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So I've added the 3-prong plug to my garbage disposal, and now I'm on to the GFCI outlet installation. The breaker was not GFCI protected, nor is there a GFCI outlet on the loop. I've gotten to the point where I need to know which is my line, and which is my load given this was a switched box.

Given the following was what I found, I'm pretty certain the LINE is coming off the left and the LOAD heads off to the right. Updated the drawing to reflect the actual situation.

Loads of line and load confusion.

Am I right or should I try each pair on the GFCI as the line on their own before wiring it up as I believe it should?

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Just wire the GFCI between the wiring and the disposal so that it's on the switch (often, you would daisy chain the switch off of the GFCI and have the switched device hardwired, but that doesn't solve your problem). So, in your diagram, the load side is where the current is coming in, the line side goes to the switch, and the disposal is obvious. What you'll have is:

              Switch (I'm guessing this is "line?" in your diagram)
               | | |
Line           W G B           GFCI   Plug  Disposal
Black ---------+ | +--B------- Black  =----B
Green -----------+----G------- Green  =----G
White ----------------W------- White  =----W

In the switch, the input is the white, even though it's hot in the wiring, and the output of the switch is black, since that's what becomes hot to the next device. When possible, I like to have my switch connected to a black and red wire, black always being hot, and red being switched. But that's usually not convenient, so this is the best an electrician can do.

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  • You know, this is what I've just done. And I'm glad because I can't believe how hard those wires were to bend round the poles... – user7116 Jul 30 '11 at 19:32
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    Many thanks, works well. – user7116 Jul 30 '11 at 20:11
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    I am concerned that if you did use the left side wires (pic) as your feed, then the black hot is on the GFI and your neutral is switched. This would be very wrong. Did you check any of the conductors with a volt meter to see which pair was the actual feed and which one went to the switch?. In your pic, the source/feed should be on the right, switched hot, uninterrupted neutral. Check by measuring from the narrow slot of your recpt to earth ground. (green) If the voltage remains in either switch position, you have a switched neutral. – shirlock homes Jul 30 '11 at 21:24
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    I'm not concerned about reverse polarity (white and black), I am concerned about an open neutral when the switch is in the off position. To check this with your 3 lite tester, plug it in the GFI with switch on. All three lites should be on. Turn off the switch and all three lites should go off. If you have a switched neutral, the tester will have two lites still on and read as "open neutral". If this is the case, you will need to rewire so hot is switched, not the neutral as your pic shows. Good Luck, let me know what you find. – shirlock homes Jul 31 '11 at 0:57
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    So, to put him at ease, with the GFCI reset and the switch and panel on, plug in your outlet tester; you should get two amber lights and no red light (usually indicating proper outlet wiring). Now, turn off the switch for the disposer. The GFCI should NOT trip, and ALL lights on the outlet tester should go out. If some lights are still on, you have most likely switched the neutral; this is unsafe. Then, switch it back on; again, the GFCI should not trip, and the two amber lights should come back on. – KeithS Aug 1 '11 at 15:06

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