I live in an older home with 2 prong plugs. I want to update these with 3 prong. My plan is to put a GFCI at the first outlet in the circuit and then regular 3 prong outlets downstream from there.

I can figure out which outlets are on the same circuit by flipping the circuit at the breaker and noting which turn off. My question is, how do I figure out which outlet is "first" in the circuit?

  • 1
    Before you go that route, are you sure your electrical boxes are not grounded? In many installations with two-prong plugs, there is a bare copper ground wire screwed to the back of the metal box.
    – Edwin
    Commented Jun 27, 2015 at 13:46
  • 2
    Forgo the aggravation, and simply install a GFCI breaker instead.
    – Tester101
    Commented Jun 27, 2015 at 13:49
  • @Tester101, I would personally endure the aggravation and install the GFCI in the room just for the sake of convenience. Typically, I find GFCI breakers convenient only for circuits located in the same room as the breaker panel (normally the garage).
    – mjohns
    Commented Jun 27, 2015 at 14:51
  • @Edwin, I was reading this article on This Old House that talked about that. Which was interesting. I got my multimeter and set it to VAC, stuck the red in the short prong hole and touched the screw. On one outlet I got a reading of around 50, on another I got a reading of 1 or 2. Are they both grounded?
    – mikeazo
    Commented Jun 27, 2015 at 15:08
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    It's hard to say. You never know what condition the old outlets are in. You really have to take the old outlets out, and look in the back of the boxes. If you have a wire screwed to the box, and it's fastened well, then hook up a new self-grounding outlet. From the short prong to the grounding prong should give you full voltage (about 120V).
    – Edwin
    Commented Jun 27, 2015 at 15:24

1 Answer 1


Usually it's physically obvious - depending on how exposed the wiring is (typically in the basement, if at all) it can be easier or more difficult to be sure, but it will almost always be the one closest to the breaker box. In some cases the wire routing won't be so straightforward, but usually it is.

I'm assuming you are capable of safely working with exposed wires, though I'd never suggest (and don't myself) doing anything more than check for voltage on them when exposed and live. Always turn off the breaker and check for voltage before actually manipulating wires.

So you pick the closest outlet on the circuit. You turn off the breaker and open the box. You disconnect one set of wires positioning them safely or temporarily taping them, and turn the circuit back on. if all other outlets are dead, you got the right outlet. If some other outlets are live, do the same thing at one of them. Ultimately you need to not merely find the outlet, but find which set of wires are the supply from the breaker (that will be connected to the "line" terminals on the GFCI) and which are the wires leading to the other outlets (that will be connected to the "load" terminals on the GFCI.)

  • Sounds simple enough. The side of the basement where the breaker box is is unfinished, so I can see the wires. I'm sure that will help quite a bit. I should be able to find which are the supply pretty easily.
    – mikeazo
    Commented Jun 27, 2015 at 15:11

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