Quote Originally Posted by chaz7979 View Post
I noticed those 2 hole outlets at HD today... how do they even allow those things to be sold? I thought you had to have a ground???
They are permitted "for replacement purposes only." They are not permitted in any installation where a circuit is being modified. If modification is necessary -- say, you add an outlet -- then the whole circuit must be brought up to code.

To have a single GFI protect an entire circuit (or any number of outlets downstream) you must ensure that you can hook up both the downstream hot and neutral wires to it, and that NO other circuits share the neutral. GFIs work by making sure the current flowing through the hot and neutral wires are equal. You can have nuisance tripping if you have weak insulation on your K&T that is "bleeding off" electricity. But this shouldn't be seen as a nuisance, it should be seen as a symptom of a big problem that needs to be addressed.

If you aren't sure that your neutral serves only one circuit, or if the hot and neutral run different paths, you can simply install a GFI at each location, connecting only the "line" side. This will then protect YOU from faulty equipment plugged directly into the GFI.