Electrical Distribution Planning Guide

Introduction

Electrical distribution is the backbone of any data center facility. It determines the facility's capacity, reliability, and efficiency. This guide covers the complete power path from the utility service entrance through medium-voltage distribution, transformation, UPS conditioning, and low-voltage distribution to the rack-level PDU.

Medium-Voltage Service Entrance

The medium-voltage service entrance is the point where utility power enters the facility. Design considerations include the utility voltage level (typically 12.47kV, 13.8kV, or 34.5kV in North America), service configuration (radial, loop, or dual-feed), and the metering and protection arrangement required by the serving utility.

For facilities requiring high reliability, dual utility feeds from independent substations or feeders provide the first level of redundancy. The switchgear configuration must support automatic transfer between feeds with defined transfer time requirements.

Transformation Strategy

Transformers step voltage from the medium-voltage service level to the low-voltage distribution level used within the facility (typically 480V in North America or 400V in many international applications). Key selection criteria include:

• Transformer Type: Dry-type (cast resin) transformers are standard for interior installation. Liquid-filled transformers may be used for outdoor pad-mounted applications or very large capacity requirements.
• Impedance: Transformer impedance affects available fault current and voltage regulation. Impedance values must be coordinated with upstream and downstream protection devices.
• Efficiency: DOE 2016 or equivalent efficiency standards. High-efficiency designs reduce heat load on the cooling system and improve overall PUE.
• K-Rating: K-rated transformers are recommended for data center applications to handle harmonic currents generated by IT power supplies. K-13 or K-20 ratings are typical.

UPS Topology Options

The UPS system conditions incoming power and provides battery-backed continuity during utility outages and generator start/transfer sequences. The choice of UPS topology affects efficiency, reliability, footprint, and maintenance procedures.

Redundancy Configuration

Redundancy configuration determines how many independent power paths serve the IT load and how the system behaves during component failures or maintenance events.

• N (No Redundancy): Single power path with no backup at the distribution level. Lowest cost but any single failure causes load loss.
• N+1 (Concurrent Maintainable): One additional capacity unit beyond the minimum required. Allows maintenance on one unit without reducing available capacity below the load requirement.
• 2N (Fault Tolerant): Two completely independent power paths, each capable of supporting the full load. Any single failure on either path does not affect the load.
• 2N+1: Two independent paths plus one additional unit on each path. Provides fault tolerance plus concurrent maintainability on either path.

Protection Coordination

Protection coordination ensures that overcurrent protective devices (circuit breakers and fuses) operate in the correct sequence to isolate faults while minimizing the extent of load disruption. A coordination study is required for every data center electrical system and must be updated whenever significant changes are made to the distribution configuration.

Key elements include time-current coordination curves for all devices in series, arc-flash hazard analysis per NFPA 70E, selective coordination requirements per NEC Article 700 (where applicable), and zone-selective interlocking for faster fault clearing in critical paths.

Backup Generation

Diesel generators provide extended runtime capability beyond UPS battery duration. Generator system design addresses capacity sizing, fuel storage and delivery, starting and transfer sequences, paralleling requirements, and emissions compliance.

• Generator sizing: match UPS input requirements plus mechanical loads with appropriate load step acceptance
• Fuel storage: on-site storage for minimum 24-48 hours at full load, with fuel supply contracts for extended outages
• Transfer sequence: automatic transfer switch (ATS) operation with defined transfer time and retransfer delay
• Paralleling: multiple generators paralleled for capacity and redundancy, with load sharing and synchronization

Monitoring and Metering

Electrical monitoring provides the data necessary for capacity management, efficiency tracking, billing, and fault diagnostics. Metering is installed at every major distribution tier from utility service through rack PDU.