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Home > Archive > Electrical Engineering > February 2008 > Contact surface area & household power transmission
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| Author |
Contact surface area & household power transmission
|
|
|
| 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?
| |
| Gerald Newton 2008-02-19, 9:25 pm |
| On Feb 19, 5:47=A0pm, "Joe" <sla...@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.
| |
| Tim Perry 2008-02-20, 3:25 am |
|
"Joe" <slayte@hotmail.com> wrote in message
news:neqdnVapSNFACSbanZ2dnUVZ_oqhnZ2d@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.
| |
| Paul Hovnanian P.E. 2008-02-20, 3:25 am |
| Tim Perry wrote:
>
> "Joe" <slayte@hotmail.com> wrote in message
> news:neqdnVapSNFACSbanZ2dnUVZ_oqhnZ2d@comcast.com...
> than
> circuit.
> (most?)
> same
> contact
> a
> in
>
> 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 mailto:Paul@Hovnanian.com
------------------------------------------------------------------
If you're not part of the solution, you're part of the precipitate.
| |
| phil-news-nospam@ipal.net 2008-02-20, 9:25 am |
| On Tue, 19 Feb 2008 19:47:54 -0700 Joe <slayte@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 / spamtrap-2008-02-20-0802@ipal.net |
|------------------------------------/-------------------------------------|
| |
| Dave Martindale 2008-02-20, 8:25 pm |
| "Paul Hovnanian P.E." <paul@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
| |
| Paul Hovnanian P.E. 2008-02-20, 9:25 pm |
| Dave Martindale wrote:
>
> "Paul Hovnanian P.E." <paul@hovnanian.com> writes:
>
>
>
> 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/Co...Interupters.htm
--
Paul Hovnanian mailto:Paul@Hovnanian.com
------------------------------------------------------------------
Incorrigible punster -- Do not incorrige.
| |
|
| Paul Hovnanian P.E. wrote:
> Dave Martindale wrote:
>
> 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/Co...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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