GFCI and Shared Neutrals
I thought I would write about my experience on wiring a GFCI. I am not
an electrician but a normal DIY amateur. I do have some electrical
education way back when, but it was way back when though :).
Anyway I looked onto the Internet and found a pretty good explanation
GFCI's at Leviton's web site (e-z Learn). One thing to note is that on
web presentation and on their instructions for GFCI if your receptacle
has 2 or 4 wires coming into it, not counting the ground wires then it
DIY job. If you do not have that number of wires call an electrician.
OK I thought how hard can this be? It looks so simple every where you
on the Internet. But as with most of these things the devil is in the
So just to be safe I went overboard on the test equipment. Mainly
am a techie and sometimes you just never know and a piece of test
will bail you out of a tight spot. Any way it turns out it was a good
Note that the list below is what I got, the best choice for you will
what you feel most comfortable with design and budget wise.
1) Volt Tick -- Fluke 1AC II AC Voltage detector
A pen looking like tester. It will find on contact through plastic
insulation wires with voltage. Using electro static fields it will
find hot wires.
2) Volt meter -- Fluke 117, a digital volt meter
3) Wire stripper / Cutter (Klien tools make good but expensive ones)
4) Electrical tape
5) Wire nuts -- Red and Yellow denote the most often used sizes
6) GFCI -- I got the Leviton Smart Lock from Home Depot because
it was the most handily available. Slightly cheaper in sets of three.
7) Socket tester -- One with three lights usually. Tells you if you
your socket/GFCI wired correctly. Some will even test GFCI's
by inducing a small Ground fault current to "pop" the GFCI
8) 6 feet of 14 Gauge wire in each color Black, White, Green, Red
9) Screw driver, flat blade or Philips
OK armed to the teeth ready to do battle. One of the key safety tips I
is not to work alone. Make sure you have someone there to keep an eye
They do not have to be technically competent, but they should know
main breaker is and know how to call 911 :)
Put in the receptacle tester and it showed that the receptacle is
correctly. So at least that is a good start I thought. In the other
outlett on the receptacle I plug in a light. The test and the light
Go to the basement and hit the breaker. Come back and lights are off.
test the receptacle with volt meter and volt tick. Dead no voltage.
Remove the face plate and then check again for any voltage. None
Carefully loosen the holding wires of the receptacle and pull it out.
Uh Oh ... I see seven wires not including grounds coming into this
box. And the receptacle has one white wire handing off it, two black
and no ground. Not the standard game plan at all.
Hmmm .... I think. What is this ... I carefully again test for hot
with the volt meter on any exposed wire surfaces dead nothing. Then
with the volt tick. It lights I have a hot wire in there ... Yikes!
What I have is three cables coming into the junction box:
1) 4 Wire connection to the main breaker panel (Edison
Feed). This cable
has a Red and Black hot wires, a white neutral wire
a bare earth wire. The Red and Black wires carry 120 V
on a 180 degree phase differential. The circuit is known
as an Edison Circuit or also known as a shared
The red wire on this cable that is still hot in this case.
2) A NM Romex cable (Bedroom A feed)
with a Black, White and Ground wire.
The red hot cable is connected to the black cable on
this and feeds a Bedroom room A.
3) A NM Romex cable (Bedroom B feed)
with a Black, White and Ground Wire.
This feeds bedroom B. The Black wire from the 4 wire
connection feeds this Bedroom B.
All the grounds are connected together and are connected to the metal
in wall receptacle box. All the neutrals are connected together. The
receptacle is earthed to the junction box by its attachment screws,
hence there is no earth lead to the receptacle. Note: By removing the
receptacle out from the junction box, I have effectively removed its
The Hot wire is the red wire. It is hot because it is the other phase
of the Edison circuit that shares the neutral. Go down to the breaker
panel and hit a few more breakers. Eventually the red wire is no
Note if I did not have the Volt Tick, I may not have noticed it being
(I therefore really recommend getting one of these, cost about $US
The Edison circuit was controlled by two single breakers, one for
each phase of the Edison circuit.
Also the volt tick allows you to test for hot wires without having to
expose the end of the copper wire which can cause electrical shocks.
You can test for hot wires while everything is still insulated with
wire nuts and or electrical tape. I always test again with a
volt meter once the bare copper is revealed.
I also check the neutrals for hot voltage with the volt tick. Note a
neutral can become hot because it may not be connected to earth. For
example in my junction box. From Bed Room A and Bedroom B and the
circuit all the neutrals where joined together via a wire nut. It I
disconnected the wire nut, and not hit the breaker for the red wire,
one of these neutrals would then become hot if bedroom A had any
OK. I finally have a breaker box which is really powered down. I also
know what the wiring is. Note half the battle is realizing exactly
you are dealing with when you have zero documentation.
I wiring in the GFCI to protect Bedroom B's receptacles. It is correct
that you cannot use a standard GFCI on a shared neutral circuit. So
did I do it? A Edison shared neutral circuit can be split into two
non-shared neutral circuits. I do this and one split goes to bedroom A
and the other to Bedroom B. On the bedroom B split I place the GFCI.
as far as the GFCI is concerned it is no longer sitting on a Edison
Shared Neutral circuit. It is sitting on its own branch with it's own
I wire it up, and also connect a earth lead to the GFCI. Note that
installation this is not required because I have a metal receptacle
that is earthed. Attaching the receptacle via metal screws to the box
it. Note I usually put a piece of electrical tape around the GFCI to
up the terminals.
After carefully pushing the wires around the receptacle box, I have
enough room to put the GFCI in, tighten up the screws and replace the
receptacle face plate.
Now for the moment of truth. Turn the power back on and reset the
O.K. test the outlets on the GFCI, and the downstream receptacles with
the three light tester. O.K. Also test the sockets of the other
in Bedroom A. In each case re check the voltages of the sockets. They
123.5 volts for ground and neutral to hot, and close to zero neutral
ground in both bedrooms A and B.
Note if you get a reading of 220 volts anywhere, that
means you have an error and have connected across the two phases of
Edison Circuit. This is bad. So turn off at the breaker panel.
I check the GFCI, it resets normally and when tripped shutsdown the
Note on the Leviton GFCI, if you have Line and Load wires reversed the
GFCI will not have power at the socket, even though the green LED on
front of the GFCI will light.
OK after all that I am done. Somewhat more that I had bargin for, but
least everything is working and voltages are in order. I guess
what you get on the Internet is the glossed over version "this is easy
version of things.