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.
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/.
Best Answer
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.