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I pulled an outlet out of the box to determine how far upstream it was on the circuit in preparation for installing a GFCI. Three hots and three neutrals connected to the outlet.

I disconnected all hot wires and used a non-contact detector to determine which is the line from the circuit breaker. I verified the other two were hot loads going elsewhere (i.e., not energized when I turned on the breaker -- and everything else on the circuit was off, so I got the outlet furthest upstream). I figured, OK, they're splitting the circuit left and right around the room to reach the other outlets, etc.

However, when I connect just one of the load hots, everything downstream gets power, and the other load hot that's not yet connected also has power. Ergo, the separate hot load wires connect with each other somewhere I can't see, but I have no idea why or where, since hooking up just one got power to everything.

I've never seen this before. Can anyone tell me what's going on?

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    I am told that in the UK the practice is to use ring circuits in which the power to any given point comes through two wires. If I understand it correctly, the ends of the ring are in the panel connected by two different breakers. This works well in the UK where there is only one voltage, 230 V.This is not allowed in the US, but I don't know whether codes allow a ring on a 120 V circuit protected by one single-pole breaker. – Jim Stewart Nov 15 '16 at 23:23
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    A non contact tester will show a voltage on an open wire if run in parallel with a live wire. Voltage is induced from the live wire to the wire run in parallel it only takes a short distance to develop enough of a field to trick a non contact meter or tester. High impeadance bolt meters will also read a voltage but usually show a lower voltage than the live circuit. – Ed Beal Nov 15 '16 at 23:45
  • Thanks. This is helpful. I'll try testing with a volt meter next. Might take me some time as this is at our vacation condo. – part-timer Nov 15 '16 at 23:59
  • Where on the planet are you? – ThreePhaseEel Nov 16 '16 at 0:25
  • California, USA – part-timer Nov 16 '16 at 5:05
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sounds like the circuit is making a complete circle. they ran wire around the room and came back to the same box again. go to the "last" box on the run. There should only be one wire. Wire in, no wire out.

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One possibility is it was wired by a Briton. They are fond of "ring circuits" where the circuit makes a circle. Huge no-no anywhere else in the world, we use a physical tree topology and there should never be an electrical loop.

There's another possibility, that you'll only find by looking. It may have been initially wired with a receptacle on a switch. In this case, it is common to "split" receptacles by breaking off the tab between screws - in this case each half of the receptacle can be (must be) fed separately, and it can no longer be used as a handy way to splice 2 wires. It's common for amateurs to go through and replace all the receptacles, get on a roll and completely forget to look for those subtle little broken-off tabs. Now they are bridging the switched and unswitched lines, defeating the switch, and relevant to you, providing a second current path. It's not quite a loop, but will display that same "why does energizing one hot energize the other" symptom.

Search the room for switches or former switches, particularly ones which seem to do nothing.

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