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I just bought a leviton GFCI and the instructions say if there are two load lines in the box, do not connect the GFCI in that box. I can't understand why this is a problem, especially since the device has holes for two sets of wires (plus screw terminals) on the load side. Is this an issue of GFCI volume vs box size?

This a kitchen outlet; my alternative would be to cut into the supply line & put the GFCI there, but this would be in the basement and inconvenient.

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don't understand that instruction as Leviton would not have any idea what size box your are using. It is common practice to attach two load lines using the back wire ports as long as they are the same wire size. Can you quote the instruction from the directions in context? – shirlock homes Jun 30 '12 at 11:35
Their advice is to "don't install it, instead talk to an electrician" – Joe Philllips Jul 2 '12 at 20:32

You can connect two loads to in the same box. I don't see any valid safety or utility issue here.

I suspect that Leviton advises against it because of ground potential differences in the multiple load lines. I would speculate that if the potential difference is significant enough it could cause a trip of the GFCI if it is grounded to the same box as the loads.

It's not uncommon for a weekend-warrior to ground load lines from the GFCI separately and cause a trip. This may erroneously lead the user to think there is a problem with the GCFI receptacle when, in fact, this is precisely the intended behavior.

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If all your ground paths aren't at the same potential, you're doing it wrong, no? – Jeremy W. Sherman Jul 22 '12 at 19:44
@JeremyW.Sherman It's not uncommon for different grounds within a structure to have slightly different potentials. It can stem from improper grounding at the panel or a box somewhere missing it's ground. – Matthew Jul 23 '12 at 14:30

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