by Mark Lamendola
Special thanks to Fred Hartwell for helping me to understand this. We can all thank Fred for Example D3(a) in Annex D of the NEC, which is the reference for this article.
A raceway may run through a high temperature area,
while its terminations are in a low temperature area. Many industrial
processes result in extensive heat at the ceiling level (where you would
run a raceway). But fans, ventilation, and the natural thermal gradient
sharply reduce the temperature at the operator's level.
An example would be a set of open thermal pots for melting aluminum. Along the ceiling, the temperature stays at about 140 DegrF. in moderate weather. Worst case, you are going to see 165 DegrF on a really hot day (but the pot operators are working at 80 to 90 DegrF). Further, the pot area is surrounded by air curtains or doors to cordone it off from adjacent areas of the plant.
So you are running a 480V feeder from the service entrance to a lighting transformer located in an area adjacent to the potting area, and your feeders pass through the potting area. There are no terminations in the potting area and you are using THHN, so you can use the 90 Degr column. You know you need 1 AWG to handle the 160A load you calculated. But when you apply the correction factor, you find you need 500 kcmil.
Now you apply the correction factor to your THHN for temperature at the transformer. But the worst case is you are going to see 95 degrees on a really hot day. But since you've applied the correction factor to your THHN for the worst case of the potting ceiling area and sized the conductor accordingly, you can apply the 75 degree column to that same THHN. The 500 kcmil has an ampacity of 357A--so you are actually oversized.
But suppose you had not used the 90 degree column. That means you would have had to run 600 kcmil to accomplish the same thing. Even at 500, you are talking about special tools and some very strenuous work. But using a wire that is 20% larger now makes the job 200% harder. How do you support the raceway needed for so much extra conductor--how many additional holes must you drill? And how do you make bends, given that your bender isn't large enough?
The difference between the cost of the first installation (using the 90 degree column) and the cost of the second installation is huge. It's not just a larger wire size running in the "same 1-inch EMT" you were going to run, anyhow. I haven't even calculated the raceway size, but I would guess you are going from 4-inch raceway to 6-inch.
Even with a smaller load, these numbers can make a huge difference.
Now, perhaps, you can see why we find 90 degree conductors in various NEC tables.