iwire Wrote:
sgreany
The question you asked has absolutely nothing to do with the ambient temperature.
electricman2 nailed it, there is nothing for the NEC to reevaluate, the rule is a good rule.
First read carefully the name of Table 310.16
quote:Allowable Ampacities of Insulated Conductors Rated 0 Through 2000 Volts, 60°C Through 90°C (140°F Through 194°F), Not More Than Three Current-Carrying Conductors in Raceway, Cable, or Earth (Directly Buried), Based on Ambient Temperature of 30°C (86°F)
The rating of 8 AWG in the 75 C column is 50 amps.
Now if the ambient temperature is 30 C as the table is designed around and if the cable or raceway has three 8 AWGs each loaded to 50 amps the temperature of the conductors will approach 75 C.
75 C is 167 F and yes that is warm but it is within the design limitations of any 75 C insulation.
Connect this conductor that is close to 75 C to a 60 C rated breaker and the heat transfer from the conductors to the breaker is very likely to cause the breakers thermal overload to trip the breaker.
Bob
I believe that the actual problem with the temperature rating of terminals is whether they will continue to conduct the current safely rather than the effect the heat would have on adjacent parts. When the testing laboratories test terminals they subject them to repeated thermal cycling. The temperature rating of the terminal is the maximum temperature it can be repeatedly subjected to without loosening or failing physically which would cause arcing and destructive heating of the terminal and the conductor insulation. In other words you can only allow a certain temperature rise in a terminal before the stresses induced by thermal expansion will cause the terminal to fail.
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Tom H
