I was replacing a receptacle on what I'll call circuit 1, which is a 20-amp
appliance circuit in the kitchen. The breaker was off, confirmed with outlet
tester. The wire nut came off of the neutral bundle, I went to put it back
on, touched two of the NEUTRAL (white) wires, and got a shock. I got my
multimeter and found 120 volts across two neutral wires. I found this VERY
But the plot thickens: my wife then asked when the microwave would be back
on. She was right...the microwave was off, but the microwave is on a
different circuit (also a 20 amp appliance circuit), which I'l call circuit
2. I tested an outlet on circuit 2, and the outlet tester indicated that hot
& ground were switched.
So, I turned off circuit two, and the hot neutral in circuit one went away.
Turned both circuits back on, and everything works fine.
This seems very strange to me...can anyone shed some light on this?
Welcome to Multiwire circuits. You are on a circuit that shares a neutral. Your
life would be easier in the future if you bought a 2 pole breaker and put #1
and #2 on it or just put a handle tie between #1 and #2.
| I was replacing a receptacle on what I'll call circuit 1, which is a 20-amp
| appliance circuit in the kitchen. The breaker was off, confirmed with outlet
| tester. The wire nut came off of the neutral bundle, I went to put it back
| on, touched two of the NEUTRAL (white) wires, and got a shock. I got my
| multimeter and found 120 volts across two neutral wires. I found this VERY
| But the plot thickens: my wife then asked when the microwave would be back
| on. She was right...the microwave was off, but the microwave is on a
| different circuit (also a 20 amp appliance circuit), which I'l call circuit
| 2. I tested an outlet on circuit 2, and the outlet tester indicated that hot
| & ground were switched.
| So, I turned off circuit two, and the hot neutral in circuit one went away.
| Turned both circuits back on, and everything works fine.
Turn circuit 2 back on, but unplug the microwave oven. See if the
voltage is still present in the neutrals.
| This seems very strange to me...can anyone shed some light on this?
Sounds more like a case of neutral crossover. Someone could have wired
the neutral of circuit 2 into circuit 1.
It is unlikely that ground was switched with hot (things would get really
nasty if it were). The indication you are getting is probably because
there is voltage between neutral and ground (you'd get this if ground was
really hot), and no voltage between hot and neutral (you'd get this if
hot was really ground. Hence the reading says hot and ground are switched.
But that's not the only thing that can cause this. If hot is hot, and ground
is ground, and neutral is hot (same phase as the other hot), you'd get the
same reading. You may even get a false reading due to capacitive voltages
when there is no load. If you were doing this reading with the neutral
bundle all taken apart, I would not be surprised by it.
The big question is where does the wiring for circuit 2 go? If it passes
through the box for the receptacle on circuit 1, that could have been why
the neutrals were cross connected.
Are circuit 1 and circuit 2 on opposite phases? There would be 240 volts
between hots of the 2 circuits, when live, if so. You can tell by checking
their position in the breaker box. Phases alternate vertically, but are
the same horizontally. If they are on opposite phase, and especially if
they go through the same receptacle box, this could be a case of shared
neutral. Such a circuit SHOULD be wired via a single 2-pole common trip
breaker to avoid the shocking experience you had. But they are very often
not wired that way.
So let us know more about the circuits you have.
Thangs Greg, a handle tie is now on my Home Depot list.
This answers a lot of questions for me, including why a 3-wire cable was
coming into that box. However, I'm still having trouble figuring out why
there was voltage across the two neutrals. Since the neutral is shared,
there is only ONE neutral in this circuit--how can there be another neutral
that is out of phase?
Yes, apparently this is a shared neutral circuit. I was not aware of any
such thing before I read Greg's post. A three-wire cable runs into the box,
with red being tied to a 2-wire cable going back out of the box, so I
presume that black is hot for my circuit (circuit 1) and red is hot for the
microwave circuit (circuit 2), with a shared neutral.
They are on top of each other in breaker box, so out of phase as you
suggested. Voltmeter confirms: 240 volts between hot 1 and hot 2.
Planning on getting a handle tie for the breakers per Greg's recommendation.
| This answers a lot of questions for me, including why a 3-wire cable was
| coming into that box. However, I'm still having trouble figuring out why
| there was voltage across the two neutrals. Since the neutral is shared,
| there is only ONE neutral in this circuit--how can there be another neutral
| that is out of phase?
When you opened the bundle, the neutral wire coming from the receptacle
the microwave is plugged in to is no longer grounded. It is in fact
connected to the hot wire through the microwave oven (or any other load
on that circuit). Remove (unplug) all the loads, and then it is just
an isolated piece of wire.
Any high impedance device (voltmeter, neon light tester, or your body)
will get nearly a full 120 volts. A low impedance device (a light bulb
with a high wattage) will get a lot fewer volts. Current is limited by
the sum of all impedances, though that won't stop you from getting a
shock since it doesn't take much current to do that (2 milliamps).
| Yes, apparently this is a shared neutral circuit. I was not aware of any
| such thing before I read Greg's post. A three-wire cable runs into the box,
| with red being tied to a 2-wire cable going back out of the box, so I
| presume that black is hot for my circuit (circuit 1) and red is hot for the
| microwave circuit (circuit 2), with a shared neutral.
| They are on top of each other in breaker box, so out of phase as you
| suggested. Voltmeter confirms: 240 volts between hot 1 and hot 2.
| Planning on getting a handle tie for the breakers per Greg's recommendation.
Be sure your breakers are qualified (UL listing) for a handle tie.
Not all are. Certain older breakers will not only not fully trip
the other, but can also cause a mechanical jam that can prevent a
trip even on a later fault current. If in doubt, a replacement
2-pole breaker (some come with integrated handle tie and some come
with a single handle) is the safe way to go (leaving you with 2
spare 1-pole breakers for your next new circuit that will surely
not be done as a shared neutral).
When our kitchen was overhauled I added two shared neutral circuits, 4 sets
of outlets on 4 breakers, When you take the cover off of the outlets there
is a label that says shared neutral cirxuit. Also labeled breaker box the
same way. .
| When our kitchen was overhauled I added two shared neutral circuits, 4 sets
| of outlets on 4 breakers, When you take the cover off of the outlets there
| is a label that says shared neutral cirxuit. Also labeled breaker box the
| same way. .
I would not do a shared neutral unless:
1. Every device that could interrupt the circuit (breaker, GFCI, etc)
had a common switch/trip, factory made and UL listed that way.
2. In a common cable sheath or conduit.
3. Never split where one circuit goes to one box and a different circuit
goes to another. This means each box must have all of the multiwire
circuit so it is obvious.
4. Double neutral if three phase.
I would not recommend ever sharing a neutral even on single phase
services. Handle ties for (2)120V circuits are not cool. Even on
single phase services the phase 'legs' or hots alternate vertically in
the panel. So any breakers that may be handle tied have 240V between
them. If the neutral is lost between the panel and the shared point of
connection you can easily have 240V across 120V equipment connected to
these circuits. Plus since this is single phase you can easily
overload the neutral ampacity wise. Three phase neutral sharing across
individual circuits is even more hazardous since the the potential
fault current is much higher than with single phase.
In short the proper fix for this is to run an additional neutral,
which since you are dealing with cable means running a new cable for
one of the circuits. I would not handle tie these breakers!
If there is 240 volts between the two breakers that supply a multiwire
branch circuit then the current in the neutral will be the difference of
the currents in the two legs rather than the sum. The use of a handle
tie or a two pole breaker assures that when the circuit is opened
manually in order to work on it that you will not end up opening a
neutral that is carrying the current of the other leg of the circuit.
Multiwire branch circuits have been used safely and effectively for
decades and there is nothing wrong with there continued use by qualified
persons. If you don't understand multiwire branch circuits then call an
electrician because you have no business doing any electric work.
I use plain Jane Usenet to access the groups.
This message was displayed in alt.engineering.electrical
This is the only message that has been echoed to Usenet.
Can someone tell me where the original thread originated from?
On Mon, 27 Sep 2004 17:53:08 GMT, Kilowatt put forth the notion that...
This was an old thread, and has probably been deleted from your server.
You should be able to look up the original thread at g00gle.
On Mon, 27 Sep 2004 09:54:10 GMT, email@example.com put forth the
You'd be hard pressed to find an electrical contractor that doesn't use
3-wire feeds for 120 volt home runs. It costs too much to run separate
neutrals, and it clutters up the panel and the space available getting
into it. In 120/208 wye systems, contractors share one neutral with all
three phases. If they're phased properly in the panel, there's no
problem with doing it that way.
Shared neutrals have been quite common for some time, and as you point out
usually aren't problematic when installed properly, and used with resistive
However, where shared neutrals are used with harmonic rich loads (switching
power supplies, electronic ballasts, some HID Lighting, etc.) problems with
the neutral conductor can, and likely will occur. Triplen harmonics can be
additive in the neutral with true RMS current levels significantly higher
than any single phase conductor even where the current measured on each
phase conductor appears to be well balanced, and the conductors are
Isn't the triplen harmonic problem only applicable for 3-phase wye
systems? iiuc These wouldn't add in a 120/240 centre tapped
single phase system. Since the subject is Home wiring... I'm
assuming this is what we have.
The subject was home wiring, but the post I responded to made reference to a
208Y120 3 Phase system as one where the neutral was commonly shared. A
shared neutral in a 3 phase system regardless of the load type used to be
the norm. Now that more has been learned in regards to additive harmonics on
shared neutrals of such systems more attention has focused on wiring
practices and components used, especially where a large portion of the known
load is harmonic rich. A few examples of that are cabling systems with an
oversized neutral available, and K rated transformers.
Remove the two fish in address to respond
I did not really state that correctly, you are right about neutral
currents roughly, the current relationship you describe is accurate
for straight up resistive loads only. Most loads on 120v branch
circuits are more complex and can have harmonic issues. As far as
being qualified, I am a licensed master electrician with I over 25
years experience, admittedly mostly industrial, not much residential.
Many of the higher quality job specs I have worked with addressed this
issue and required dedicated neutrals for all branch circuits where a
neutral is required. In the last 15 years or so I have been more
involved with service and less with installation and I have seen many
examples of bad events from shared neutrals on 120v branch circuit, a
few folks injured, and tens of thousands of dollars in equipment
destroyed because of neutral failures on these circuits. I have not
installed such a circuit in many years and never will again. If you
insist on using these circuits at least install a single breaker that
is designed to open two circuits rather than handle ties on two single
circuit breakers, a weak or damaged spring in one and both may fail to