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?
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Axial heat transfer.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

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.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Tim Perry wrote:

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: snipped-for-privacy@Hovnanian.com
------------------------------------------------------------------
  Click to see the full signature.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

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
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
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/Construction/Siemens-Introduces-Combination-ARC-Fault-Circuit_Interupters.htm
--
Paul Hovnanian mailto: snipped-for-privacy@Hovnanian.com
------------------------------------------------------------------
  Click to see the full signature.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Paul Hovnanian P.E. wrote:

http://www2.sea.siemens.com/News/Construction/Siemens-Introduces-Combination-ARC-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--

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
| 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 |
  Click to see the full signature.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Polytechforum.com is a website by engineers for engineers. It is not affiliated with any of manufacturers or vendors discussed here. All logos and trade names are the property of their respective owners.