Proper Sizing for PDU's/Power in Rack

I use the HP Power Advisor and the APC UPS Selector tool to provide a good estimate of power requirements. The APC tool has a great database of common telco/server equipment, and provides a way to build in some headroom for expansion.

The way I tackle this problem is to ignore the nameplate information and actually measure the current draw of each piece of equipment. There a are a number of ways to do this but the simplest solution for me is to use a power lead which has been modified so that one of the conductors forms an accessible loop of wire outside the cable. A clamp meter is then used on that wire. Measurements are taken both at startup and while running, under both normal and heavy loads. With this information I can then work out realistic power requirements. If you're unable/unwilling to do this yourself talk to an electrician about it.