Electrical: Multiwire Circuits and GFCIs

[Chris Padilla]

So I'm now into wiring up my garage (mah-hal) and as code requires, I need to protect all the 120 V circuits with a ground-fault circuit interrupter.

All my 120 V (20 A) circuits are multiwired (3-wire Edison if you like) and so I just picked the box at the beginning of a run and wired the GFCIs in: red/white to one GFCI outlet and black/white to the other GFCI outlet.

Guess what?! Only one hot will work at a time. In other words, I can't have both the 'red' GFCI on and the 'black' GFCI on at the same time! Having one 'on' won't allow the second one to turn 'on.'

Well, some of you may be like, "Well, duh!" but it took me some head scratching and thinking last night as I turned into bed.

I haven't tried it yet (will this evening depending upon feedback here) but I think that each hot needs its own neutral (white) back to the service panel. You can't share neutrals in a typical multiwire situation with GFCIs. Am I correct? Running another neutral is no big deal, fortunately.

If I am wrong, how do I get my GFCIs wired in properly in my situation?

Thanks! :)

[Jamie Buxton]

Chris --
You're right, you can't use a shared neutral in a GFI. The GFI works by comparing the current in its hot leg with the current in its neutral leg. When they're not identical, the GFI concludes that something is wrong. If you have a shared neutral, it may have current in it which is not coming from the hot which is passing through the GFI. The only fix is to seperate your neutrals.

[Jeff Sudmeier]

We ran into that problem on a garage that we wired a few years ago. It took us a LOT longer to figure out than it did you! :)

[Chris Padilla]

Jeff, I found a better explanation of how GFCIs work than what Leviton provided and once I understood that, it seemed to me that each hot needed its own neutral...just checking here with the collective! I'm glad to hear that the couple marbles still rolling around in my head are worth something! :)

Thanks, Jamie! :D

[Ken Fitzgerald]

Chris......in my earliest discussions here about wiring my new shop that possibility came up. Interestingly the local electrical inspector suggested that type of wiring and told me I had to use GFCI's as the first outlet on each circuit. I elected to do it the "old fashion way". Now seeing your report, I am glad I did! . Can you just run a second neutral or do you have to redo the whole thing?Good luck with your wiring

[Chris Padilla]

Ken,

I'm lucky (or brilliant! ;) ) in that I ran all conduit so I just need to fish the additional line through the whole branch!:mad: It is easy...but will be a tad time-consuming but what is new with that on my garage gut??!! :rolleyes: :mad:

[Tyler Howell]

The candle always shines brighter when you light it yourself.;)
You do understand that you can protect other outlets with one GFI??
Just to keep the math down I always run one neutral per circuit GFI or not.

[Chris Padilla]

Tyler,

Yes, I just need to put the GFCI at the beginning of the branch and all the rest downwind are all set. Of course, you must be careful to wire the power in to the 'line' side of the GFCI and the rest of the outlets go on the 'load' side of the GFCI.

It was an interesting situation last night in my garage sitting there clicking the resets and only being able to have one on a time (had a radio in a downwind outlet that kept turning off and on, off and on) and wondering what the heck I did wrong!

[Tyler Howell]

I've found a couple installed by pros that screwed that one up.

Just a heads up. You may want to have a few extra on hand. The manufacture in the past only guaranteed one (real) GF operation.
Regular testing is good.
Play safe.

[Chris Padilla]

It says "Test Monthly" on them! :)