Power infrastructure
A rack PDU (power distribution unit) is the last stage of the power path: it takes one feed and splits it to the equipment in a rack. There are four types, basic, metered, monitored and switched, and they differ in what they let you see and control. Upstream, a floor PDU takes the UPS output and turns it into the branch circuits that feed the racks.
It distributes power and nothing else. Vertiv describes basic rack PDUs as entry-level, low-cost units that provide reliable power distribution to the equipment in a rack or cabinet, in a range of electrical and receptacle configurations. There is no display and no network connection, so the only way to know the load on a basic PDU is to measure it upstream or with a clamp meter. That is fine for a small, stable rack; it is a blind spot in a room where loads change often.
A local display of the power drawn. Vertiv notes that metered PDUs let you see power consumption at the unit, which is useful when equipment is installed or moved, to keep a circuit from being overloaded. Because the reading is local only, Vertiv recommends metered units for highly secure rooms that must keep the power infrastructure air-gapped from the network. Someone still has to walk to the rack to read it.
Remote visibility, and for switched units, remote control of each outlet. A monitored PDU reports power use over the network, at the unit level or outlet by outlet, and raises alarms when a user-defined threshold is crossed; Vertiv recommends them for high-density rooms that want to monitor or improve power usage effectiveness. A switched PDU adds the ability to turn each outlet on or off, or reboot it, from a remote console. That matters for sites with nobody on the floor: a hung server can be power-cycled without a visit. Vertiv also notes that smart PDUs can schedule power cycling of unused equipment to save energy, and that some keep reporting through an out-of-band serial or KVM link if the main network fails.
| Type | What you see | What you control | Best fit |
|---|---|---|---|
| Basic | Nothing at the unit | Nothing | Small, stable racks |
| Metered | Load on a local display | Nothing | Secure rooms kept off the network |
| Monitored | Load remotely, by unit or outlet, with alarms | Nothing | Dense rooms tracking capacity and efficiency |
| Switched | Load remotely, by unit or outlet | Each outlet on, off or reboot | Rooms with no one on site |
Mostly IEC C13 and C19 outlets, on supplies from single-phase to three-phase. Vertiv lists rack PDUs from 100V single-phase to 240/415V three-phase, at current ratings from 10A to 120A, and names IEC320 C13 and C19 as the most common outlet standard worldwide, rated for output voltages up to 250V. Two details prevent common outages. Locking or retention outlets keep a cord from being pulled out by accident, which would drop the load. Colour-coded alternating outlets make it easier to spread the load across circuits and phases; Vertiv warns that unbalanced loads risk overloading one circuit while capacity is stranded on another.
Vertical, in most racks, because it does not use equipment space. Vertiv notes that horizontal PDUs sit inside the rack and typically take 1U or 2U with 8-16 outlets, while vertical PDUs mount at the back or side of the rack and can hold up to 54 outlets. Horizontal units still suit short racks and wall-mount cabinets. In either case, the choice follows how many cords the rack needs and where the cords can be routed without blocking airflow.
A cabinet on the data-centre floor that takes the UPS output and distributes it to many racks as branch circuits. The Vertiv Liebert TFX, for example, is a floor-mounted PDU offering single- or two-stage distribution in small to mid-sized data centres and server rooms, with up to three 42-pole panelboards, a transformer, and power monitoring. Its isolated panelboards let monitoring components be serviced while the rest of the unit stays live, which is the kind of detail that decides how much of a room goes down for maintenance. The floor PDU is also where the UPS output is converted to the voltage the racks need and where each rack circuit gets its breaker.
By giving each cord of a dual-corded server a separate path back to a separate UPS. The Uptime Institute describes Tier III and Tier IV sites as having dual power paths for concurrent maintainability; in the rack, that means an A-side and a B-side PDU, each fed from a different floor PDU and UPS. Either side can then be serviced or fail without the server losing power, provided neither side is loaded beyond what it can carry alone. Single-corded equipment cannot use two paths by itself; it needs a transfer switch, covered in ATS and STS. The UPS redundancy guide explains the UPS side of the same design.
Mostly checks, not repairs. A rack PDU has few serviceable parts, so maintenance means confirming loads against the circuit rating, checking that cords are seated and locked, reviewing alarms and thresholds on monitored units, and keeping the firmware of networked units current. A floor PDU is closer to switchgear: its breakers, transformer and connections are inspected and scanned like any distribution equipment, usually in the same visit as the data-centre UPS maintenance it sits behind. Battery strings on the UPS side are a separate item; batteries for data-centre UPS covers sizes and formats.
A metered PDU shows the load on a local display only; a monitored PDU reports it over the network, by unit or by outlet, with alarms.
Turning individual outlets on, off or rebooting them from a remote console, for example to restart a hung server without a site visit.
Vertiv puts horizontal units at 8-16 outlets in 1U or 2U, and vertical units at up to 54 outlets.
A rack PDU splits one feed to the equipment in a rack; a floor PDU takes the UPS output and distributes it to many racks as branch circuits, often with a transformer.
For dual-corded equipment on a redundant design, yes: an A-side and a B-side, each on a separate path back to a separate UPS.
Sources, checked 2026-10-04:
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