# Rack Power Distribution Unit At the end of the electrical chain is a vertical strip inside the rack with outlets down its length. It is the humblest piece of equipment in the building and the one that finally hands power to a machine. In a modern facility it is also an instrument: it measures what every outlet draws, reports it continuously, and can switch individual outlets on and off from a console. > [!note] The one-sentence version > The last few feet of the electrical system, with a meter on every outlet and a switch behind each one. ## Why it exists Power arriving at a rack must be divided among many machines, protected so one machine's fault affects only itself, and measured so that capacity can be managed. Dividing and protecting could be done with ordinary breakers; measuring at the outlet is what makes the modern unit worth its cost. Capacity in a data hall is allocated in kilowatts, and a planner who knows the actual draw of every machine can fill a rack to its real limit rather than to a conservative estimate. Measurement converts guesswork into usable capacity, and usable capacity is the scarcest commodity in the building. ## Prerequisites - **Basic, metered, and switched:** a basic unit distributes; a metered unit measures at the inlet or at each outlet; a switched unit can energize and de-energize outlets individually. - **Phase balance:** three-phase supply divided among outlets so that the three phases carry similar current; imbalance wastes capacity and stresses upstream equipment. - **Derating:** the convention of continuously loading a circuit to no more than eighty percent of its rating, which is why a rack's usable capacity is below its nameplate. - **Inlet and outlet standards:** the connector families that determine what plugs into what, a mundane detail that governs whether a given server can be installed at all. - **Out-of-band access:** the ability to command the unit over the management network, independent of the servers it feeds; see [[wiki/Baseboard Management Controller|Baseboard Management Controller]]. ## How it works Two units per rack, one for each power path, mount vertically at the rear corners and are fed from separate busways or branch circuits; see [[wiki/Redundant Power Feeds|Redundant Power Feeds]]. Each server's two power supplies connect to opposite units, so either can carry the machine alone. Internal current transformers measure each outlet, a controller aggregates and reports those measurements, and relays allow remote switching. Telemetry flows into the facility's monitoring systems, where it is used for capacity planning, billing, anomaly detection, and the unglamorous but frequent task of proving which outlet actually feeds a given machine. Remote power cycling of a single outlet, available from a console, is one of the most-used recovery tools in operations; see [[wiki/Site Reliability Engineering|Site Reliability Engineering]]. ## In rack-scale systems The highest-density racks change this picture. Where a rack is delivered as one integrated system drawing two hundred kilowatts, power enters through a busbar and a shelf of rectifiers rather than through outlet strips, and metering moves into the power shelf and the trays; see [[wiki/High-Voltage Direct Current Distribution|High-Voltage Direct Current Distribution]] and [[wiki/Busbar Busway|Busbar Busway]]. The function survives and relocates: something still divides, protects, and measures, and it is now part of the rack's own architecture rather than an accessory bolted into it. ## Stack position - **Upstream:** [[wiki/Busbar Busway|busway]] or branch circuits from the uninterruptible power system. - **Downstream:** servers, switches, and storage. - **Function and vendor:** a commodity with many qualified suppliers; the differentiators are measurement accuracy, management software, and connector choice. - **Degraded case:** without per-outlet measurement, racks are filled to estimates rather than to measured draw, and a meaningful share of purchased capacity goes unused. ## Lineage and trajectory The rack power strip began as a plain outlet bar, acquired metering as capacity became the binding constraint, acquired switching as remote operations became standard, and is now being absorbed into integrated rack architectures at the top of the density range. The trajectory follows density: higher currents, direct-current variants, finer measurement, and tighter integration with the thermal and compute telemetry that operations teams read together. ## Relationships - **fed by:** [[wiki/Busbar Busway|Busbar Busway]] · [[wiki/Uninterruptible Power Supply|Uninterruptible Power Supply]] - **feeds:** [[wiki/Compute Tray|Compute Tray]] · [[wiki/Data Center Rack|Data Center Rack]] - **reports to:** [[wiki/Observability|Observability]] · [[wiki/Site Reliability Engineering|Site Reliability Engineering]] - **configured for:** [[wiki/Redundant Power Feeds|Redundant Power Feeds]] - **superseded in dense racks by:** [[wiki/High-Voltage Direct Current Distribution|High-Voltage Direct Current Distribution]] - **source articles:** [[articles/Vertically Integrating an AI Superpower from AI Factory to Citizen|Vertically Integrating an AI Superpower from AI Factory to Citizen]] · [[articles/The Gigawatt Is the New Measure of War|The Gigawatt Is the New Measure of War]] - **collection route:** [[collections/American Infrastructure|American Infrastructure]] · [[collections/American|American Collection]] ## Evidence posture - **Established:** basic, metered, and switched unit classes; per-outlet measurement and remote switching; phase balancing; the eighty-percent continuous loading convention; dual-unit installation for A and B paths; integrated power shelves in rack-scale systems. - **Strongly indicated:** measured per-outlet telemetry materially increasing usable rack capacity against estimate-based planning; operator practice is consistent and published quantification is limited. ## Sources - Manufacturer technical literature on rack power distribution and metering, as of October 2026. - Open Compute Project rack power specifications, as reviewed October 2026.