What is a PDU? Rack power distribution units: role, types and specs

What is a PDU? It depends on the context. In a data center or server room, PDU stands for power distribution unit, usually meaning the rack PDU (also written rPDU): a strip mounted in the rack that is the last stage of the power chain. One branch circuit comes in from upstream and the PDU splits it into anything from a handful to several dozen outlets that feed the power supplies of servers, switches and storage. It also provides overcurrent protection at that stage, and intelligent models meter current and energy and can switch individual outlets on and off remotely. In networking and telecommunications, PDU stands for protocol data unit, the unit of data each protocol layer passes along; the two share nothing but the abbreviation. This article covers the rack PDU first and then lists the other meanings.

Where a PDU sits in the data center power chain

Power passes through several stages between the utility and a server’s power supply:

Utility supply / generator
  → Switchgear (with an automatic transfer switch, ATS)
  → UPS
  → UPS output distribution
  → Floor-standing PDU, remote power panel (RPP) or busway tap-off box
  → Rack PDU (the PDU this article is about)
  → Power supplies in servers and switches

A rack PDU’s input is one branch circuit, protected by a breaker in the floor PDU, the RPP or a busway tap-off box. A dual-feed rack has two PDUs, one on a branch from the A feed and one from the B feed. So how much equipment a PDU can carry depends first on the size of the circuit feeding it.

This is also where the term gets confusing. In data center design documents, “PDU” on its own often means the floor-standing PDU: a cabinet, usually with a transformer, that takes UPS output and splits it into dozens of branch circuits, rated from tens to hundreds of kVA. The strip in the rack is then called a rack PDU or rPDU to tell them apart. When you read an equipment list or a design that mentions a “150 kVA PDU”, it is the floor unit, not the strip.

A rack PDU does four jobs:

1. Distribution: splits one circuit into many outlets, one cord per device;
2. Protection: a breaker for the whole PDU or for each outlet bank, so an overload on one bank trips only that bank;
3. Metering: shows or reports current, power and energy, the data behind rack power management and billing by kWh;
4. Control: a switched PDU can turn individual outlets off and on remotely, or stagger them at power-up.

A basic PDU does only the first two, a metered PDU adds the third, and only intelligent models do all four.

Basic, metered and intelligent PDUs: what is the difference?

Vendors name their ranges differently, so the clearest way to classify them is by what you can see and what you can control:

Type	Common name	Local display	Remote readings	Metering level	Remote outlet switching	Typical use
Basic	Basic	None, at most indicator lights	No	None	No	Racks with a steady load where nobody needs to see the current
Metered	Metered	Yes, current for the whole PDU or each phase	No	Whole PDU, phase or bank	No	Checking the headroom on site before racking
Monitored	Monitored, or metered with a network port	Yes	Yes, web interface and SNMP	Whole PDU, phase or bank	No	Colocation racks that need continuous data or billing by kWh
Outlet-metered	Outlet Metered	Yes	Yes	Each outlet	No	Shared racks where each device’s consumption must be known
Switched	Switched	Yes	Yes	Whole PDU or bank	Yes	Unstaffed sites and devices that need a remote power cycle
Switched with outlet metering	Switched with Outlet Metering	Yes	Yes	Each outlet	Yes	Both of the above

The last four are usually grouped together as intelligent, smart or managed PDUs. A few notes:

• What an intelligent PDU reports: typically voltage, current, real power, apparent power, power factor and cumulative energy (kWh). Some models take external sensors such as temperature, humidity and door contacts, and send alerts by SNMP trap or email when a threshold is crossed; check the product datasheet.
• Metering accuracy: if the readings are used to bill customers by kWh, check the accuracy class stated in the datasheet, and compare the readings with the facility’s main meter and branch circuit monitoring to confirm the difference is acceptable.
• Why switching matters: devices without a BMC, such as older servers, some switches and PCs, can be recovered from a hang by cutting and restoring power remotely. Staggered outlet power-up spreads the inrush when a whole rack is energized, and switching off unused outlets stops unauthorized equipment being plugged in. A remote power-off cuts the device’s power outright, so confirm which machine is on the outlet before you do it.

How to read a PDU spec

A PDU spec usually comes as one long line, for example:

0U vertical, single-phase 32 A input, IEC 60309 plug (2P+E), 3 m input cord; outputs 20 × C13 + 4 × C19 in 2 banks, each protected by a 16 A breaker; metered per PDU, with network port.

A North American equivalent might read: 0U vertical, 208 V single-phase, NEMA L6-30P plug, 24 A maximum; 20 × C13 + 4 × C19 in 2 banks, each with a 20 A breaker. Field by field:

Field	Meaning	What to check
0U vertical	Mounts at the back of the rack beside the rails and uses no rack units	Whether the rack’s depth and side space can take it; if not, use a 1U horizontal model
Single-phase 32 A input	The most the PDU can draw from upstream	The upstream branch breaker must not be larger than the PDU’s input rating
IEC 60309 plug (2P+E)	The input connector; three pins for single-phase	Whether the facility provides industrial receptacles, local wall outlets or terminal connections
3 m input cord	Reach from the PDU to the upstream receptacle	Overhead or underfloor feed, and whether the cord reaches without a big coil
20 × C13 + 4 × C19	Outlet types and counts	C13 for typical servers and switches, C19 for high-wattage power supplies
2 banks, 16 A breaker each	Outlet grouping and per-bank protection	Keep each bank under 80% of its breaker rating
Metered per PDU, network port	Metering granularity and communication	Whether your management system can collect it over SNMP or a similar protocol

The input rating decides how much equipment one PDU can carry. Planning at 80% is required for continuous loads under the US National Electrical Code and is common practice elsewhere; that is why North American PDUs on a 30 A plug are labeled 24 A:

Input	Calculation	Apparent power	At 80%
Single-phase 16 A, 230 V	230 V × 16 A	3.68 kVA	2.94 kVA
Single-phase 32 A, 230 V	230 V × 32 A	7.36 kVA	5.89 kVA
Three-phase 16 A, 400 V	3 × 230 V × 16 A	11.04 kVA	8.83 kVA
Three-phase 32 A, 400 V	3 × 230 V × 32 A	22.08 kVA	17.7 kVA
Single-phase 20 A, 120 V	120 V × 20 A	2.4 kVA	1.92 kVA
Single-phase 30 A, 208 V	208 V × 30 A	6.24 kVA	4.99 kVA
Three-phase 30 A, 208 V	1.732 × 208 V × 30 A	10.81 kVA	8.65 kVA
Three-phase 60 A, 208 V	1.732 × 208 V × 60 A	21.62 kVA	17.3 kVA

On a 400 V three-phase PDU each outlet bank is wired line to neutral and gets 230 V; on a North American 208 V three-phase PDU the outlets are usually wired line to line at 208 V. Either way, each bank sits on a particular phase, so spread the load evenly across the banks. In a dual-feed rack the two PDUs’ capacities do not add up: if one feed fails, the other must carry the whole rack, so usable capacity is that of one PDU.

Common input connectors are wall plugs on small single-phase models (NEMA 5-15P and 5-20P in North America, Schuko and other national plugs elsewhere); locking NEMA plugs such as L5-30P, L6-30P and L21-30P; IEC 60309 industrial connectors, with three pins (2P+E) for single-phase and five (3P+N+E) for three-phase wye, color-coded blue for 200–250 V and red for 380–480 V; and hardwired terminals on high-current or permanently installed units. Outlets are mostly IEC 60320 C13 and C19, rated 10 A and 16 A under IEC (15 A and 20 A under North American listings), and some PDUs add local outlets.

PDU vs UPS, ATS, RPP and power strip

Several pieces of power equipment in a data center have similar names and sit close together. Tell them apart by where they sit in the chain and whether they have a battery or a transfer switch:

Equipment	Position in the chain	Battery or transfer	Main job
UPS	After the switchgear, before the floor PDUs	Battery; keeps supplying power when the utility fails	Backup power and voltage and frequency regulation
Floor PDU, remote power panel (RPP)	After the UPS output, usually one per row or room section	Neither	Splits UPS output into branch circuits for each rack, with breakers and branch circuit monitoring
Rack-mount ATS	In the rack, with one input from the A feed and one from the B feed	Transfer switch, no battery	Two inputs, one output, so single-supply devices can use both feeds
Rack PDU	In the rack, the last stage	Neither	Distribution to each device, bank protection, metering and control
Power strip	Homes and offices	Neither	Temporary outlets, sized for a household circuit; not suitable inside a server rack

Some products combine functions, such as a PDU with a built-in ATS: two inputs, automatic transfer inside and one row of outlets, useful in racks with many single-supply devices.

Other meanings of PDU: protocol data unit and more

Searches for “PDU meaning” mix up several entirely different things:

Field	Full name	What it means
Data center power	Power distribution unit	The rack PDU described here, and the floor-standing PDU in design documents
Computer networking	Protocol data unit	The unit of data at each protocol layer: a frame at the data link layer, a packet at the network layer, a segment for TCP or a datagram for UDP at the transport layer
SNMP	Protocol data unit	The operation types in SNMP messages, such as GetRequest, GetNextRequest, GetBulkRequest, SetRequest, Response, SNMPv2-Trap and InformRequest
5G mobile networks	PDU session	The connection set up between a device and a data network
Electric vehicles	Power distribution unit	The high-voltage junction box that distributes traction battery power to the motor controller, air conditioning compressor and other components

In a data center two of these meet: an intelligent PDU is usually monitored over SNMP, and every GetRequest the management system sends is itself an SNMP PDU. Before connecting a PDU to monitoring, confirm that SNMP works with the net-snmp tools:

# Read the PDU's system description and uptime to confirm SNMP is reachable (use your actual community string)
snmpget -v2c -c public 10.30.0.21 SNMPv2-MIB::sysDescr.0 SNMPv2-MIB::sysUpTime.0

# Walk the vendor's private MIB subtree to find the OIDs for current, power and energy
snmpwalk -v2c -c public 10.30.0.21 1.3.6.1.4.1

The OIDs for current, power and other readings are defined in the vendor’s MIB files, so follow the vendor documentation. Keep the PDU’s management port on the management network only, change the default community string and web password, and use SNMPv3 where the PDU supports it.

Four questions before you choose a rack PDU

1. What circuit does the facility provide to this rack? Single- or three-phase, the voltage, 16 A or 32 A (20 A, 30 A or 60 A in North America), receptacle or terminals: this decides the PDU’s input rating and connector.
2. What power inlets do the devices in the rack have? C14 inlets need C13 outlets, C20 inlets need C19 outlets; count them and leave spares.
3. Do you need to read current remotely or bill by kWh? If so, choose a monitored or outlet-metered PDU and confirm that your management system can collect its data.
4. Do any devices need a remote power cycle? If there is older equipment without a BMC, or the site is unstaffed, choose a switched PDU.

Mounting space decides between 0U vertical and 1U horizontal, and can be checked last.

Making PDU data useful

An intelligent PDU earns its keep when its data is collected continuously, not when someone glances at its display on a walk through the room. Toplink DCIM brings the intelligent PDUs in the racks under one system, meters their power live and summarizes power use per data center on its dashboard. Which devices sit in which rack and rack unit is recorded in rack and asset management, so the admin panel shows what each rack holds and how much headroom the room has left against its power capacity. PDUs cover the rack level; UPS, cooling and other facility data belong to facility monitoring, and how the two kinds of system divide the work is explained in what is DCIM.