Power
Single-Phase Power
The standard electrical supply for most residential and small commercial environments — 120V or 240V delivered on a single alternating current waveform.
In North America, the standard utility supply is single-phase at 120/240V split-phase: two hot conductors 180° out of phase with each other and a neutral. Devices that need 120V connect between one hot and neutral. Devices that need 240V (dryers, HVAC, some PDUs) connect between the two hots.
For small IT environments — a handful of servers in a closet, a small NAS and switch — single-phase power is typically sufficient. Most standard UPS units, PDUs, and server power supplies accept single-phase input.
The limitation shows up as power density grows. On a 20A circuit at 120V, the maximum load is 2,400W (derated to ~1,920W at 80% for continuous loads). A rack of 1U servers with 750W power supplies can exceed a single circuit quickly, requiring multiple circuits per rack.
Single-phase power also creates an unbalanced load problem at scale — large single-phase draws create uneven loading across the three phases of the upstream supply, which utility companies and building engineers care about.