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allowable ampacities for conducter

1K views 11 replies 7 participants last post by  n175h 
#1 ·
on the chart of allowable ampacities of conducters what determines which column you use? i live in south alabama and am getting conflicting reports on whether to use the 75 or 90 degree celcius column.
 
#4 ·
You must also take into consideration the temp rating for the terminal connections.
A good example would be a panel being fed with THHN/THWN wire.
THHN is listed in the 90 degree column, but the panel lugs are not rated for 90 degrees. So you must use either the 60 or 75 degree column just because the panel lugs are not rated for 90 degree.

Like Joed mentions above. There is more to this article than meets the eye. It is also the most misunderstood and misused articles in the NEC.
 
#6 ·
Exactly what is it that you are trying to size here?
 
#7 ·
Typically, in new installtions:
For THHN/THWN conductors in conduit you would use the 75 deg C column.
For NM cable you are limited to the 60 deg C column.
 
#9 ·
This throws another wrench into it. Conductor sizing for a residential service is also different. It does not use the same chart as feeders or branch circuits.

This is why when folks ask what size wire for XX amps it is impossible to say without knowing all the details.
 
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