I didn't find discussion of altitude in my cable application books either nor on a couple cable websites.
Transformers and switchgear get derated at high altitudes, but I have not seen it done for cables. It makes sense, the less dense air has less cooling capability, but how much?
Adjustments for altitude of transformers and switchgear are made for insulation dielectric strength, not for loading. Cable insulation dielectric strength doesn't depend on air pressure.
Using different ANSII tables I would derate the current as follows;
1000m = 100%
3000m = 96%
6100m = 90%
This is the current derating for fuses, enclosed air disconnects, metal enclosed and metal clad switchgear. Basically, it accounts for the reduces cooling effect of the less dense air.
As already pointed out, the insulation strength does not depend on air pressure so it would not de-rate. But, any place relying on through air clearances should be derated as follows;
1000m = 100%
3000m = 80%
6100m = 56%
So, say you require 3/8" clearance minimum for 480V. At 6100m this would be very close to 3/4" required.
Thanks for the answers. I find it sort of surprising that cable derating for elevation does not show up in the NEC or in some other relevant standard. Now I have to use good engineering judgment, and use my not-so-deep wisdom to talk someone into buying bigger by some not-too-clear degree. I might as well go find a brick wall to talk to, without a standard to back me up.
I'd guess that it is not addressed in the NEC because it has never been an actual problem. Most of the NEC is based on hindsight - fixing problems that have already occurred.
"Theory is when you know all and nothing works. Practice is when all works and nobody knows why. In this case we have put together theory and practice: nothing works... and nobody knows why! (Albert Einstein)
