Electrical Question Unrelated to Woodworking

On Tue, 28 Sep 2004 00:23:36 GMT, "toller" <[email protected]> wrote:

>It sounds like a multiwire circuit. Essentially you are using a 240v
>circuit for two 120v circuits.
>It saves a little wire, and cuts the voltage drop a bit.
>It is perfectly legal if done properly.
>
>I found exactly what you feared in my circuit box. Both circuits were on
>the same leg; if they were ever heavily used it could have caused a fire.
>Many people use a 240v breaker, both for the reason you describe (with a
>240v breaker you have to wire it correctly) and to prevent surprises from
>just shutting off one of the circuits and thinking both are off, but it is
>not required.

Actually, I think the current code DOES require a two pole breaker now
for a multiwire circuit. For both of the reasons you describe.


- -
LRod

Master Woodbutcher and seasoned termite

Shamelessly whoring my website since 1999

http://www.woodbutcher.net

This is called a multiwire circuit and is fairly common. Since the two hots
are 180 degrees out of phase with each other, the current flow through one
hot to neutral tends subtracts form the current flow from the other hot to
neutral. Normally these should be on the same 2 pole breaker or two adjacent
breakers that are tied together so that when turning off one circuit the
other circuit is off as well.

"DonkeyHody" <[email protected]> wrote in message
news:[email protected]...
>I wanted to borrow some electricity to install a light in a closet. A
> 3-way switch for the hall light was conveniently located on the
> opposite side of the same wall. In the box for the 3-way switch I
> found 4 each two conductor w/ground romex wires. The grounds were all
> tied together. Three of the insulated wires went to the switch as
> expected. Three neutrals were tied together, and two hots were tied
> together. But the two hots that were tied together were not on the
> same breaker with the light switch. They were on another breaker on
> the other side of the box, making 240 volts between the switch and the
> other two hots. So I had two different circuits on different breakers
> sharing a common neutral that is apparently the only neutral path back
> to the box. This is all original wiring in a house built in 1976. I
> know it's all original because the wires were spray painted along with
> the walls.
>
> Is this a common practice I haven't run across before? Does the code
> allow it?
>
> I realize that since the circuits are on different phases, the common
> neutral carries only the DIFFERENCE in the loads on the two circuits,
> not the combined load of both. But it still doesn't feel quite right
> to me. What if somebody moved one of the circuits to a breaker on the
> same side of the box. Then the common neutral would carry the
> combined load and overheat.
>
> DonkeyHody
> "Every man is my superior in that I can learn from him." - Thomas
> Carlyle

It is against code to have a bare light bulb type fixture in a closet, in
fact in any area inhabited area. A fixture where the bulb is protected from
damage is another story.

"Edwin Pawlowski" <[email protected]> wrote in message
news:[email protected]...
>
> "DonkeyHody" <[email protected]> wrote in message
> news:[email protected]...
>>I wanted to borrow some electricity to install a light in a closet.
>
> In at least some areas, it is against code to have a light fixture in a
> closet. It may be allowed in some walk-in closets. You may want to check
> into it before going ahead.
>

"Mark L." <[email protected]> wrote in message
news:[email protected]...
> Bob,
> I was just wondering what happens if the neutral that is feeding the 2
> opposing (out of phase) hot legs were to be disconnected? 240V across
> both of the legs (not each)? Which IF the loads were of equal resistance,
> would result in 120V across each leg. But I'm guessing that that would be
> a long shot to have each legs loaded the same. Does this make sense?
> Mark L.

Thats about the size of it. its not unheard of for a neutral wire to come
loose and end up with 100 volts on one leg and 140 on the other. The
symptom tells you its probably an open neutral.

By the way, someone said something about 4 neutrals being bonded together.
Presumably the poster meant they were wire nutted together somewhere out in
the field wiring, as I beleive the code requires that neutral bar
connections can only be a single wire. Ground connections on the other hand
are allowed 2 or 3 wires per connection point (depends on label on panel).

>
> Bob Peterson wrote:
>
>> This is called a multiwire circuit and is fairly common. Since the two
>> hots are 180 degrees out of phase with each other, the current flow
>> through one hot to neutral tends subtracts form the current flow from the
>> other hot to neutral. Normally these should be on the same 2 pole breaker
>> or two adjacent breakers that are tied together so that when turning off
>> one circuit the other circuit is off as well.
>

John McCoy <[email protected]> writes:
>[email protected] (DonkeyHody) wrote in
>news:[email protected]:
>
>> Then the common neutral would carry the
>> combined load and overheat.
>
>If I understood you correctly, there's three neutrals tied together
>in the box. Since the neutrals are all tied together at the breaker
>box too, it doesn't make any difference which side the 3 hots come
>from - regardless of how you slice it, there's 3 return paths for
>the 3 loads.
>
>John


Not necessarily. He had 4 cables entering the work box:


Assumptions:

12-3 feeding power from panel to work box on two breakers
12-2 providing common for 3-way switch
12-2 to light fixture
12-2 to string of outlets.

One of the 12-3 current conductors feeds the switch, the
other feeds the string of outlets. Here there
will be three grounded (neutral) conductors bonded together,
but only the 12-3 grounded conductor is a return path to
the panel (well, the grounding conductor is, too).

scott

"DonkeyHody" <[email protected]> wrote in message
news:[email protected]...
>I wanted to borrow some electricity to install a light in a closet.

In at least some areas, it is against code to have a light fixture in a
closet. It may be allowed in some walk-in closets. You may want to check
into it before going ahead.

[email protected] (DonkeyHody) wrote in
news:[email protected]:

> Then the common neutral would carry the
> combined load and overheat.

If I understood you correctly, there's three neutrals tied together
in the box. Since the neutrals are all tied together at the breaker
box too, it doesn't make any difference which side the 3 hots come
from - regardless of how you slice it, there's 3 return paths for
the 3 loads.

John

[email protected] (Scott Lurndal) wrote in news:6ii6d.3924$nj.2946
@newssvr13.news.prodigy.com:

> One of the 12-3 current conductors feeds the switch, the
> other feeds the string of outlets. Here there
> will be three grounded (neutral) conductors bonded together,
> but only the 12-3 grounded conductor is a return path to
> the panel (well, the grounding conductor is, too).

Ah - yes, I wasn't reading what he wrote as being configured like
that, but your assumptions sound a lot more likely. I would not
be pleased with wiring like that (and would probably rewire it,
which would take longer than anticipated & require more bad
language to be invested).

John

In article <[email protected]>, [email protected] (DonkeyHody) wrote:
>John,
>My terminology might not be entirely accurate. I'm refering to
>everything on the breaker side of the load as "hot" and everything on
>the return side of the load as "neutral". Four cables enter the box.
>Cable #1 carries two "hot" wires coming from another 3-way switch
>controlling the hall light, but they alternate which one is hot
>depending on the position of the other 3-way. Cable #2 carries the
>"hot" from the switch to the hall light and provides a neutral return
>from the hall light back to the switch box. Cable #3 brings in a hot
>from the opposite phase in the breaker panel and provides the only
>neutral return to the breaker panel from the switch box. Cable #4
>carries power from cable #3 to a receptacle (or two or three) and
>provides a neutral return back to the switch box. All 4 bare ground
>wires are tied together. All 3 neutral wires are tied together, but 2
>of these are returns from loads #2 and #4 to the box leaving only the
>third neutral as a path back to the breaker panel.

All is OK so far, since (as you stated) the two breakers are on opposite legs
of the service.
>
>I forgot to mention that ALL WIRES ARE #14, on 20 Amp breakers, which

This is emphatically NOT OK. For safety and code compliance, 14-gauge wire
*requires* 15 amp breakers.

>increases my level of discomfort with having this shared neutral.

Actually, the shared neutral is the least of your worries in this circuit.
With the two breakers being on opposite legs, the shared neutral never carries
any more current than the *difference* in current between the two hot legs.
For example, if one hot leg is drawing 9 amps, and the other is drawing 7, the
current in the neutral is only 2 amps. In the worst case (with no load at all
on one side), the neutral carries the same current as the loaded hot leg.
That's still bad, of course, because the breakers are oversized for the wire.

>I'll at least tie the two breakers together, since they are
>conveniently adjacent,

Tying them together is good -- but replacing them with breakers of the proper
rating for the wire (i.e. 15 amps) and tying *those* together is better.
Installing a 15 amp double-pole breaker is even better still.

This is important for at least two reasons:

1) It keeps the two breakers on opposite legs. If they were ever placed on the
same leg, the current in the shared neutral would equal the *sum* of the
currents on the separate hots. In the worst case, even with breakers properly
sized to the circuit conductors, this could be *double* the current that the
neutral conductor is rated to carry -- a serious fire hazard.

2) It ensures that *all* conductors in the shared circuit are de-energized at
once. If the two breakers are independent, it's possible to shut off only one
leg, leaving anyone performing maintenance on the _other_ leg exposed to an
electrocution hazard.

>but rewiring would be cussworthy because 1)This
>closet is on an outside wall where headroom in the attic drops to
>nothing, and 2) I had an additional 8 inches of insulation blown in
>over the tops of the ceiling joists, making both the joists and the
>wires hard to find.

Yep -- so replace the breakers with 15s, tied together, and everything will be
fine.

--
Regards,
Doug Miller (alphageek-at-milmac-dot-com)

Get a copy of my NEW AND IMPROVED TrollFilter for NewsProxy/Nfilter
by sending email to autoresponder at filterinfo-at-milmac-dot-com
You must use your REAL email address to get a response.

It sounds like a multiwire circuit. Essentially you are using a 240v
circuit for two 120v circuits.
It saves a little wire, and cuts the voltage drop a bit.
It is perfectly legal if done properly.

I found exactly what you feared in my circuit box. Both circuits were on
the same leg; if they were ever heavily used it could have caused a fire.
Many people use a 240v breaker, both for the reason you describe (with a
240v breaker you have to wire it correctly) and to prevent surprises from
just shutting off one of the circuits and thinking both are off, but it is
not required.

"DonkeyHody" <[email protected]> wrote in message
news:[email protected]...
>I wanted to borrow some electricity to install a light in a closet. A
> 3-way switch for the hall light was conveniently located on the
> opposite side of the same wall. In the box for the 3-way switch I
> found 4 each two conductor w/ground romex wires. The grounds were all
> tied together. Three of the insulated wires went to the switch as
> expected. Three neutrals were tied together, and two hots were tied
> together. But the two hots that were tied together were not on the
> same breaker with the light switch. They were on another breaker on
> the other side of the box, making 240 volts between the switch and the
> other two hots. So I had two different circuits on different breakers
> sharing a common neutral that is apparently the only neutral path back
> to the box. This is all original wiring in a house built in 1976. I
> know it's all original because the wires were spray painted along with
> the walls.
>
> Is this a common practice I haven't run across before? Does the code
> allow it?
>
> I realize that since the circuits are on different phases, the common
> neutral carries only the DIFFERENCE in the loads on the two circuits,
> not the combined load of both. But it still doesn't feel quite right
> to me. What if somebody moved one of the circuits to a breaker on the
> same side of the box. Then the common neutral would carry the
> combined load and overheat.
>
> DonkeyHody
> "Every man is my superior in that I can learn from him." - Thomas
> Carlyle

try alt.home.repair

Bob,
I was just wondering what happens if the neutral that is feeding the 2
opposing (out of phase) hot legs were to be disconnected? 240V across
both of the legs (not each)? Which IF the loads were of equal
resistance, would result in 120V across each leg. But I'm guessing that
that would be a long shot to have each legs loaded the same. Does this
make sense?
Mark L.

Bob Peterson wrote:

> This is called a multiwire circuit and is fairly common. Since the two hots
> are 180 degrees out of phase with each other, the current flow through one
> hot to neutral tends subtracts form the current flow from the other hot to
> neutral. Normally these should be on the same 2 pole breaker or two adjacent
> breakers that are tied together so that when turning off one circuit the
> other circuit is off as well.

[email protected] (DonkeyHody) wrote in message news:<[email protected]>...
> I wanted to borrow some electricity to install a light in a closet. A
> 3-way switch for the hall light was conveniently located on the
> opposite side of the same wall. In the box for the 3-way switch I
> found 4 each two conductor w/ground romex wires. The grounds were all
> tied together. Three of the insulated wires went to the switch as
> expected. Three neutrals were tied together, and two hots were tied
> together. But the two hots that were tied together were not on the
> same breaker with the light switch. They were on another breaker on
> the other side of the box, making 240 volts between the switch and the
> other two hots. So I had two different circuits on different breakers
> sharing a common neutral that is apparently the only neutral path back
> to the box. This is all original wiring in a house built in 1976. I
> know it's all original because the wires were spray painted along with
> the walls.
>
> Is this a common practice I haven't run across before? Does the code
> allow it?
>



> I realize that since the circuits are on different phases, the common
> neutral carries only the DIFFERENCE in the loads on the two circuits,
> not the combined load of both. But it still doesn't feel quite right
> to me. What if somebody moved one of the circuits to a breaker on the
> same side of the box. Then the common neutral would carry the
> combined load and overheat.
>
> DonkeyHody
> "Every man is my superior in that I can learn from him." - Thomas
> Carlyle

Whew!!!!!! I thought "Wood related electrical question" he was going
to ask the impedance and dielectric constant of wood.

John,
My terminology might not be entirely accurate. I'm refering to
everything on the breaker side of the load as "hot" and everything on
the return side of the load as "neutral". Four cables enter the box.
Cable #1 carries two "hot" wires coming from another 3-way switch
controlling the hall light, but they alternate which one is hot
depending on the position of the other 3-way. Cable #2 carries the
"hot" from the switch to the hall light and provides a neutral return
from the hall light back to the switch box. Cable #3 brings in a hot
from the opposite phase in the breaker panel and provides the only
neutral return to the breaker panel from the switch box. Cable #4
carries power from cable #3 to a receptacle (or two or three) and
provides a neutral return back to the switch box. All 4 bare ground
wires are tied together. All 3 neutral wires are tied together, but 2
of these are returns from loads #2 and #4 to the box leaving only the
third neutral as a path back to the breaker panel.

I forgot to mention that ALL WIRES ARE #14, on 20 Amp breakers, which
increases my level of discomfort with having this shared neutral.
I'll at least tie the two breakers together, since they are
conveniently adjacent, but rewiring would be cussworthy because 1)This
closet is on an outside wall where headroom in the attic drops to
nothing, and 2) I had an additional 8 inches of insulation blown in
over the tops of the ceiling joists, making both the joists and the
wires hard to find.

Thanks to everybody for your help.

DonkeyHody

"We should be careful to get out of an experience only the wisdom
that is in it - and stop there; lest we be like the cat that sits down
on a hot stove-lid. She will never sit down on a hot stove-lid
again---and that is well; but also she will never sit down on a cold
one anymore." - Mark Twain


>
> If I understood you correctly, there's three neutrals tied together
> in the box. Since the neutrals are all tied together at the breaker
> box too, it doesn't make any difference which side the 3 hots come
> from - regardless of how you slice it, there's 3 return paths for
> the 3 loads.
>
> John

[email protected] (DonkeyHody) writes:
> So I had two different circuits on different breakers
>sharing a common neutral that is apparently the only neutral path back
>to the box. This is all original wiring in a house built in 1976. I
>know it's all original because the wires were spray painted along with
>the walls.
>
>Is this a common practice I haven't run across before? Does the code
>allow it?

Yes and yes.

>
>I realize that since the circuits are on different phases, the common
>neutral carries only the DIFFERENCE in the loads on the two circuits,
>not the combined load of both. But it still doesn't feel quite right
>to me. What if somebody moved one of the circuits to a breaker on the
>same side of the box. Then the common neutral would carry the
>combined load and overheat.

yes, and probably not. How often are both current carrying legs at
full capacity, especially in a house?

scott

On Tue, 28 Sep 2004 02:31:15 GMT, "Edwin Pawlowski" <[email protected]>
wrote:

|
|"DonkeyHody" <[email protected]> wrote in message
|news:[email protected]...
|>I wanted to borrow some electricity to install a light in a closet.
|
|In at least some areas, it is against code to have a light fixture in a
|closet. It may be allowed in some walk-in closets. You may want to check
|into it before going ahead.

Depends on the size of the closet. I have two light fixtures and a
skylight in mine. BTW, a skylight is a bad idea in a closet, at least
in sunny AZ. SWMBO had a brand new red silk blouse stored for several
months before deciding to wear it. One sleeve was badly faded from
the sunlight. I wound up putting some window tinting film on a sheet
of plastic and using it as a filter.
|