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Posted by Existential Angst on November 7, 2009, 11:26 am
> Existential Angst wrote:
>>>>> Do you know *why* you can use just one neutral to serve two hots with
>>>>> this
>>>>> kind of circuit?
>>>> If I didn't, would you tell me?
>>> Apparently you don't, so permit me to enlighten you. Code requires that
>>> all
>>> ungrounded (i.e. hot) conductors of a multiwire (Edison) circuit have a
>>> *single* disconnecting means -- so unless you can find a 20-pole
>>> breaker,
>>> you're going to have a tough time meeting Code with that monster circuit
>>> you
>>> proposed.
>>> The reason Code requires this is straightforward: the neutral wire
>>> carries
>>> current. If one hot leg of a multiwire circuit is left powered on, and
>>> any
>>> load on that leg is in use, the current drawn by that load is flowing in
>>> the
>>> neutral -- presenting a risk of fatal electric shock to anyone servicing
>>> the
>>> other leg of that circuit.
>> So are you saying that in a simple 3 wire bx cable, with the two hots
>> sharing a neutral, you'd need a two-pole breaker?
> yup, exactly. Or if they were two single pole breakers they'd have to be
> next to each other with a handle tie.
OK, one more clarification:
Said 3-wire bx cable, with two hots, one neutral, each hot on a sep. svc
leg, and each hot powering a 120V appliance, right?
Say, one hot for the microwave, and the other hot for the toaster, with
each neutral from that appliance's outlet/box being pigtailed together to
one neutral going back to the breaker panel. Good so far?
You would then put those two hots on a double pole breaker? So that if the
toaster goes out, the microwave goes out as well? Hmmmmm......
--
EA
> nate
> --
> replace "roosters" with "cox" to reply.
> http://members.cox.net/njnagel
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Posted by Doug Miller on November 7, 2009, 12:49 pm
>OK, one more clarification:
>Said 3-wire bx cable, with two hots, one neutral, each hot on a sep. svc
>leg, and each hot powering a 120V appliance, right?
>Say, one hot for the microwave, and the other hot for the toaster, with
>each neutral from that appliance's outlet/box being pigtailed together to
>one neutral going back to the breaker panel. Good so far?
Good so far.
>You would then put those two hots on a double pole breaker? So that if the
>toaster goes out, the microwave goes out as well? Hmmmmm......
Code requirement as of the 2008 NEC.
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Posted by Existential Angst on November 7, 2009, 12:33 pm
> Existential Angst wrote:
>>>>> Do you know *why* you can use just one neutral to serve two hots with
>>>>> this
>>>>> kind of circuit?
>>>> If I didn't, would you tell me?
>>> Apparently you don't, so permit me to enlighten you. Code requires that
>>> all
>>> ungrounded (i.e. hot) conductors of a multiwire (Edison) circuit have a
>>> *single* disconnecting means -- so unless you can find a 20-pole
>>> breaker,
>>> you're going to have a tough time meeting Code with that monster circuit
>>> you
>>> proposed.
>>> The reason Code requires this is straightforward: the neutral wire
>>> carries
>>> current. If one hot leg of a multiwire circuit is left powered on, and
>>> any
>>> load on that leg is in use, the current drawn by that load is flowing in
>>> the
>>> neutral -- presenting a risk of fatal electric shock to anyone servicing
>>> the
>>> other leg of that circuit.
>> So are you saying that in a simple 3 wire bx cable, with the two hots
>> sharing a neutral, you'd need a two-pole breaker?
> yup, exactly. Or if they were two single pole breakers they'd have to be
> next to each other with a handle tie.
From http://en.wikipedia.org/wiki/Shared_neutral, it would appear that wiki
agrees with you! The article also references the term Edison Circuit.
Split phase shared neutral
In split phase house wiring, for example, a duplex receptacle in a kitchen
is typically connected with a cable that has three conductors, in addition
to ground. The three conductors are usually colored red, black, and white.
The white serves as a common neutral, while the red and black each feed,
separately, the top and bottom hot sides of the receptacle. Typically such
receptacles are supplied from a ganged breaker, i.e. a breaker in which the
handles are tied together for a common trip, so that if one kitchen
appliance malfunctions and pops the breaker, the other side of the duplex
receptacle will be shut off as well. This is called a multiwire circuit.
-------------------------------------
Man, this is news to me!
It seems, tho, there are two separate issues at work here:
1. The current carrying demand on a neutral conductor
2. A personnel-type issue, ito breaker panel voltages.
The split-phase nature of the two hots addresses (1), while the double-pole
breaker addresses (2).
I personally never thought (2) was such a big deal, as any wire dangling in
the air can be hot, *including* a plumbing connection to the street, say,
when the water meter is disconnected.
But, http://www.phy.ornl.gov/divops/ESH/98-2.html ( a link from the above
wiki site) apparently frowns on edison circuits, and indeed suggests double
pole breakers as one remedy.
AND, a double pole breaker also has another safety feature that just dawned
on me, quite related to (1):
It discourages moving one of those neutral-sharing hot wires in the
breaker panel willy-nilly, possibly putting both hots on the same svc leg,
thus overloading the neutral.
But here's what I have experienced:
More often than not, in opening a junction box, esp. the larger 5" ones,
I'll see oodles of neutrals wire-nutted together, irrespective of the phase
of the hot leg. Apparently there is a wide-spread cavalier attitude toward
the lowly neutral.
I suspect some of this cavalier attitude toward neutrals comes from the
notion that the ground, or other mystery neutrals, are picking up some of
the slack anyway, so no biggie. When in fact it could be a biggie.
And consider this:
For X number of circuits, there should X number of neutrals on the neutral
bar, MINUS the number of edison circuits. This would seem to be check out
pretty well on bx/romex systems, but how about separate wires pulled through
EMT? It seems that there could be much more abuse of the hot/neutral ratio
in pulled wires. I've seen pretty scarce neutral bars in some breaker
panels!
--
EA
> nate
> --
> replace "roosters" with "cox" to reply.
> http://members.cox.net/njnagel
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Posted by Nate Nagel on November 7, 2009, 12:40 pm
Existential Angst wrote:
>> Existential Angst wrote:
>>>>>> Do you know *why* you can use just one neutral to serve two hots with
>>>>>> this
>>>>>> kind of circuit?
>>>>> If I didn't, would you tell me?
>>>> Apparently you don't, so permit me to enlighten you. Code requires that
>>>> all
>>>> ungrounded (i.e. hot) conductors of a multiwire (Edison) circuit have a
>>>> *single* disconnecting means -- so unless you can find a 20-pole
>>>> breaker,
>>>> you're going to have a tough time meeting Code with that monster circuit
>>>> you
>>>> proposed.
>>>> The reason Code requires this is straightforward: the neutral wire
>>>> carries
>>>> current. If one hot leg of a multiwire circuit is left powered on, and
>>>> any
>>>> load on that leg is in use, the current drawn by that load is flowing in
>>>> the
>>>> neutral -- presenting a risk of fatal electric shock to anyone servicing
>>>> the
>>>> other leg of that circuit.
>>> So are you saying that in a simple 3 wire bx cable, with the two hots
>>> sharing a neutral, you'd need a two-pole breaker?
>> yup, exactly. Or if they were two single pole breakers they'd have to be
>> next to each other with a handle tie.
>
> From http://en.wikipedia.org/wiki/Shared_neutral, it would appear that wiki
> agrees with you! The article also references the term Edison Circuit.
>
> Split phase shared neutral
> In split phase house wiring, for example, a duplex receptacle in a kitchen
> is typically connected with a cable that has three conductors, in addition
> to ground. The three conductors are usually colored red, black, and white.
> The white serves as a common neutral, while the red and black each feed,
> separately, the top and bottom hot sides of the receptacle. Typically such
> receptacles are supplied from a ganged breaker, i.e. a breaker in which the
> handles are tied together for a common trip, so that if one kitchen
> appliance malfunctions and pops the breaker, the other side of the duplex
> receptacle will be shut off as well. This is called a multiwire circuit.
>
> -------------------------------------
>
> Man, this is news to me!
>
> It seems, tho, there are two separate issues at work here:
> 1. The current carrying demand on a neutral conductor
> 2. A personnel-type issue, ito breaker panel voltages.
>
> The split-phase nature of the two hots addresses (1), while the double-pole
> breaker addresses (2).
> I personally never thought (2) was such a big deal, as any wire dangling in
> the air can be hot, *including* a plumbing connection to the street, say,
> when the water meter is disconnected.
> But, http://www.phy.ornl.gov/divops/ESH/98-2.html ( a link from the above
> wiki site) apparently frowns on edison circuits, and indeed suggests double
> pole breakers as one remedy.
>
> AND, a double pole breaker also has another safety feature that just dawned
> on me, quite related to (1):
> It discourages moving one of those neutral-sharing hot wires in the
> breaker panel willy-nilly, possibly putting both hots on the same svc leg,
> thus overloading the neutral.
>
> But here's what I have experienced:
> More often than not, in opening a junction box, esp. the larger 5" ones,
> I'll see oodles of neutrals wire-nutted together, irrespective of the phase
> of the hot leg. Apparently there is a wide-spread cavalier attitude toward
> the lowly neutral.
>
> I suspect some of this cavalier attitude toward neutrals comes from the
> notion that the ground, or other mystery neutrals, are picking up some of
> the slack anyway, so no biggie. When in fact it could be a biggie.
>
> And consider this:
> For X number of circuits, there should X number of neutrals on the neutral
> bar, MINUS the number of edison circuits. This would seem to be check out
> pretty well on bx/romex systems, but how about separate wires pulled through
> EMT? It seems that there could be much more abuse of the hot/neutral ratio
> in pulled wires. I've seen pretty scarce neutral bars in some breaker
> panels!
>
well, you don't need neutrals for "pure" 240VAC circuits, such as a
water heater, air conditioner, air compressor, etc...
nate
--
replace "roosters" with "cox" to reply.
http://members.cox.net/njnagel
|
|
Posted by Robert Green on November 7, 2009, 2:51 pm
> Existential Angst wrote:
> >> Existential Angst wrote:
> >>>>>> Do you know *why* you can use just one neutral to serve two hots
with
> >>>>>> this
> >>>>>> kind of circuit?
> >>>>> If I didn't, would you tell me?
> >>>> Apparently you don't, so permit me to enlighten you. Code requires
that
> >>>> all
> >>>> ungrounded (i.e. hot) conductors of a multiwire (Edison) circuit have
a
> >>>> *single* disconnecting means -- so unless you can find a 20-pole
> >>>> breaker,
> >>>> you're going to have a tough time meeting Code with that monster
circuit
> >>>> you
> >>>> proposed.
> >>>> The reason Code requires this is straightforward: the neutral wire
> >>>> carries
> >>>> current. If one hot leg of a multiwire circuit is left powered on,
and
> >>>> any
> >>>> load on that leg is in use, the current drawn by that load is flowing
in
> >>>> the
> >>>> neutral -- presenting a risk of fatal electric shock to anyone
servicing
> >>>> the
> >>>> other leg of that circuit.
> >>> So are you saying that in a simple 3 wire bx cable, with the two hots
> >>> sharing a neutral, you'd need a two-pole breaker?
> >> yup, exactly. Or if they were two single pole breakers they'd have to
be
> >> next to each other with a handle tie.
> > From http://en.wikipedia.org/wiki/Shared_neutral, it would appear that
wiki
> > agrees with you! The article also references the term Edison Circuit.
> > Split phase shared neutral
> > In split phase house wiring, for example, a duplex receptacle in a
kitchen
> > is typically connected with a cable that has three conductors, in
addition
> > to ground. The three conductors are usually colored red, black, and
white.
> > The white serves as a common neutral, while the red and black each feed,
> > separately, the top and bottom hot sides of the receptacle. Typically
such
> > receptacles are supplied from a ganged breaker, i.e. a breaker in which
the
> > handles are tied together for a common trip, so that if one kitchen
> > appliance malfunctions and pops the breaker, the other side of the
duplex
> > receptacle will be shut off as well. This is called a multiwire circuit.
> > -------------------------------------
> > Man, this is news to me!
> > It seems, tho, there are two separate issues at work here:
> > 1. The current carrying demand on a neutral conductor
> > 2. A personnel-type issue, ito breaker panel voltages.
> > The split-phase nature of the two hots addresses (1), while the
double-pole
> > breaker addresses (2).
> > I personally never thought (2) was such a big deal, as any wire dangling
in
> > the air can be hot, *including* a plumbing connection to the street,
say,
> > when the water meter is disconnected.
> > But, http://www.phy.ornl.gov/divops/ESH/98-2.html ( a link from the
above
> > wiki site) apparently frowns on edison circuits, and indeed suggests
double
> > pole breakers as one remedy.
> > AND, a double pole breaker also has another safety feature that just
dawned
> > on me, quite related to (1):
> > It discourages moving one of those neutral-sharing hot wires in the
> > breaker panel willy-nilly, possibly putting both hots on the same svc
leg,
> > thus overloading the neutral.
> > But here's what I have experienced:
> > More often than not, in opening a junction box, esp. the larger 5" ones,
> > I'll see oodles of neutrals wire-nutted together, irrespective of the
phase
> > of the hot leg. Apparently there is a wide-spread cavalier attitude
toward
> > the lowly neutral.
> > I suspect some of this cavalier attitude toward neutrals comes from the
> > notion that the ground, or other mystery neutrals, are picking up some
of
> > the slack anyway, so no biggie. When in fact it could be a biggie.
> > And consider this:
> > For X number of circuits, there should X number of neutrals on the
neutral
> > bar, MINUS the number of edison circuits. This would seem to be check
out
> > pretty well on bx/romex systems, but how about separate wires pulled
through
> > EMT? It seems that there could be much more abuse of the hot/neutral
ratio
> > in pulled wires. I've seen pretty scarce neutral bars in some breaker
> > panels!
> well, you don't need neutrals for "pure" 240VAC circuits, such as a
> water heater, air conditioner, air compressor, etc...
> nate
Agreed. While that's technically true, most of the 240VAC equipment in an
average home is not pure 240VAC. In my panel they run neutrals to power the
oven timers, water heater igniters and other circuits needing only 110VAC.
I think the primary reason NEC demands "tied" breakers, is, as said before,
to prevent electricians from thinking the entire split-receptacle is
disconnected. Another problem is that if they are on separate breakers,
someone could move the wire from one side of the panel to the other and thus
overload the single neutral. Usually, the best and easiest fix is pulling
two runs of Romex to the receptacle so that each circuit has its own
neutral. Not sure, but believe the NEC considers that "parallel circuits."
But it still doesn't get around the issue of an outlet being hot when you
think it's not. Even testing an outlet is no guarantee that it's dead. A
broken wire or bad connection could make an outlet read dead but once you
got your fingers down past the break --- ZAP!
I believe the problem most people have with shared neutrals is that it seems
like they are adding 1 plus 1 and getting 1 as a result. However, since
they are out-of-phase, you're really adding 1/2 plus 1/2 and coming out with
1. When the first circuit's amplitude is at its peak, the other phase is at
the bottom, and the two phases cancel each other out. It doesn't seem to be
common sense that by adding a load to the other half of unbalanced load that
you'd actually be reducing the current in the shared neutral wire, but
that's how it works. I think. (-: At least that's how my friend who
designs 240VAC gear explained it me.
--
Bobby G.
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