How many PDUs per rack?

One connects to power feed A and the other to feed B, so the rack stays on if one circuit fails. Small racks with only a few devices can run on one PDU.

Quick guide

• Small office rack, non-critical gear: 1 PDU.
• Standard server rack with dual-power-supply servers: 2 PDUs (A/B).
• High-density racks above 10 kW: 4 PDUs, or 2 three-phase PDUs.

Step 1: Add up the load

List the maximum watts for each device. Add them, then add 20 percent for growth. Example: 4,000 W of equipment plus growth equals about 4,800 W.

Step 2: Apply the 80 percent rule

Electrical codes in the US limit continuous load to 80 percent of a circuit's rating. A 208 V, 30 A single-phase circuit supplies about 6,240 VA, so plan for roughly 4,990 VA of continuous load per PDU. With A/B redundancy, each PDU should be able to carry the whole rack alone if the other fails, so split the load with that in mind.

Step 3: Count outlets

Count the plugs you need. Servers usually use C13 outlets. Larger devices may need C19. Add spare outlets for future gear.

Step 4: Pick the type

• Basic: distributes power only.
• Metered: shows load on a display.
• Switched: lets you turn outlets on and off remotely.
• Vertical (0U): mounts on the rear rail and saves rack space.

Rule of thumb: buy two PDUs sized so either one can carry the full load.