@Stevenal- on thermal trip that is correct. An equation with log or feed back starting at the actual conductor temperature is much more accurate. However for those calculating EFLI in the field in much easier to just use the final conductor short circuit temperature or there about.
5 or 10 second disconnection times with EGCs and tap conductors could actually have me using 150*C (THHN) and 250*C (XLPE) conductor temps.
A 60 amp circuits will result in the EGC melting under acceptable worse case breaker clearing times, a 100 amp circuit has a 50% chance of the EGC melting before the breaker opens and a 200 amp circuit has a 100% chance of the EGC melting (breaking the the path back to the source) before the breaker ever opens.
Based upon these time current curves of a common brand name UL listed breaker one could make the inference that Table 250.122 assumes a clearing time faster than 10 seconds (and as such the intent {goal} of the code) is that a ground fault clear in less than 10 seconds.
For a real world 60 amp circuit trip should not exceed 5 seconds. Going by a typical time current curve 480 amps is indicated for a 5 second trip giving a max Zs of 0.25 ohms at 120 volts and 0.577 ohms at 277 volts.
With all this said I would give anything for a research lab like this:
Asking around we came up with a guess that at 30*C there would be multiplier of 0.823. At 30*C the AC resistance would be 17.7% lower.
Thus, we obtain:
14 AWG = 3.1 ohms per 1000 feet 75*C = 2.5513 ohms at 30*C
12 AWG = 2.0 ohms per 1000 feet 75*C = 1.646 ohms at 30*C
10 AWG = 1.2 ohms per 1000 feet at 75*C = 0.9876 ohms at 30*C
Here are the two equation that I am (mostly) basing my inferences on:
&
Some slight changes being that I am assuming 115 volts (lower end of the acceptable bandwidth) and that the fault current must be 125% over the breaker's minimum magnetic pickup as derived from the attached time current curves. Also I am not concerned with the "home run" instead the whole circuit to its furthest point but ending at cord caps and light fixture leads. 30*C vs 25*C in UL's research.
GFL = ground fault loop
15 amp breaker:
14-14 = 124.672 feet = 0.6361513472 ohms GFL =/= 188 amps at 120 volts, 179 amps at 114 volts
12-14 = 196.85 feet = 0.826238505 ohms GFL =/= 145 amps at 120 volts, 138 amps at 114 volts
10-12 = 314.961 feet = 0.8294812896 ohms GFL =/= 145 amps at 120 volts, 137 amps at 114 volts
20 amp breaker:
12-14 = 164.042 feet = 0.6885334866 ohms GFL =/= 174 amps at 120 volts, 166 amps at 114 volts
10-12 = 229.659 feet = 0.6048299424 ohms GFL =/= 198 amps at 120 volts, 188 amps at 114 volts
Assuming 2000 amps of fault current Line to Neutral at the panel board we obtain a Ze of about 0.05 ohms...
So adding 0.05 ohms to 0.8294812896 = 131 amps at 115 volts
Adding 0.05 ohms to 0.6885334866 = 156 amps at 115 volts
20 x 6 = 120 amps ==== 125% of 120 = 150 amps (UL's parallel arc mitigating threshold)
IMO, 500 amps (0.25 ohms Ze) at the panelboard is nowhere near as prevalent as its being argued in front of the code making panels. Further, I wish I could also test the magnetic pickup of modern breakers.
It is my opinion that both the external loop impedance and magnetic pickup of circuit breakers are being wittingly exaggerated in an effort to justify the mandate of AFCIs vs a practical table limiting the run length of 15 and 20 amp 120 volt residential circuits.
And yes, I know Table 68 in the CEC references voltage drop, but behind the scenes voltage drop is of the least concern to anyone involved.
