Electrical Wiring in the Home/Sharing a neutral wire
Hi Bob,I plan to bring 2-20 amp circuits to a part of my house using 12-3 wire. Will sharing the neutral wire on both circuits be a problem using GFCI outlets? Thanks for any advice.
Multi-wire or split circuits were quite common back in the 70's when I first got in the field. But that's also when GFCI's - in the early stages - first started becoming Code required. We quickly found that GFCI's and split circuits simply did NOT work well together because of the nature of the trip mechanism inside the GFCI - designed to detect any imbalance of ~ 5 milliamps or more between the load wire and the neutral. On a split circuit - the neutral wire is shared between 2 circuits and carries the imbalance of the load on the 2 circuits. For example - if the red wire load is supplying one room...and loads are on only on that circuit...the GFCI will work fine. BUT - as soon as you turn on something on the black wire circuit in the next room...the neutral is now offset...and the GFCI sees that as an imbalance and mistakenly thinks it's a fault on that circuit and trips out that GFCI...what we call "nuisance tripping". (the same thing also happens on AFCI's...now pretty much required on any circuit in the home that isn't already GFCI protected). So - as the Code evolved and started requiring GFCI's in more places - and now AFCI's...we rarely ever run any 3 wire or split circuits now to completely avoid this problem. You didn't say what these two 20 amp circuits will be supplying...but odds are they may fall in an area that is now Code required to be either GFI or AFI protected....and circuits with shared neutrals will not operate well in either protection scenario....I highly recommend you either run two separate 12-2 circuits - one dedicated to each load area...or a run of 12-4 (allowing you to have a separate neutral now for each circuit)and abandon the 12/3....and eliminate the nuisance tripping headache.
The choice is up to you Steve...but I Highly recommend you do not use the 12/3 or share any neutrals for proper protection and far fewer headaches in any area that now requires GFI or AFI protection.
Hope this helps...if you have further questions - please follow-up. (NOTE: I am currently in recovery from an open-heart surgery that did NOT go well....as I continue to struggle to recover...I am often gone for extended periods for medical treatments - and will need to mark myself as "away" here at this site to avoid overflow of questions ) You can still reach me directly by email at:
and I will get back to you as soon as I am well enough to do so.
Owner / Master Electrician
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