Right, the usual GFCI does not against electrical arcs. The National
Electrical Code now requires arc fault circuit interrupters (AFCI) in
certain rooms of homes to reduce such fires. Wikipedia has a good summary
of what GFCIs can and cannot do at:
http://en.wikipedia.org/wiki/Arc-fault_circuit_interrupter Arcing that
results from loose connection at outlets and switches or broken wires are a
major cause of house fires.
So it seems like the ground fault is especially useful near water
faucets and wet places.
But the arc fault seems especiallly useful everywhere, not just
bedrooms. Does that mean every circuit breaker should be arc fault?
I don't advise trying to use current technology on older wiring systems.
Aside from Edison circuits, you find anomalies caused by mixed neutrals
and the like. You certainly could test each circuit with an AFCI, and if
it holds, you're good to go
Even after looking it up last year, I never did understand what an
Edison circuit is or how it's different from any other circuit.
I know approximately what a mixed neutral is. I don't think my home
described below is likely to have the anomalies you speak of.
My house is from 1979, has quite a bit of empty space in the breaker
box, and there has only been one change, one added circuit, to the
attic to power the attic ceiling light (with receptacle), the roof
fan, and the outdooor floodlight. Along the way it goes by the far
wall of the laundry room where it powers a ceiling light and 5 double
receptacles (no more than 2 sockets are used at any one time, a lamp
and one power tool or another.) The circuit breaker has never
Do you think it's likely I'll have problems changing to AFCI?
I think you'd be OK, it's houses that predate the 50's where I find all
manner of rube wiring. In your case the only probable issue would be 3
wire circuits, which are two circuits which share a common neutral. AFCI
breakers can't be connected to these circuits. If you open your panel
and find any cables that have a red, black, and white wire, you won't be
able to protect them with an AFCI.
Aha! Now I understand what 'common ground' means. My former house
was built in 1980, and had common grounds (3-wire) circuits. I
understood how they worked, but I did not realize the danger involved.
On 6/27/2013 6:32 AM, email@example.com wrote:
These circuits, which are called multiwire branch circuits, three wire
circuits, or Edison circuits, are common "neutral" in that two hot legs,
of different potential share one neutral. Your typical residential
electric service, which is 120/240 is an Edison circuit. When installed
properly, they're no more dangerous than a two wire circuit
On Wednesday, June 26, 2013 9:39:36 PM UTC-4, micky wrote:
An Edison circuit allows you to save running one neutral wire.
Let's say I'm putting in new outlets, a long run from the
panel. There are enough outlets that I would need to pull
two romex cables, ie one hot, one neutral, one ground. Each
cable would serve half those outlets.
With an Edison, I can pull one romex with two hots, one neutral,
one ground. The one neutral is shared by the two groups of
outlets. I've traded two cables for one and reduced the
number of conductors by one, all of which save some $$.
On Thu, 27 Jun 2013 06:27:09 -0700 (PDT), " firstname.lastname@example.org"
It also reduces combined voltage drop by up to half if both circuits
are equally loaded because the current on the neutral is virtually
That is another reason to use them on long runs.
They do make 2 pole AFCIs now to replace the two pole breaker you
should be using on a multiwire circuit.
From my understanding of MWC's (which could easily be wrong) if the shared
neutral fails for any reason, you can potentially end up with much more than
120VAC where you don't want it. If a neutral fails in normally wired pair
of outlets, there's usually no risk of voltage escalation or imbalance.
However a failer neutral in a MWC can play out like this:
There are a couple of other considerations that affect the cost analysis,
too. I have always been able to buy 12/2 w/g Romex on sale for much less
than 12/3 w/g. So much less, in fact, that there was no difference in the
price between running two 12/2 cables and using a MWC using 2/3. I assume
that's a function of how much more 12/2 is manufactured than 12/3. It took
a little longer to install and staple, but I really prefer separate
neutrals. I assume that the different in installation time is a much bigger
concern to builders of large tracts of homes.
MWC's require (at least I am pretty sure NEC and CEC say so) a double pole
breaker requiring de-energizing both circuits to do repairs. Not a big
issue, but at times it's been nice to have a nearby live outlet to plug
tools into when I am working on a different circuit in the room. Not sure
if the DP breakers with a single handle for MWC's come in "dual skinny"
format, either, which is another concern for folks wanting to add new
circuits to an older box.
I do have one multiwire circuit in the panel to power my XTB X10
repeater/coupler/amplifier but nothing else is plugged into that outlet,
which has about 6' of wire running to the nearby circuit panel. Even that
outlet is wired with two separate runs of 12/2 w/g Romex.
I am sure there are labor cost savings for developers building a series of
houses, but in my case MWC's didn't make much sense except for the special
case of the X10 device that by design has to straddle both hots to
communicate between the phases.
Split circuit receptacles were required by code in kitchens before
GFCIs were required - and the ONLY way to put GFCI protection on these
split kitchen outlets is to use a 2 pole panel mounted device.
Current code prefers non-split 20 amp GFCI outlets within a 59 inch
radius of the sink, but still allows split 15 circuits
It's basically a 120/240 two-phase circuit with two hots and a neutral.
The advantage of using it is that it requires two less wires to be
pulled than two separate 120VAC circuits, because only one neutral and
one ground is required rather than two of each. The reason that you can
get away with it is that if you are only using 120VAC loads, but they
are evenly split between the two hot legs, the current in the neutral
will actually be zero! This is because the two 120VAC hots are 180
degrees out of phase (they add to a potential of 240VAC) it's very
similar to the way a three phase system works with 480VAC/277VAC if you
are familiar with that but with only two "phases."
Apparently this was devised by Edison as a means to provide two DC
circuits with only three wires (I guess the two "phases" in that case
would have been one wire at the positive nominal voltage relative to
ground and the other would be at negative nominal voltage relative to
ground) and someone appropriated the terminology to include the
analogous AC configuration... which makes sense as we all know that
Edison was a proponent of DC not AC. (Tesla and Westinghouse were the AC
The problem with using an AFCI on an Edison circuit is that you would
need a two pole breaker, which do exist but are unsurprisingly more
expensive than a single pole unit (or even two single pole units!) An
alternative would be two single pole breakers, but the manufacturer
would have to list them as being acceptable to use with a handle tie and
I haven't researched it enough to determine if there are any like that
I'm assuming what he means by a "mixed neutral" is one where neutrals
from one circuit are cross-connected to another circuit. That would be
a code violation and has been for ages - when neutrals from different
circuits are connected together you can no longer assure that the
currents in a cable sum to zero which has been a requirement for a very
So... you shouldn't have that situation, and if you do, might as well
find out about it now and fix it (should not require opening any walls
unless you have hidden splices which would be another violation...)
replace "roosters" with "cox" to reply.
A handle tie might be acceptable if AFCI breakers only detected arc
faults. But they also include ground fault protection, which requires
the neutral to go through the breaker, which means you need a 2-pole
breaker for an Edison circuit.
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