Free Air conductor calculations

[Mr. Wizard]

Bear with me ya'll, I've never really had to do many free-air conductor installations, so here goes...

We have an 800 amp main breaker, feeding a loadcenter 380' away, 3 phase, 480 volts. An electrical contractor is coming in to do some upgrades to the Information Services department, and we have to temp in the loadcenter to keep the servers running (we have to do the work while the circuit is energized). The answer I've come up with is paralleling two 350's per phase with a #2 egc. We're basically going to run the conductors on the floor until the EC is done. I just think that two 350's/phase is a bit overkill, but since I know I could be wrong (or right?), I'd ask you guys and girls to shine a little light on this. Thanks in advance for any input

 

[don_resqcapt19]

I don't see laying on the floor as a "free air" application.

 

[Mr. Wizard]

Unfortunately there is no way to run the conductors overhead. My concern, other than the size of the wire, was about the conductors getting damaged due to being on the floor. But it didn't occur to me that the conductors being on the floor wouldn't be considered free air. Is it due to the fact the conductor isn't 100% surrounded by air? If they're not free air, but not in conduit, on what would I base my calculations? I'm in no way trying to bump heads with you - I have no doubt you're right, but if you could show me the error of my ways I'd greatly appreciate it.

 

[don_resqcapt19]

I don't know if I am correct. It is just my opinion that to use the free air ampacity, the conductor needs to be completely in free air.

 

[Smart $]

Quite likely the termination temperature limitations of 110.14(C) will not permit you to use the free-air ampacity for anything other than derating.

PS: Parallel 350's are undersized for an 800A main.

 

[texie]

If it was my call to make, I wouldn't allow THHN or THW to just be laid on the floor. Besides, what will you do with all that abused wire when you are done? IMHO a better choice would be to rent 4/0 type W cable with camlocks from Aggreko , GE or CAT. Just run 3 or 4 sets per phase. It's safer and I'll bet cheaper in the long run. You should not pay the "rack rate" for this much, especially at this time of year with not to many big disasters going on.