What Is a Power Distribution Unit (PDU)?

A Power Distribution Unit (PDU) is a device that distributes electrical power to multiple pieces of equipment within a server rack or data center. While a simple power strip serves a similar function at the consumer level, enterprise PDUs are designed for far greater reliability, capacity, and often remote management capability.

Choosing the wrong PDU can result in blind spots in power monitoring, circuit overloads, or an inability to remotely recover locked-up equipment. Getting it right from the start is critical.

PDU Form Factors

Horizontal PDUs

These mount in standard rack unit spaces (typically 1U or 2U) at the top or bottom of a rack. They're straightforward and inexpensive but consume valuable U-space and often have a limited number of outlets.

Vertical PDUs

Vertical PDUs mount along the rear vertical rails of a rack and take up zero U-space. They typically offer 20–42 outlets and are the preferred choice in high-density environments. Most enterprise-grade PDUs are vertical.

The Four Types of PDUs

1. Basic PDUs

Basic PDUs distribute power and nothing more. They have no monitoring, no switching, and no remote management. They're appropriate for test labs, edge sites, or very small deployments where power management isn't a concern. Cost is their primary advantage.

2. Metered PDUs

Metered PDUs include an onboard ammeter or display that shows real-time current draw at the inlet. This lets technicians physically verify that a circuit is not being overloaded. A practical step up from basic PDUs, metered units are widely used in small-to-medium data centers.

3. Monitored PDUs

Monitored PDUs add network connectivity (typically via an embedded network card) so that power readings — voltage, amperage, kilowatts, and often individual outlet-level data — can be viewed remotely via SNMP, web interface, or integration into a DCIM platform. This is the most common type in mid-to-large enterprise data centers.

4. Switched PDUs

Switched PDUs take monitored functionality further by allowing individual outlets to be turned on, off, or rebooted remotely. This is invaluable for recovering hung servers, cycling power on network equipment, or enforcing power sequencing. The added cost is often justified in mission-critical environments.

Key Specifications to Evaluate

  • Input voltage and phase: Single-phase (120V/208V) or three-phase (208V/415V) depending on your electrical infrastructure
  • Amperage rating: Common ratings are 20A, 30A, and 60A; match to your circuit breaker and load
  • Outlet count and type: C13, C19, NEMA 5-20R — verify compatibility with your equipment power cords
  • Outlet-level metering: Critical for chargeback in multi-tenant environments
  • Redundant feeds: Dual-corded PDUs support A+B feed redundancy for uptime
  • Environmental monitoring: Some PDUs support temperature and humidity sensor ports

PDU Redundancy: A/B Feed Configuration

Most enterprise servers have dual power supplies. Best practice is to connect PSU-A to a PDU on Circuit A, and PSU-B to a separate PDU on Circuit B, fed from a different UPS or upstream breaker. This ensures that a single PDU failure or circuit fault does not take down the server. Never connect both power supplies of a critical server to the same PDU.

Sizing Your PDU Correctly

The golden rule: never load a circuit above 80% of its rated capacity (the National Electrical Code 80% rule). For a 30A circuit, your maximum continuous load should not exceed 24A. Add up the nameplate power draw of all equipment on a PDU and stay well within this limit, accounting for peak load spikes.

Summary: Which PDU Type Do You Need?

Environment Recommended PDU Type
Home lab / test bench Basic or Metered
Small business server room Metered or Monitored
Enterprise data center Monitored or Switched
Colocation / multi-tenant Switched with outlet-level metering