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Posted By: MAH 3wire and GFI's - 03/20/11 05:42 AM
First off I'm an apprentice, so go easy smile

I recently learned that my supply authority will not allow 2 separate GFI devices to be fed from a 3 wire circuit. For example a 14/3 supplying 2 GFCI devices in a single box with the neutrals tapped in the box before the GFCI devices, the resulting answer to supplying these would be to run 2 runs of 14/2 to supply the devices. I have searched the code book for a couple of days now and have come up empty, maybe its a combination of rules that lead to this interpretation but I fail to see the difference of the joint in the box and the neutral bar in the panel.
Posted By: EV607797 Re: 3wire and GFI's - 03/20/11 08:18 AM
Wow, that doesn't make any sense. The functionality of the GFI doesn't happen until it receives a feed. A shared neutral ahead of them shouldn't matter. I'm positive that the GFIs aren't going to know the difference.
Posted By: sparky Re: 3wire and GFI's - 03/20/11 11:34 AM
Would this be in a location an AFCI is required?

Posted By: MAH Re: 3wire and GFI's - 03/20/11 08:53 PM
I will explain the particular situation to clarify

I built a display panel for our company for a local home show, and the inside of the panel was open for display but I did not terminate the receptacles that were in 1110(handy) boxes because the panel was never to be energized and only for show. I ran a 2 wire to a standard receptacle that was fed from an AFCI breaker, and I ran a 3 wire to another handy box which was originally going to be a split receptacle and in my haste I mounted the GFCI receptacle to it(but not terminated). When the inspectors came around to inspect the booths they pointed out the 3 wire to a single 1110 box with a GFCI receptacle and we were told to change it even though it was a de-energized panel with no practical way to energize it easily. So me on the defensive asked why we cannot supply 2 GFCI devices from a 3-wire and the only answers I have gotten are 1 You are not supposed to, 2 Its against code. I have looked for a code but have come up with nothing saying one way or another, but I'm concerned that my understanding of GFCI's may be flawed when I fail to see the issue and the people I learn from and the authorities are telling me this is not an accepted trade practice

Oh and the issue of box fill in an Handy box was never brought up so its not a misunderstanding of the violation

Thank you for the replies
Posted By: twh Re: 3wire and GFI's - 03/21/11 05:25 AM
It's a perfectly correct installation. The trade practice is that the inspector must quote a rule.
Posted By: MAH Re: 3wire and GFI's - 03/21/11 06:14 AM
I appreciate the input, when I was told this it was as if someone had told me the sky was yellow. I won't use this to argue with my journeyman or the supply authority, but at least I feel competent again with my understanding of how this wiring method works. I'll save that argument for when I'm a j-man haha

Thanks everybody, I'm enjoying this forum very much
Posted By: sparky Re: 3wire and GFI's - 03/21/11 12:11 PM
1 You are not supposed to, 2 Its against code.

that takes a lot of us down memory lane MAH

the trade really isn't as obligatory in it's tutalage to the up and comming as it should be

used to get me down a lot when i wore your shoes

what i often countered with was politely asking 'how would you like to see it ?' , hopefully prying an explanation out

Posted By: jdevlin Re: 3wire and GFI's - 03/21/11 02:28 PM
Unless it is a kitchen counter receptacle that is required to be split I think the inspector was wrong.
Posted By: pooL8 Unbalanced Current - 03/22/11 10:43 AM
I know from experience that it works just fine. I put in two outside gfcis this way at the house 2 doors down from me 3 years ago. But I also fed lights from the gfcis.
No problems yet.

How I see it is... if Gfcis compare the current at their hot terminal to the current at their neutral terminal, then what lies beyond isn't even readable. Proof of that is when you wire a plug off of from a Gfci's line. It doesn't protect that plug, cause it doesn't detect what's outside of it, unless you hook to loadside ofcourse.
Same principal as tapping a smaller wire to a BBH. The wire only sees what the heater puts out, not the entire circuit current

We have been told:
"The neutral carries the unbalanced current of a multiwire branch circuit" ...true
"Gfci's trip on a 4mA unbalanced current"...true

And that's where lots of people stop, I think... and a norm developed. Two different unbalanced currents thought of as the same?

Wouldn't this situation be the same as using a 2-pole Gfci breaker to feed two different branch circuits on a 3-wire?

Posted By: mikesh Re: Unbalanced Current - 03/22/11 11:24 PM
As long as the GFCI outlets are after the split in circuits (out in the field) then the outlets will work correctly. You cannot use a 1 pole GFCI breaker and a normal 1 pole breaker on the other phase and have the GFCI breaker hold.
Posted By: mersadrad Re: Unbalanced Current - 03/24/11 03:17 AM
I would say that GFCI just filters current. So Line from panel to the load to the shared neutral. What happ when power travels from opposite side. Unbalanced load flows only behind GFCI. If in CEC doesn't say it clearly, well you can and can't. That right inspectors reserve for themselfs.
If you don't see something a lot in practice people are sceptical about it. GFCI should be checked every month. Not for use everywhere.
In parrallel circuits:


If you split neutral before comming to GFCI you just changed current...

...or I am just talking nonsense
Posted By: ghost307 Re: Unbalanced Current - 03/24/11 12:40 PM
A GFCI doesn't filter anything. It measures the current leaving the device on the phase wire (black) and the current returning to the device on the neutral wire (white) and then compares the two. In normal operation they should always be the same since electricity flows in a closed circuit.

If the two current measurements differ by more than a prescribed maximum amount (4-6 milliamperes), a contact inside the GFCI opens and shuts off the current flow.
So if 9 amperes is traveling through the heating elements in the toaster and 1 ampere is flowing up your arm, through your heart, down your other arm and into the faucet on the kitchen sink, that 1 ampere (1,000 milliampere) difference sensed by the GFCI will open the circuit at the device rather than stand there cooking yourself to death.
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