Power Distribution Units for Server Cabinets

What is a PDU?

A Power Distribution Unit (PDU) is a device designed to distribute reliable network power to multiple devices within a rack or cabinet. It's essentially a sophisticated, heavy-duty power strip built for the high-demand, critical environment of a data center or server room.

A basic PDU takes a single input from a power source (like a wall outlet or a UPS) and provides multiple outlets for equipment like servers, switches, and storage arrays.

Types of PDUs for Cabinets

Choosing the right type is the first and most critical step.

1. Basic PDU

• Function: Provides simple power distribution without any monitoring or control features.

• Features: Multiple outlets, sometimes with different amperage ratings (e.g., NEMA 5-15R, IEC C13/C19).

• Best For: Non-critical environments, small network cabinets, or where budget is the primary constraint. They are often the most affordable option.

2. Metered PDU (or Monitored PDU)

• Function: Provides real-time monitoring of power parameters via a physical display (often an LCD or LED screen) on the unit itself.

• Features: Displays key metrics like amperage (A), voltage (V), frequency (Hz), and power utilization (kW/kVA). This allows you to see how much power your rack is drawing, helping to prevent circuit overloads.

• Best For: Environments where capacity planning and avoiding overloads are important. It provides visibility without remote access.

3. Switched PDU (or Smart PDU)

• Function: Provides both monitoring and remote control capabilities. This is the most feature-rich common type.

• Features: Includes all metered features but adds remote management via a network connection (LAN/IP). You can:

• Remotely power cycle individual outlets to reboot locked-up equipment.

• Sequentially power on/off outlets (staggered startup to prevent inrush current overloads).

• Set up alerts and notifications for power events (e.g., high current threshold).

• Best For: Remote data centers, lights-out facilities, or any environment where IT staff need to manage power remotely for efficiency and quick problem resolution.

4. Automatic Transfer Switch (ATS) PDU

• Function: Provides redundancy by accepting power from two separate sources (e.g., two different UPS units or utility lines).

• Features: If the primary power source fails, the ATS PDU automatically and seamlessly switches the connected equipment to the secondary source without interruption.

• Best For: High-availability environments where maximum uptime is critical, such as for core network switches or mission-critical servers.

5. Hot Swap PDU

• Function: Similar to an ATS PDU but allows you to manually switch the input power source without shutting down connected equipment. This is useful for maintenance.

• Features: A front-panel switch or control to change between two power inputs.

• Best For: Maintenance scenarios where you need to work on one power feed (e.g., a UPS) while keeping equipment running on the other feed.

Key Selection Criteria

When choosing a PDU, consider these factors:

1. Power Input:

• Plug Type: What are you plugging the PDU into? Common types include NEMA 5-15P (standard wall plug), NEMA L5-20P (120V twist-lock), NEMA L6-30P (208V twist-lock), or IEC 60320 (common in data centers).

• Voltage & Phase: 120V (common in North America) vs. 208V/230V (more efficient for data centers). Single-phase vs. Three-phase (for high-density racks).

• Amperage: Match the PDU's amp rating to your circuit breaker (e.g., 15A, 20A, 30A, 50A). Do not exceed the circuit's capacity.

2. Outlet Types & Number:

• IEC C13: The standard "kettle plug" for servers and network gear.

• IEC C19: A higher-amperage plug for larger equipment like UPS units or powerful servers.

• Quantity: How many devices do you need to power? Plan for future expansion.

• Type: Ensure the outlets match your equipment power cords. The most common are:

• Outlet Layout: Vertical PDUs mount on the sides of the rack, saving space. Horizontal PDUs mount in 1U/2U of space like a server.

3. Form Factor & Mounting:

• Vertical (0U): Mounts vertically on the left or right side posts of the rack. Ideal for maximizing space for equipment.

• Horizontal (1U, 2U): Mounts in the rack like a standard server or switch. Necessary if your rack doesn't have vertical mounting space.

4. Management Features:

• Do you need basic power, just monitoring (Metered), or full remote control (Switched)?

• Consider the management interface: Web interface, SNMP, Telnet/SSH, or integration with data center infrastructure management (DCIM) software.

5. Additional Features:

• Power Monitoring: Accuracy of the current (amp) and power (kW) readings.

• Environmental Monitoring Ports: Some advanced PDUs have ports to connect sensors for temperature and humidity.

• Breakers: Individual outlet breakers can provide extra protection.

• UL Certification: Ensure the PDU is certified by a recognized safety laboratory like UL.

Typical Setup in a Cabinet

In a standard cabinet setup for redundancy, you would have two PDUs:

• PDU A connected to UPS A / Power Source A.

• PDU B connected to UPS B / Power Source B.

Dual-corded equipment (like most servers) would then plug one cord into each PDU. This creates an A-B power path for full redundancy. If one power path fails, the equipment continues to run on the other.

Summary Table

Recommendation: For most modern server cabinets, starting with at least Metered PDUs is advisable for basic capacity planning. Switched PDUs are worth the investment for any cabinet that requires remote management or is in a location without always-on IT staff.