A power distribution unit (PDU) is a device that takes electrical power from a single circuit and distributes it to multiple pieces of equipment. The term most often refers to the rack PDU, a strip of outlets mounted inside a server rack or colocation cabinet that servers, storage and network devices plug into. Many rack PDUs also measure power draw and, in some models, let you switch individual outlets on and off remotely. In large data centers the same name is also used for floor-standing units that distribute power from the facility’s main supply to rows of racks.
At a glance
- Splits one incoming circuit into many outlets for the equipment in a rack.
- Comes in basic, metered, monitored and switched versions; features vary by manufacturer.
- Cabinets with redundant power usually have two PDUs, one per circuit, feeding dual-power-supply equipment.
- Metered PDUs are a common source of the power data used to manage rack power density.
What problem it solves
A colocation cabinet or server rack typically receives power on one or two circuits, but holds many devices, each with its own power cord. Something has to connect them safely, within the circuit’s rating, using the right plug and outlet types. That is the basic job of a PDU.
The more advanced versions solve two other problems. First, visibility: without metering you cannot easily tell how close a cabinet is to its power limit, which risks tripped circuits or paying for unused capacity. Second, remote control: when a device hangs and will not respond over the network, a switched PDU lets you power-cycle it remotely instead of waiting for remote hands or a site visit.
How it works
Input and output. A rack PDU has an input cord with a plug matched to the circuit the facility delivers, such as a particular voltage, amperage and single- or three-phase supply. It provides a set of outlets, commonly the connector types used by server and network power supplies. In cabinets, many are mounted vertically at the back, taking up no rack units, though horizontal models that occupy one or two rack units also exist.
Feature levels. Basic PDUs distribute power only. Metered PDUs show total current or power on a local display. Monitored PDUs report usage over the network, often per outlet, and can send alerts as load approaches a threshold. Switched PDUs add remote on, off and reboot control for individual outlets. Manufacturers use different names for these levels, so compare features rather than labels.
Redundancy. In a cabinet with two power feeds from separate paths, typically backed by an uninterruptible power supply (UPS) upstream, each feed connects to its own PDU. Dual-corded equipment plugs into both, and the load is planned so either PDU can carry it alone. If the combined load exceeds what one circuit can carry, losing one feed can overload the other.
Management. Network-connected PDUs are usually managed through a web interface, management protocols or a data center infrastructure management tool. Because remote power control is powerful, access to it should sit on a management network, often alongside out-of-band management, with proper authentication.
For help specifying power for a cabinet, see our Colocation solution page.
When it matters for buyers
- Moving into colocation. Confirm whether PDUs are included and that their plugs, outlets and ratings match your equipment.
- Adding dense equipment. Higher-draw servers may need PDUs and circuits with higher ratings or three-phase power.
- Running sites with few or no staff. Switched PDUs reduce the need for site visits to reboot hung equipment.
- Tracking power costs. Per-outlet metering shows which systems use the most power and whether committed power is being used.
Questions to ask vendors
- Are PDUs included with the cabinet, available to rent, or bring-your-own?
- What circuits are delivered (voltage, amperage, phase, plug type), and what PDU ratings match them?
- Are PDU power readings available to us through a portal or network access, and how often are they updated?
- Can we use switched PDUs, and how is remote access to them secured?
- How is load balanced across our two feeds so the loss of one does not overload the other?
How it differs from a UPS
A PDU distributes power; an uninterruptible power supply (UPS) protects it. The UPS sits upstream and keeps power flowing from stored energy when the utility supply fails or degrades. The PDU sits in the rack and splits whatever power it receives among the connected devices. A PDU does not provide backup power by itself, so equipment plugged into it is protected only if the circuit feeding it is UPS-backed.