Alternatives for providing electrical power to high density racks in data centers and network rooms are explained and compared. Issues addressed include quantity of feeds, single-phase vs. three-phase, number and location of circuit breakers, overload, selection of plug types, selection of voltage, redundancy, and loss of redundancy. The need for the rack power system to adapt to changing requirements is identified and quantified. Guidelines are defined for rack power systems that can reliably deliver power to high density loads while adapting to changing needs.