Rackspace is typically at a premium at most data centers.

I'm more surprised that a data centre will apparently provide more power to a rack than is safe to use.

My info may be dated, but power density has gone up a ton over time. I'd expect a lot of datacenters to have plenty of space, but not much power. You can only retrofit so much additional power distribution and cooling into a building designed for much less power density.

This is my experience as well. We have 42u racks with 8 machines in them because we cant get more power circuits to the rack.

yep this was the case for us.