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I installed a new outdoor circuit. The line comes out of the house and into the LINE side of the GFCI outlet. I connected a wire to the LOAD side and ran it to two standard outlets downstream. My tester says the circuit is wired properly at all the outlets; the tester button appropriately trips the GFCI when used.

Here's the problem: devices plugged directly into the GFCI outlet work fine. However, when I plug the same devices (I tested a light, a fan, and a grounded shop vac) into the downstream outlets, they trip the GFCI.

I'm stumped. Help?

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    You probably have a ground-fault in the wiring after the GFCI, and the GFCI is working properly. While rare, sometimes GFCIs actually trip because of ground-faults. – Tester101 Jul 2 '13 at 18:20
  • "My tester says the circuit is wired properly at all the outlets" -- including GFCI? Did you test the trip button as well as your tester trip button on the GFCI itself in addition to the new receptacles? – Jacob S Jul 2 '13 at 18:20
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    A standard outlet tester cannot detect a neutral-ground short. By looking at the connectivity and voltage differential, neutral and ground will appear identical. – BMitch Jul 2 '13 at 18:29
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Check for a neutral to ground short somewhere on the load side of the GFCI and downstream outlets. Also, make sure there isn't a shared neutral with any other circuit and that you don't have a connection from the neutral load directly to the line side of the GFCI (in other words, the whites cannot all be pigtailed together, the line and load neutrals must be separated by the GFCI).

  • Deleted my answer -- should have read @BMitch's answer better. Agreed -- my money is on LOAD neutral connected to LINE neutral or a neutral-ground short. – Jacob S Jul 2 '13 at 18:28
  • This is the answer. Since neutral is close to ground voltage, virtually no current will flow on a neutral-ground fault on an unloaded circuit. Once you load it, some return current is going via ground instead of neutral, and that trips it immediately. – Harper May 6 '18 at 17:03
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Check reversed hot/neutral at downstream outlets. Also check for leakage paths to ground through abraded/punctured cable.

Wire up a short stub outlet in place of your downstream cabling and verify GFCI vs your cabling.

There are some related tips here, related to keeping Load and Line neutrals separate (hots too)

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The easiest way to have this happen is to have connected and be feeding only the downstream hot or the downstream neutral through the GFCI outlet, but not both. A downstream interconnection or swapping of the grounded and grounding conductors, aka the neutral and the safety ground will also cause this to happen. Buildings constructed prior to the adoption of the 1965 National Electrical Code when the separate grounding conductor was added are particularly likely to have an unexpected interconnection.

A GFCI outlet interrupts the circuit when the current in the ungrounded (hot) conductor and the grounded (neutral) conductors isn't exactly balanced equal and opposite (except for a small permitted leakage). Any of the previously described wiring errors will result in an unbalanced current current through the GFCI causing it to trip.

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