Once you have located a space that is big enough and convenient to the administrators and/or users11.5 and that can hold all the systems you plan to put there without overstressing the physical structure, it is time to think about electrical power and air conditioning. You must think about the two together, because the amount of one you require determines the amount of the other you require, and you need a lot more of both than you think you do.
A rule of thumb to use in estimating your power requirements is to assume perhaps 150 Watts per (Intel or AMD) node. This is only a rule of thumb - if you get dual CPU nodes with all the memory they can hold and a big power supply and add a big, fast disk and a CD ROM and four network cards and a video card and an extra fan, you might need twice that. Certain alpha nodes tend very power hungry. Basically, heat tends to rise at least roughly proportional to clock, so as CPU clocks continue to increase, so will heat production. On the other hand a ``stripped'' diskless node, running a relatively low-clock CPU, might end up only drawing 50 or 60 Watts.
However, you are in for a big surprise. The numbers above are for RMS average power consumption. If PC's had a ``power factor'' close to unity, as do (for example) electric incandescent lights, one could easily take those numbers and figure out how many nodes can go onto a single 20 Amp, 120 Volt circuit.
It also wouldn't matter much how the multiple circuits that feed a power pole in a cluster room were wired. One could wire the poles with three phases of a three phase Wye supply (described in more detail below) that shared a common neutral. Everything would be safe, and easy, and would work reliably for a long time.
Alas, modern switching power supplies have a power factor that isn't too close to 1. In addition, there are details of the way that they work that make a huge difference in the number of systems you can safely plug into a single circuit, the longevity of the nodes themslves, the safety of the supply power lines, and more. For that reason, it is time for a fairly detailed excursion into Your Friend, Electricity.