because you didn't use the LOAD terminals.

  • Thanks Harper, that's helpful. I did actually end up using the load terminals on the first receptacle to provide protection to my dishwasher and garbage disposal. – Aaron B. Aug 14 '18 at 15:38

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