Harold: From SB (verbal) re: AFCI: Additions & new construction; AFCI Utilize an existing circuit, extend an existing circuit, use an existing "spare", it will be AFCI & 'sparky' owns the whole thing. Basically, this type of situation is left up to our (AHJ's) discression on "extend" & "existing".
The EC's I spoke to seem to have a grip on it, but time will tell.
