# Medium-Voltage Distribution Between the substation and the rack lies a distribution network running at thousands of volts. Medium voltage is the compromise that makes a large campus practical: high enough that the conductors stay a manageable size across long internal distances, low enough that the equipment can live inside buildings and be maintained by site electricians. > [!note] The one-sentence version > Carry the power across the campus at a few thousand volts so the copper stays small, then step it down at the last possible moment. ## Why it exists Current determines conductor size, and conductor size determines cost, weight, and the labor of installation. A hundred megawatts delivered at four hundred volts would require conductors of absurd dimension and copper losses to match. Delivered at thirteen thousand volts, the same power travels in cables a person can carry. The engineering question on every campus is therefore where to convert, and the answer has been steadily moving: the step-down point keeps migrating closer to the load as densities rise. ## Prerequisites - **Medium voltage:** by convention, roughly one thousand to thirty-five thousand volts. - **Switchgear:** the assembly of breakers, protection, and metering that sections the network; see [[wiki/Switchgear|Switchgear]]. - **Feeder:** a circuit running from a switchgear position to a downstream load or transformer. - **Radial, loop, and selective designs:** a radial feeder serves loads from one direction; a loop can be fed from either end; a primary-selective arrangement gives each transformer two sources. Reliability rises with each step, and so does equipment count. - **Arc flash:** the hazard of an energized fault; it governs clearances, protective settings, and the equipment a technician wears. Arc-resistant switchgear and remote racking exist to keep people out of the hazard zone entirely. - **Unit substation:** a packaged medium-voltage breaker, transformer, and low-voltage switchboard, installed as one assembly near the load. ## How it works The campus substation's medium-voltage bus feeds out through switchgear to feeders running in underground ductbanks or overhead cable tray to each data hall. At the hall, a unit substation or a transformer serving an uninterruptible power supply steps the voltage down for distribution to the rack. Protection is coordinated so that a fault on one feeder is cleared by that feeder's breaker, leaving everything else running; see [[wiki/Failure Domain|Failure Domain]]. Each layer is normally arranged so that any single element can be removed from service for maintenance without interrupting the load, which is what makes concurrent maintainability possible. ## The migration toward the rack As rack power has climbed from a few kilowatts to well past a hundred, the economics of the last conversion have shifted. Designs now deliver medium voltage deeper into the building, place transformers and rectifiers adjacent to or inside the rack row, and in the most recent rack-scale systems distribute direct current along a rack-height busbar; see [[wiki/Busbar Busway|Busbar Busway]] and [[wiki/High-Voltage Direct Current Distribution|High-Voltage Direct Current Distribution]]. Each step removes a conversion stage, and every conversion stage removed is both efficiency recovered and equipment that no longer has to be bought, cooled, and maintained. ## Stack position - **Upstream:** [[wiki/Electrical Substation|Electrical Substation]] and [[wiki/Power Transformer|Power Transformer]]. - **Downstream:** [[wiki/Uninterruptible Power Supply|uninterruptible power]], [[wiki/Rack Power Distribution Unit|rack power distribution]], and the servers. - **Function and vendor:** the function is standard power engineering with many qualified suppliers; the design choices are about reliability class and how close to the load the conversion happens. - **Degraded case:** conversion too early means heavy copper and avoidable losses; conversion without redundancy means a single breaker becomes a single point of failure for a whole hall. ## Lineage and trajectory Campus distribution practice came out of industrial plants and hospitals, where continuity mattered before computing existed. The data center inherited those arrangements and then pushed them, first toward higher reliability through redundant feeds, then toward higher density through conversion closer to the load. The trajectory, as of October 2026, runs toward fewer conversion stages, direct-current distribution inside the rack, and solid-state protective devices that interrupt faults faster than mechanical breakers can. ## Relationships - **fed by:** [[wiki/Electrical Substation|Electrical Substation]] - **built from:** [[wiki/Switchgear|Switchgear]] · [[wiki/Power Transformer|Power Transformer]] - **feeds:** [[wiki/Uninterruptible Power Supply|Uninterruptible Power Supply]] · [[wiki/Busbar Busway|Busbar Busway]] · [[wiki/Rack Power Distribution Unit|Rack Power Distribution Unit]] - **configured for:** [[wiki/Redundant Power Feeds|Redundant Power Feeds]] · [[wiki/Failure Domain|Failure Domain]] - **trending toward:** [[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:** the medium-voltage range by convention; radial, loop, and primary-selective topologies and their reliability ordering; arc-flash hazard management through arc-resistant equipment and remote operation; unit substations; protective coordination; the relationship between delivery voltage and conductor size. - **Established:** rack power densities past one hundred kilowatts in current rack-scale systems, driving conversion closer to the load. - **Plausible:** solid-state protective devices displacing mechanical breakers in data center distribution; products exist and broad adoption remains to be shown. ## Sources - Electrical engineering standards practice for medium-voltage distribution, protection, and arc-flash safety, as of October 2026. - Public rack-scale system documentation on in-rack power distribution and density, as of October 2026.