Protecting multiple outlets with one GFCI; is it possible with this wiring scheme?

I have a family-room circuit with 5 outlets and a one-year-old who thinks it's funny to try to stick things in them. So I thought it would be a good idea to add GFCI protection (and tamper resistance). I know that in theory you only need one GFCI box to protect the whole circuit, the trick is to determine which receptacle is farthest upstream and replace that one, making sure you get line vs. load correct. So my plan was this: take all five boxes out of the wall, disconnect and separate the wires, flip the breaker back on, and walk around with my non-contact voltage tester looking to see which black wire still has voltage on it, implying it is the most upstream. Well, it turned out that when I did this, every single black wire at every single outlet still had voltage. I'm sure this wasn't a false positive because none of the white wires triggered the tester. I take this to mean that there is one main wire that circles the perimeter beneath the floor, and right below each outlet there must be a junction box with a separate wire going up to the receptacle. Is my reasoning correct, both regarding how to find the first receptacle and what it means that the technique didn't work? If so, does it imply that I can't protect this circuit by replacing a single receptacle, and would need to either install a new outlet farther upstream or use a GFCI breaker instead?

• How many wires are in each box? Are there wires pushed back inside the box, that are not attached to the receptacles? – Tester101 Oct 15 '15 at 13:23
• It's fairly common practice to connect receptacles using pigtails. So if you only disconnected the wires attached to the receptacle, the circuit would not be broken. Look in the back of the box, for a bundle of black wires. – Tester101 Oct 15 '15 at 13:25
• @Tester101 No pigtails or pushed-back wires. One box had two sets of wires coming in, the rest only had one. – dlf Oct 15 '15 at 13:26
• You could always protect the entire circuit, by installing a GFCI breaker. Though you'll probably also want (if you don't already have) AFCI protection as well, so you'll need a dual function circuit interrupter (DFCI). – Tester101 Oct 15 '15 at 13:30

You may be able to find an outlet further upstream which supplies power to the other outlets, but in comparing the price of a combination GFCI/AFCI breaker (~$40) with a single tamper-resistant GFCI outlet GFCI (~$20), I personally wouldn't spend any more time investigating for the additional $20 and would just swap out the breaker. • The junction boxes, if they exist, are most likely in the crawlspace beneath the room. It's kind of a pain getting in there and moving around, but I foresee a visit in my future. – dlf Oct 15 '15 at 13:31 • And is it true that if my picture is correct, the single GFCI trick won't work since, no matter which one I swapped out, each other receptacle would have a current path that doesn't go through it? – dlf Oct 15 '15 at 13:36 • GFCI would have to be the "first" one on the breaker and all the other outlets would have to be connected to the "load" terminals on the outlet. Using a GFCI breaker ( or DFCI like @Tester101 mentioned) is your best and easiest bet. – JPhi1618 Oct 15 '15 at 15:18 • @dlf It definitely depends on the wiring inside of the junction boxes and the outlets. If the diagram is correct and you have easy access to fish a new wire to the first outlet, you could potentially change the wiring inside the junction box so that its GFCI protected outputs protect the other outlets. As mentioned in my answer and also stated by Tester101, a combination GFCI/AFCI breaker is the easiest option if an extra ~$20 is not a significant factor. – statueuphemism Oct 15 '15 at 15:20