Rack Powering Options for High Density

Overview

The paper gives guidance for 120/208 V rack power distribution in North America: number of feeds, single- versus three-phase whips, breaker placement, connector choice, voltage selection and redundancy. Its starting point is that rack power must adapt to IT refreshes every 2-3 years and to dynamic loads that can swing to about 200% of baseline consumption.

Three-phase whips to every rack are therefore recommended as the flexible default. Tables compare whip types by kW deliverable and cost per kW. Whip options span roughly 2.4-36 kW per rack; 208 V 30 A three-phase gives about 8.3 kW and 50 A three-phase about 14.4 kW.

As density rises, the number of single-phase whips becomes impractical: above about 11 kW per rack, 50 A three-phase is preferred over 30 A. The paper notes that 208 V covers more than 97% of IT equipment and about 95% ships with 120 V cords. Cited Uptime surveys put average rack density at 5.6 kW in 2017 and 8.4 kW in 2020.

The scope is limited to North American voltages, so the same logic must be adapted to 230/400 V environments elsewhere. As a practical rule, rack PDUs should be loaded to no more than 80% of rating, and racks above about 17 kW call for a higher distribution voltage.

Key points

1. Branch-circuit capacity: 120 V 20 A = 2.4 kW; 208 V 20 A = 4.2 kW (1-4 circuits give 2.4/4.8/7.2/9.6 or 4.2/8.3/12.5/16.6 kW).
2. Rack PDUs should be loaded to no more than 80% of rating.
3. Whip options deliver roughly 2.4-36 kW per rack; 208 V 30 A 3-phase about 8.3 kW, 50 A 3-phase about 14.4 kW.
4. Above about 11 kW per rack single-phase whips become impractical; prefer 50 A 3-phase over 30 A.
5. 208 V covers over 97% of IT equipment; about 95% ships with 120 V cords.
6. Cited Uptime surveys: average rack density 5.6 kW (2017) to 8.4 kW (2020).
7. Dynamic IT power can reach about 200% of baseline consumption.

Using it in design

• In the rack power module, catalogue whip types, compute kW per whip as V x A x sqrt3 x 0.8 (three-phase) or V x A x 0.8 (single-phase), and default each rack to two three-phase whips (A/B).
• Warn when rack kW exceeds what the selected whips can deliver after the 80% derate.
• Suggest higher-voltage distribution (415/240 V) when a rack exceeds about 17 kW.
• In 230/400 V regions, adapt the same calculation to the local phase voltage.

Who should read it

Electrical engineers and IT infrastructure planners designing high-density rack power.

Related standards

• NEC Article 210/645
• IEC 60320
• NEMA WD 6
• UL 60950/62368

Publication details

Authors

Paul Lin

Publisher

Schneider Electric

Year

2021

Document number

WP 29

Document language

English

Source type

Manufacturer

Access

Free to read

Licence

Copyright retained; citation only — the publisher’s terms apply to the original document

Citation