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On Tue, 19 Feb 2008 18:59:48 -0800 (PST), Gerald Newton <...@electrician2.com> wrote:
On Feb 19, 5:47 pm, "Joe" <...@hotmail.com> wrote:
I've been pondering a question lately that I figure you guys can help me
with.
There are, based on my casual observation, many places in household wiring
where the connections within a circuit have a much smaller contact area th=an
the cross-sectional area of the wire that is required for the same circuit=.
A good example would be the 'quick connect' slots in the back of a
receptacle. Love them or hate them, they are allowed by code in many (most=?)
areas, and it seems to me the contact area is extremly small--perhaps
several orders of magnitude smaller than the cross-sectional area of a 14
gauge wire.
Another example would be a 100A breaker feeding a subpanel. The breaker
feeds cable the size of a pencil, but clips to the hot bus bar with the sa=me
tiny tabs as a 15A breaker (which seem to have a comparatively small conta=ct
area when compared to the diameter of 2 AWG cable).
So, it seems to me that, in a circuit running at capacity, there would be =a
high concentration of current flowing through the very small contact area =in
these apparent 'bottlenecks'...why don't they overheat?
Axial heat transfer.
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On 20 Feb 2008 14:05:01 GMT, phil...@ipal.net wrote:
On Tue, 19 Feb 2008 19:47:54 -0700 Joe <...@hotmail.com> wrote:
| I've been pondering a question lately that I figure you guys can help me
| with.
|
| There are, based on my casual observation, many places in household wiring
| where the connections within a circuit have a much smaller contact area than
| the cross-sectional area of the wire that is required for the same circuit.
|
| A good example would be the 'quick connect' slots in the back of a
| receptacle. Love them or hate them, they are allowed by code in many (most?)
| areas, and it seems to me the contact area is extremly small--perhaps
| several orders of magnitude smaller than the cross-sectional area of a 14
| gauge wire.
|
| Another example would be a 100A breaker feeding a subpanel. The breaker
| feeds cable the size of a pencil, but clips to the hot bus bar with the same
| tiny tabs as a 15A breaker (which seem to have a comparatively small contact
| area when compared to the diameter of 2 AWG cable).
|
| So, it seems to me that, in a circuit running at capacity, there would be a
| high concentration of current flowing through the very small contact area in
| these apparent 'bottlenecks'...why don't they overheat?
They do get hotter. It's called a hotspot. It can also happen with wire,
especially stranded. However, when the heat is sourced at just a spot like
that, it can be carried along the bus, wire, or tab, and dissipate into the
surrounding environment a lot easier than if the entire length were heating
due to a big overload.
--
|---------------------------------------/----------------------------------|
| Phil Howard KA9WGN (ka9wgn.ham.org) / Do not send to the address below |
| first name lower case at ipal.net / spam...@ipal.net |
|------------------------------------/-------------------------------------|
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On Wed, 20 Feb 2008 00:57:34 -0500, "Tim Perry" <...@nospammeadelphia.net> wrote:
"Joe" <...@hotmail.com> wrote in message
news...@comcast.com...
I've been pondering a question lately that I figure you guys can help me
with.
There are, based on my casual observation, many places in household wiring
where the connections within a circuit have a much smaller contact areathan
the cross-sectional area of the wire that is required for the same
circuit.
A good example would be the 'quick connect' slots in the back of a
receptacle. Love them or hate them, they are allowed by code in many(most?)
areas, and it seems to me the contact area is extremly small--perhaps
several orders of magnitude smaller than the cross-sectional area of a 14
gauge wire.
Another example would be a 100A breaker feeding a subpanel. The breaker
feeds cable the size of a pencil, but clips to the hot bus bar with thesame
tiny tabs as a 15A breaker (which seem to have a comparatively smallcontact
area when compared to the diameter of 2 AWG cable).
So, it seems to me that, in a circuit running at capacity, there would bea
high concentration of current flowing through the very small contact areain
these apparent 'bottlenecks'...why don't they overheat?
Well they do sometimes :) I had a 15A outlet melt once. A copy machine was
plugged in further down the chain.
Mostly it is a case of I squared R where R is very small. Hopefully the
breaker will trip before the junction acts like a fuse.
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On Tue, 19 Feb 2008 22:54:22 -0800, "Paul Hovnanian P.E." <...@hovnanian.com> wrote:
Tim Perry wrote:
"Joe" <...@hotmail.com> wrote in message
news...@comcast.com...
> I've been pondering a question lately that I figure you guys can help me
> with.
>
> There are, based on my casual observation, many places in household wiring
> where the connections within a circuit have a much smaller contact area
than
> the cross-sectional area of the wire that is required for the same
circuit.
>
> A good example would be the 'quick connect' slots in the back of a
> receptacle. Love them or hate them, they are allowed by code in many
(most?)
> areas, and it seems to me the contact area is extremly small--perhaps
> several orders of magnitude smaller than the cross-sectional area of a 14
> gauge wire.
>
> Another example would be a 100A breaker feeding a subpanel. The breaker
> feeds cable the size of a pencil, but clips to the hot bus bar with the
same
> tiny tabs as a 15A breaker (which seem to have a comparatively small
contact
> area when compared to the diameter of 2 AWG cable).
>
> So, it seems to me that, in a circuit running at capacity, there would be
a
> high concentration of current flowing through the very small contact area
in
> these apparent 'bottlenecks'...why don't they overheat?
>
Well they do sometimes :) I had a 15A outlet melt once. A copy machine was
plugged in further down the chain.
Mostly it is a case of I squared R where R is very small. Hopefully the
breaker will trip before the junction acts like a fuse.
Problem with that theory: The current seen by the circuit breaker never
exceeds full load current (this isn't a short). Unfortunately, the
junction temperature can rise to levels that can result in a fire.
This is why the NEC requires Arc Fault protection. As these junctions
begin to fail, they start arcing.
--
Paul Hovnanian mail...@Hovnanian.com
------------------------------------------------------------------
If you're not part of the solution, you're part of the precipitate.
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On Thu, 21 Feb 2008 01:07:43 +0000 (UTC), dave...@cs.ubc.ca (Dave Martindale) wrote:
"Paul Hovnanian P.E." <...@hovnanian.com> writes:
Problem with that theory: The current seen by the circuit breaker never
exceeds full load current (this isn't a short). Unfortunately, the
junction temperature can rise to levels that can result in a fire.
This is why the NEC requires Arc Fault protection. As these junctions
begin to fail, they start arcing.
Problem with *that* is that the load is still in series with the arc,
and the arc current is still limited to the load current of the device.
As I understand it, current arc fault interrupters are designed to
detect only the high-current arcing you can get with a line to neutral
(or ground) fault, not low-current arcs you get with the load still in
series with the arc.
Dave
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On Wed, 20 Feb 2008 19:10:35 -0800, "Paul Hovnanian P.E." <...@hovnanian.com> wrote:
Dave Martindale wrote:
"Paul Hovnanian P.E." <...@hovnanian.com> writes:
>Problem with that theory: The current seen by the circuit breaker never
>exceeds full load current (this isn't a short). Unfortunately, the
>junction temperature can rise to levels that can result in a fire.
>This is why the NEC requires Arc Fault protection. As these junctions
>begin to fail, they start arcing.
Problem with *that* is that the load is still in series with the arc,
and the arc current is still limited to the load current of the device.
As I understand it, current arc fault interrupters are designed to
detect only the high-current arcing you can get with a line to neutral
(or ground) fault, not low-current arcs you get with the load still in
series with the arc.
Dave
This is true of the older units. The latest generation if AFCIs is
supposed to respond to hi-z series arcs.
http://www2.sea.siemens.com/News/Construction/Siemens-Introduces-Combination-AR C-Fault-Circuit_Interupters.htm
--
Paul Hovnanian mail...@Hovnanian.com
------------------------------------------------------------------
Incorrigible punster -- Do not incorrige.
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On Fri, 22 Feb 2008 15:02:33 -0600, bud-- <...@isp.com> wrote:
Paul Hovnanian P.E. wrote:
Dave Martindale wrote:
"Paul Hovnanian P.E." <...@hovnanian.com> writes:
Problem with that theory: The current seen by the circuit breaker never
exceeds full load current (this isn't a short). Unfortunately, the
junction temperature can rise to levels that can result in a fire.
This is why the NEC requires Arc Fault protection. As these junctions
begin to fail, they start arcing.
Problem with *that* is that the load is still in series with the arc,
and the arc current is still limited to the load current of the device.
As I understand it, current arc fault interrupters are designed to
detect only the high-current arcing you can get with a line to neutral
(or ground) fault, not low-current arcs you get with the load still in
series with the arc.
Dave
This is true of the older units. The latest generation if AFCIs is
supposed to respond to hi-z series arcs.
http://www2.sea.siemens.com/News/Construction/Siemens-Introduces-Combination-AR C-Fault-Circuit_Interupters.htm
As the article says, "combination" AFCIs are required as of 1-1-08 by
the 2005 NEC. Combination AFCIs detect arcs at a 5A level and will
detect series arcs. (Previously the detection level was 75A.) (Old and
new AFCIs also have ground fault detection required at 50mA, but usually
is at 30mA.)
--
bud--
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