PDU vs RPP: Selecting White-Space Power Distribution for Data Centers
The role of white-space distribution
Between the UPS output switchboard and the rack, power passes through floor-level distribution: a power distribution unit (PDU), a remote power panel (RPP), busway, or a combination. The choice affects floor space, branch-circuit length, monitoring and how easily racks can be added.
PDU and RPP compared
| Feature | Floor PDU | RPP |
|---|---|---|
| Transformer | Usually includes an isolation or step-down transformer | No transformer |
| Size | Larger footprint, heat from transformer losses | Compact, often at row end |
| Function | Feeds one or more RPPs or racks directly | Branch distribution close to racks |
| Typical use | Voltage change or a separately derived system | Shortening branch circuits from a PDU or UPS board |
| Monitoring | Input and branch metering | Branch circuit monitoring |
In many Indian facilities the UPS output is already at 415V/240V, so a transformer is not needed and RPPs fed directly from the UPS output switchboard are common. Where a separately derived neutral or isolation is required, a transformer-based PDU is used.
A/B feed design
Dual-corded IT equipment draws from two independent sources. Each rack receives an A feed and a B feed from separate RPPs or PDUs, ideally from separate UPS systems. Size each feed so that it can carry the full rack load if the other path fails, and keep per-path loading below 50% of the rated capacity under normal operation.
Branch circuit monitoring
Branch circuit monitoring measures current, power, energy and harmonic distortion per outgoing breaker. It allows operators to:
- detect overloaded circuits before a trip,
- verify that A and B feeds remain balanced,
- allocate energy to tenants in colocation facilities.
Agree CT accuracy, sampling, alarm thresholds and the communication protocol (Modbus RTU/TCP, SNMP via gateway) with the DCIM integrator. See metering architecture.
Protection and selectivity
Branch breakers must discriminate with upstream UPS output breakers, including in battery mode where fault current is limited. Check breaker curves against the UPS manufacturer's short-circuit capability. Surge protection at the UPS and distribution levels is covered in the surge protection design guide.
Selection checklist
- Rack count, kW per rack and growth plan
- Single- or three-phase rack supplies and connector types
- Whether a transformer or separately derived system is needed
- A/B source arrangement and maximum per-path loading
- Branch circuit monitoring scope and DCIM protocol
- Floor space, cable entry (top or bottom) and cooling layout
For the upstream architecture, read Tier III vs Tier IV electrical distribution. PowerPanelTech builds RPPs, PDU distribution sections and UPS switchboards; see data center power panels.
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