Anyway, knitting the thread back together. I have a theory (though I could be wrong since its half- baked in progress).
Short circuit current was brought up in this thread as possibly having played a role in Ampacity tables.
So far several things have or can been established:
1. breaker magnetic trip points have gone down for reasons still not well known.
2. 240.4 D limits the over current protection on small conductors that otherwise have higher current ratings.
3. Small conductors can see the highest short circuit currents per circular mil of copper relative to larger conductors.
4. The NEC does not require that a short circuit trip a breaker magnetically, as such would still be considered code complaint.
5. Small conductors are typically stapled across wood studs and must survive short circuits without setting studs on fire.
6. In IEC based codes where magnetic trip is essentially required, smaller conductors are be permitted for the same current rating.
Thus under the NEC, cases exist where a short circuit will heat its conductor along with the bi-metal strip. In those cases the bi metal's heating must be faster to trigger effect, that being unlatching the breaker before the copper becomes hot enough to damage. Therefore the statement can be made 'under all current conditions the bi-metal must have a combined heating characteristic and heat response characteristic which shall be faster then any conductor heating which could reach a threshold capable of thermal damage' I would however imagine (guess) its not linear despite being an inverse characteristic.
For example: 30 amps of over current initiating an unlatch in 900 cycles resulting in say a 15*C conductor rise may not exhibit the same conductor rise if the same bi-metal naturally cleared 160 amps in 9 cycles, as it could be higher then 15*C. Its just a guess, but in that case it would make sense why smaller conductors have a a lower OCPD rating vs current carrying capacity. It may also explain why OCPD manufactures decided to lower the magnetic trip values. 2,500 amps cleared via magnetic trip vs thermal trip makes a difference in any case.
Here is a paper on short circuit withstand and heating of conductors:
http://www.cooperindustries.com/con...ductor_Protection_Wire_Cable_Protection_2.pdf
Thus if the above is true, the wire must have a lower OCPD regardless of its ability to carry current 24/7 which would explain the one correlation to K factor.
..........................................................................................................
The above of course throws one thing under the buss: motor circuits.
I can legally have #14 on a 40 amp breaker. The magnetic trip value on average is 15x40= 600amps and can certainly be higher. In most runs a ground fault will rely on the bi-metal (over load protection), one that is designed around 40 ampere rated conductor.
Thus, how is this ok? Does higher current resulting in faster mi-metal heating which result in less of a temp rise? What is the relationship? As you can see in contradicts the above. Anyone know why that is? :?:dunce::?
)
