What Is a PDU? Rack PDU Types, C13–C19 Outlets and Rack Power Sizing
What a PDU (power distribution unit) is and how it differs from a power strip; basic, metered and switched PDUs, C13–C14 and C19–C20 connectors and 16 A / 32 A rack power sizing.
A PDU (power distribution unit) is a multi-outlet unit mounted in a 19-inch rack that distributes one supply line to servers and network equipment. It differs from a household power strip in its rack mounting, continuous full-load design, IEC C13–C19 outlets and, depending on model, current metering and remote switching.
> Related Category: Browse models by outlet type and current in PDU Power Distribution Units, and UPS Uninterruptible Power Supplies for the supply side.
1. PDU types
| Type | Feature | Use |
|---|---|---|
| Basic PDU | Distributes power only; may have a breaker or switch | Small network racks, office comms rooms |
| Metered PDU | Local display shows total current (A) | Load balancing, overload watch |
| Monitored PDU | Current, voltage and energy read over the network | Data centres, energy reporting |
| Switched PDU | Each outlet switched remotely | Remote sites, rebooting a hung device |
| PDU with ATS | Transfers automatically between two sources | Dual feed for single-supply devices |
By mounting: a horizontal (1U–2U) PDU fits the rack rails with 6–12 outlets; a vertical (0U) PDU sits in the rear side space, uses no rack units and offers up to 20–40 outlets.
2. Input and output connectors
| Connector | Rated current | Where used |
|---|---|---|
| Earthed household socket | 16 A | Modems, adapters, office devices |
| IEC C13 outlet / C14 plug | 10 A | Servers, switches, storage; the most common outlet |
| IEC C19 outlet / C20 plug | 16 A | High-power servers, blade chassis, large UPS units |
| CEE (IEC 60309) plug, single-phase 16 A / 32 A | 16 A / 32 A | PDU input lead, latching connection |
| CEE plug, three-phase 16 A / 32 A | 3 × 16 A / 3 × 32 A | High-density racks |
- A C14 plug fits a C13 outlet and a C20 plug fits a C19 outlet; a C14 plug does not fit C19.
- Locking C13 outlets stop plugs working loose under vibration or cable strain.
- The PDU input plug must match the site socket; for CEE plugs see IEC 60309 CEE Industrial Plugs and Sockets.
3. Rack power sizing
The power a PDU can carry is limited by input current and voltage:
| PDU input | Maximum power | Recommended continuous load (80%) |
|---|---|---|
| Single-phase 16 A, 230 V | 3.7 kW | ≈ 2.9 kW |
| Single-phase 32 A, 230 V | 7.4 kW | ≈ 5.9 kW |
| Three-phase 16 A, 400 V | 11 kW | ≈ 8.8 kW |
| Three-phase 32 A, 400 V | 22 kW | ≈ 17.7 kW |
Steps:
- Add measured or manufacturer-stated typical consumption, not nameplate ratings; a power supply label is often twice real consumption.
- Compare the total with the 80% limit of the PDU.
- Leave room to grow; provide at least 20% more outlets than devices.
Example: 8 servers × 350 W + 2 switches × 150 W = 3,100 W. That exceeds the 80% limit of one 16 A PDU (2.9 kW); use a 32 A PDU or two separate 16 A lines.
- On a three-phase PDU the outlets are grouped by phase; spread devices evenly across phases.
- Do not connect a device drawing more than 10 A to a C13 outlet; use C19.
4. A+B dual feeds
For dual-supply devices fit two PDUs per rack: feed A from one UPS and feed B from a second UPS or a separate line. One power supply of each device goes to A, the other to B.
Rule: in normal operation each PDU should stay below 40% of its rating. If one feed fails the whole load moves to the other and remains under 80%. With both PDUs at 70%, the surviving feed trips and the whole rack goes down.
For UPS sizing see Online vs Line-Interactive UPS, kVA to kW and Battery Runtime.
5. Selection notes
- Input lead and plug: 2–3 m suits most racks; the plug must match the site socket.
- Protection: an internal breaker or thermal cut-out opens only the PDU on overload. Above 16 A, outlet groups have their own breakers.
- Surge protection: some models include an SPD module; it is not needed on a UPS output.
- Earthing: bond the PDU body to the rack earth bar.
- Colour and labels: different colours for A and B PDUs or leads prevent pulling the wrong plug during maintenance.
Frequently asked questions
What does PDU mean and what does it do?
Power distribution unit. It distributes the single supply entering a rack to servers, switches and other devices; advanced models meter consumption and switch outlets remotely.
What is the difference between a PDU and a power strip?
A PDU mounts in a 19-inch rack, is built for continuous full load and offers IEC C13–C19 outlets. A power strip is for home and office use with no rack mounting or metering.
What is the difference between C13 and C19?
A C13 outlet carries 10 A and a C19 outlet 16 A, and the plug shapes differ. Standard servers and switches use C13; high-power servers and blade chassis use C19.
How many servers fit on a 16 A PDU?
The continuous limit is about 2.9 kW. At 350 W typical per server, 8 servers sit at the limit; 6–7 is safe.
What is the difference between a metered and a switched PDU?
A metered PDU only shows current. A switched PDU also turns each outlet on and off over the network, so a remote device can be rebooted without a site visit.