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Kitchen Man
Guest
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 Posted:
Sun Feb 27, 2005 12:02 am Post subject:
Re: 14-3 shared neutral |
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On Wed, 23 Feb 2005 13:38:01 -0330 in sci.electronics.basics, "Terry"
<tsanford@nf.sympatico.ca> wrote msg
<iL2Td.14401$uO.495799@news20.bellglobal.com>:
| Quote: | if there is load on only one side (leg) of the circuit, the neutral will
carry the same current as the hot lead on that side; and the other hot lead
will be carrying nothing; right?
It is a common mistake to think of the neutral as NOT carrying current; but
all our basic circuit training tells us that current has to flow from a
supply, through a load and return!
That return IS the neutral conductor and it better be intact and in good
shape! The fact that a good neutral is almost or only a volt or two above
ground potential means that it is doing it's job of returning the current to
the low voltage (neutral) side of the supply panel with little loss due to
the resistance of the conductor.
|
You're absolutely right about this. It is not right to share neutral
circuits. Think of the implications of doing wiring repairs - if a
worker disconnects a neutral wire splice while working on a disconnected
circuit, the currents carried by a shared circuit will start looking for
a place to go. The worker will likely get a little spark surprise at
the same time. I'm surprised that an electrical inspector would allow
it, is such practice really allowed by code?
--
http://www.xmission.com/~tiger885/motorbike/NART/nart.html
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Rich Grise
Guest
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| Posted:
Sun Feb 27, 2005 3:24 am Post subject:
Re: 14-3 shared neutral |
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On Sat, 26 Feb 2005 11:02:17 -0700, Kitchen Man wrote:
| Quote: | On Sat, 26 Feb 2005 13:02:19 +0000 in sci.electronics.basics, Fred Abse
excretatauris@cerebrumconfus.it> wrote msg
pan.2005.02.26.12.09.39.752208@cerebrumconfus.it>:
On Thu, 24 Feb 2005 21:48:13 -0800, John Larkin wrote:
Ah, I see now. Suppose two different computers calculate the number
"3". One adds 2+1, and the other takes the square root of 9. These 3's
are different, so there are actually two distinct kinds of "3".
That's only true for relatively large values of 3
But to get true phase shift, you have to take the limit as 3 approaches
zero.
I believe that Terry's original comment was meant to draw a distinction
between what's available on the two big black wires coming in to the
house, and the three 120 degree phases on the big transmission lines.
Some people are under the mistaken impression that two of those phases
are fed into the residence, rather than one phase split and inverted.
And yes, once you split and invert, you've got a 180 degree phase shift,
as well as a 50% attenuation, on *ahem* each leg. Or should I say bus?
|
But It Is Not A Phase Shift! It Is Merely A Polarity Inversion!
Some transvestites _look_ just like women - until you get under the
covers!
A phase shift oscillator works because the 180 degree phase shifted
negative feedback _looks_ to the opamp _exactly_ like in-phase positive
feedback. The opamp doesn't know it's in an oscillator - it's only
amplifying.
But just because it _looks_ the same, doesn't mean that it _is_ the
same.
Thanks,
Rich |
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Rich Grise
Guest
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| Posted:
Sun Feb 27, 2005 3:27 am Post subject:
Re: 14-3 shared neutral |
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On Sat, 26 Feb 2005 11:02:17 -0700, Kitchen Man wrote:
| Quote: | On Sat, 26 Feb 2005 13:02:19 +0000 in sci.electronics.basics, Fred Abse
excretatauris@cerebrumconfus.it> wrote msg
pan.2005.02.26.12.09.39.752208@cerebrumconfus.it>:
On Thu, 24 Feb 2005 21:48:13 -0800, John Larkin wrote:
Ah, I see now. Suppose two different computers calculate the number
"3". One adds 2+1, and the other takes the square root of 9. These 3's
are different, so there are actually two distinct kinds of "3".
That's only true for relatively large values of 3
But to get true phase shift, you have to take the limit as 3 approaches
zero.
I believe that Terry's original comment was meant to draw a distinction
between what's available on the two big black wires coming in to the
house, and the three 120 degree phases on the big transmission lines.
Some people are under the mistaken impression that two of those phases
are fed into the residence, rather than one phase split and inverted.
And yes, once you split and invert, you've got a 180 degree phase shift,
as well as a 50% attenuation, on *ahem* each leg. Or should I say bus?
|
OK, Got it.
If the top of the transformer is 180 degrees phase shifted from the bottom,
can you tap the transformer and pick off, say, 30 degrees, 45 degrees, 90
degrees, and so on, until you reach the top, at 180?
Of course not!
Now, do I make myself clear?
Thank you very much.
Rich
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John Larkin
Guest
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| Posted:
Sun Feb 27, 2005 3:51 am Post subject:
Re: 14-3 shared neutral |
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On Sat, 26 Feb 2005 21:27:17 GMT, Rich Grise <richgrise@example.net>
wrote:
| Quote: | On Sat, 26 Feb 2005 11:02:17 -0700, Kitchen Man wrote:
On Sat, 26 Feb 2005 13:02:19 +0000 in sci.electronics.basics, Fred Abse
excretatauris@cerebrumconfus.it> wrote msg
pan.2005.02.26.12.09.39.752208@cerebrumconfus.it>:
On Thu, 24 Feb 2005 21:48:13 -0800, John Larkin wrote:
Ah, I see now. Suppose two different computers calculate the number
"3". One adds 2+1, and the other takes the square root of 9. These 3's
are different, so there are actually two distinct kinds of "3".
That's only true for relatively large values of 3
But to get true phase shift, you have to take the limit as 3 approaches
zero.
I believe that Terry's original comment was meant to draw a distinction
between what's available on the two big black wires coming in to the
house, and the three 120 degree phases on the big transmission lines.
Some people are under the mistaken impression that two of those phases
are fed into the residence, rather than one phase split and inverted.
And yes, once you split and invert, you've got a 180 degree phase shift,
as well as a 50% attenuation, on *ahem* each leg. Or should I say bus?
OK, Got it.
If the top of the transformer is 180 degrees phase shifted from the bottom,
can you tap the transformer and pick off, say, 30 degrees, 45 degrees, 90
degrees, and so on, until you reach the top, at 180?
Of course not!
|
Of course not. If the only available vectors are 0 and 180 degrees, no
algebraic combination of the two can create any other phase angles.
Taps merely give you different amplitudes at 0 or 180 degrees.
John |
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Michael A. Terrell
Guest
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| Posted:
Sun Feb 27, 2005 5:05 am Post subject:
Re: 14-3 shared neutral |
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Kitchen Man wrote:
| Quote: |
You're absolutely right about this. It is not right to share neutral
circuits. Think of the implications of doing wiring repairs - if a
worker disconnects a neutral wire splice while working on a disconnected
circuit, the currents carried by a shared circuit will start looking for
a place to go. The worker will likely get a little spark surprise at
the same time. I'm surprised that an electrical inspector would allow
it, is such practice really allowed by code?
|
Ok, how about this? Two duplex outlets in one double box. Do you?
1. Run two 14-2/G to the box, one for each outlet.
or
2. Run one 14-3/G to the box, and connect the red wire to one outlet,
the black to the other, then connect the white wire and ground to both
outlets? As long as the current in the neutral doesn't add, there is
less current in the neutral than either supply wire.
Method one uses more material, more labor and wires can be mixed between
the devices.
Method two uses only what is needed, takes less labor t install, and the
wiring is rather obvious to anyone capable of doing a later repair.
If it doesn't meet code the inspector can't allow it.
Think about it.
--
Beware of those who post from srvinet.com!
Michael A. Terrell
Central Florida |
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Peter Bennett
Guest
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| Posted:
Sun Feb 27, 2005 6:10 am Post subject:
Re: 14-3 shared neutral |
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On Sat, 26 Feb 2005 23:05:03 GMT, "Michael A. Terrell"
<mike.terrell@earthlink.net> wrote:
| Quote: | Method one uses more material, more labor and wires can be mixed between
the devices.
Method two uses only what is needed, takes less labor t install, and the
wiring is rather obvious to anyone capable of doing a later repair.
If it doesn't meet code the inspector can't allow it.
Think about it.
|
In some cases, the Canadian electrical code _requires_ that two
circuits be run in 14/3 (or 12/3).
Duplex outlets on a kitchen counter must be split, with one of the two
sockets fed from one phase, and the other from the other phase. They
must be fed by 14/3 or 12/3 cable from a two-pole breaker.
--
Peter Bennett, VE7CEI
peterbb4 (at) interchange.ubc.ca
new newsgroup users info : http://vancouver-webpages.com/nnq
GPS and NMEA info: http://vancouver-webpages.com/peter
Vancouver Power Squadron: http://vancouver.powersquadron.ca |
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Fred Abse
Guest
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| Posted:
Sun Feb 27, 2005 6:49 pm Post subject:
Re: 14-3 shared neutral |
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On Thu, 24 Feb 2005 02:45:07 +0000, Rich Grise wrote:
| Quote: | ut they're the same signal! How can a signal be out of phase with itself?
Picking up a voltage from opposite ends of a transformer winding does not
introduce any phase shift, nor does center-tapping that winding and
grounding the center-tap.
|
OK, ground the center tap and add the signals from each end. You get
zero, not twice the original signal. Hence they are not the same signal.
| Quote: |
If you think they're out of phase, please show me the component that
delays the signal by 1/120 second.
|
Causality doesn't matter. The two signals behave exactly the same as if
you'd used two allpass networks of 90 degrees each. Show me how you would
distinguish between two sets of sinusoidal signals, one set antiphase as
the result of a delay circuit, and the other from a center tapped
transformer. It's only when signals are non-repetitive that you can
perhaps tell.
IMO. it's the use of the word "shift" that's confusing things. Use
"difference", and things get clearer.
"Antiphase" is not "in phase", by definition.
--
Then there's duct tape ...
(Garrison Keillor) |
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Kitchen Man
Guest
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| Posted:
Mon Feb 28, 2005 1:43 am Post subject:
Re: 14-3 shared neutral |
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On Sat, 26 Feb 2005 23:05:03 GMT in sci.electronics.basics, "Michael A.
Terrell" <mike.terrell@earthlink.net> wrote msg
<4221009A.B97BE5C9@earthlink.net>:
| Quote: | Kitchen Man wrote:
You're absolutely right about this. It is not right to share neutral
circuits. Think of the implications of doing wiring repairs - if a
worker disconnects a neutral wire splice while working on a disconnected
circuit, the currents carried by a shared circuit will start looking for
a place to go. The worker will likely get a little spark surprise at
the same time. I'm surprised that an electrical inspector would allow
it, is such practice really allowed by code?
Ok, how about this? Two duplex outlets in one double box. Do you?
1. Run two 14-2/G to the box, one for each outlet.
or
2. Run one 14-3/G to the box, and connect the red wire to one outlet,
the black to the other, then connect the white wire and ground to both
outlets? As long as the current in the neutral doesn't add, there is
less current in the neutral than either supply wire.
|
Maybe I've misunderstood the discussion, because I don't see a problem
with what you're saying, and possibly what I'm asking isn't clear. I've
had experience where three separate circuits, using three separate
circuit breakers, shared neutral paths via wire-nut node boxes
throughout a building. It was my impression that safe practice dictated
that each circuit (defined by a common termination in the breaker box)
should have its neutral line isolated until it also is terminated in the
breaker box.
| Quote: | Method one uses more material, more labor and wires can be mixed between
the devices.
Method two uses only what is needed, takes less labor t install, and the
wiring is rather obvious to anyone capable of doing a later repair.
If it doesn't meet code the inspector can't allow it.
Think about it.
|
I don't have a copy of the NEC, and I know that the situation I've
described caused unpredictable and, to my eyes, unsafe conditions within
the circuitry. I'd appreciate any insight anyone has to offer.
--
Al Brennan
http://www.xmission.com/~tiger885/motorbike/NART/nart.html |
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Michael A. Terrell
Guest
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| Posted:
Mon Feb 28, 2005 1:51 am Post subject:
Re: 14-3 shared neutral |
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Kitchen Man wrote:
| Quote: |
Maybe I've misunderstood the discussion, because I don't see a problem
with what you're saying, and possibly what I'm asking isn't clear. I've
had experience where three separate circuits, using three separate
circuit breakers, shared neutral paths via wire-nut node boxes
throughout a building. It was my impression that safe practice dictated
that each circuit (defined by a common termination in the breaker box)
should have its neutral line isolated until it also is terminated in the
breaker box.
I don't have a copy of the NEC, and I know that the situation I've
described caused unpredictable and, to my eyes, unsafe conditions within
the circuitry. I'd appreciate any insight anyone has to offer.
--
Al Brennan
|
I don't have a current copy of the NEC codebook. It got to expensive
for me after I became disabled.
--
Beware of those who post from srvinet.com!
Michael A. Terrell
Central Florida |
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Kitchen Man
Guest
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| Posted:
Mon Feb 28, 2005 1:55 am Post subject:
Re: 14-3 shared neutral |
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On Sat, 26 Feb 2005 21:24:23 GMT in sci.electronics.basics, Rich Grise
<richgrise@example.net> wrote msg
<pan.2005.02.26.21.24.29.841590@example.net>:
| Quote: | And yes, once you split and invert, you've got a 180 degree phase shift,
as well as a 50% attenuation, on *ahem* each leg. Or should I say bus?
But It Is Not A Phase Shift! It Is Merely A Polarity Inversion!
Some transvestites _look_ just like women - until you get under the
covers!
A phase shift oscillator works because the 180 degree phase shifted
negative feedback _looks_ to the opamp _exactly_ like in-phase positive
feedback. The opamp doesn't know it's in an oscillator - it's only
amplifying.
But just because it _looks_ the same, doesn't mean that it _is_ the
same.
|
Rich, I violently agree. Inversion is the correct term, I think I said
as much. The resultant signals, as we are so adamantly unopposing each
other about, are 180 degrees out of phase with each other. The one
thing we seem to be disagreeing about is the degree of severity of
calling it a "phase shift," since one of the signals was not caused by
some reactance to lead or lag. To me, it isn't that big a deal.
--
Al Brennan
http://www.xmission.com/~tiger885/motorbike/NART/nart.html |
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Ingvar Esk
Guest
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| Posted:
Mon Feb 28, 2005 5:56 pm Post subject:
Re: 14-3 shared neutral |
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"Rich Grise" <richgrise@example.net> wrote in message
news:pan.2005.02.26.21.24.29.841590@example.net...
| Quote: | On Sat, 26 Feb 2005 11:02:17 -0700, Kitchen Man wrote:
On Sat, 26 Feb 2005 13:02:19 +0000 in sci.electronics.basics, Fred Abse
excretatauris@cerebrumconfus.it> wrote msg
pan.2005.02.26.12.09.39.752208@cerebrumconfus.it>:
On Thu, 24 Feb 2005 21:48:13 -0800, John Larkin wrote:
Ah, I see now. Suppose two different computers calculate the number
"3". One adds 2+1, and the other takes the square root of 9. These 3's
are different, so there are actually two distinct kinds of "3".
That's only true for relatively large values of 3
But to get true phase shift, you have to take the limit as 3 approaches
zero.
I believe that Terry's original comment was meant to draw a distinction
between what's available on the two big black wires coming in to the
house, and the three 120 degree phases on the big transmission lines.
Some people are under the mistaken impression that two of those phases
are fed into the residence, rather than one phase split and inverted.
And yes, once you split and invert, you've got a 180 degree phase shift,
as well as a 50% attenuation, on *ahem* each leg. Or should I say bus?
But It Is Not A Phase Shift! It Is Merely A Polarity Inversion!
Some transvestites _look_ just like women - until you get under the
covers!
A phase shift oscillator works because the 180 degree phase shifted
negative feedback _looks_ to the opamp _exactly_ like in-phase positive
feedback. The opamp doesn't know it's in an oscillator - it's only
amplifying.
But just because it _looks_ the same, doesn't mean that it _is_ the
same.
Thanks,
Rich
Anothor way to illustrate the difference is to use a non-pure sine wave. |
Let's assume you have some strange load that causes a spike on the top of
every positive period. In a scope you will see the spike move to left or
right with a phase shift and to move to the negative side in the case of
inversion.
/Ingvar |
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