Tap Conductor Short-Circuit Current Ratings

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MiloHEES

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When applying the short-circuit formulas for conductors NEC (2014) Chapter 9, Table 8, should T1 be the temperature rating of the conductor (75C or 90C) or the ambient temperature?
 
Correct table for tap conductor short-circuit ratings

Correct table for tap conductor short-circuit ratings

Sorry, Ch. 9 Table 8 has the conductor properties where you find the cmil area used in the calculations. The correct place is Table 240.92(B)
 
The published short circuit current tables that I have seen started with the rating of the conductor and not the ambient. The reasoning is that the conductor may be operating at close to its rating at the time of the short circuit.
 
My opinion is that the conductors' continuous temperature be used since it is going to be higher than ambient. Of course this would be a higher impedance and lower short circuit current so is only really relevant for arc flash where overestimating short circuit could result in a shorter arcing time and therefore smaller arc flash

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don_resqcapt19 said:
The published short circuit current tables that I have seen started with the rating of the conductor and not the ambient. The reasoning is that the conductor may be operating at close to its rating at the time of the short circuit.
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Conductors rarely operate at maximum insulation temperature, A case can also be made to use the rated ambient temperature of the protective device.
 
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