Calculating Space and Power Density Requirements for Data Centers

Overview

Neil Rasmussen's paper builds on the earlier WP 120 and gives a step-by-step method for specifying a data centre's IT load and space. The method uses four inputs: a design-target average kW per cabinet, peak cabinet power, the uncertainty of the estimate, and the expected "managed power", the ratio of typical draw to nameplate. Rated system power and expected operating load are derived from them.

Two worked examples are included: a 40 kW server room and a 2 MW multi-room facility. In the 40 kW example the target average is 4 kW per cabinet, peak is 8 kW, uncertainty is +/-15% at 80% confidence and the managed power ratio is 70%. The result is 48 kW of rated system power, 33.6 kW of expected IT, about 15% space reserve and a density of about 115 W/ft2. As a policy, peak cabinet power is capped at twice the design average (8 kW versus 4 kW in the example).

The author's main warning is against over-specifying density. A 1,000 W/ft2 (10,764 W/m2) data centre costs about 8 times as much as a 100 W/ft2 facility, and if it is operated at 100 W/ft2 its PUE is likely to be 3 to 5. Where little is known, typical values are suggested: a spec of 120 W/ft2 (1,292 W/m2) corresponds to about 3 to 5 kW per cabinet, and the example overall target density is 5 kW per cabinet, about 160 W/ft2 (1,722 W/m2). The method makes uncertainty in the IT load an explicit input rather than hiding it, which is its strength. Its limit is that results are only as good as the average and uncertainty values the designer estimates; the paper does not produce those for you.

Key points

1. A typical spec of 120 W/ft2 (1,292 W/m2) corresponds to about 3 to 5 kW per cabinet.
2. Example overall target density: 5 kW per cabinet, about 160 W/ft2 (1,722 W/m2).
3. A 1,000 W/ft2 (10,764 W/m2) data centre costs about 8 times a 100 W/ft2 facility; operated at 100 W/ft2 its PUE is likely 3 to 5.
4. 40 kW room example: target average 4 kW/cabinet, peak 8 kW, uncertainty +/-15% (80% confidence), managed power ratio 70%.
5. Same example: rated system power 48 kW, expected IT 33.6 kW, about 15% space reserve, about 115 W/ft2.
6. Policy: peak cabinet power is capped at 2 times the design average (8 kW versus 4 kW).

Using it in design

• Use this paper as the template for the Design Basis form: target average kW per rack, peak kW per rack (default 2x average), density uncertainty (default 15%), managed power ratio (default 0.7) and reserve space (default 15%).
• Compute rated system power as racks x average x (1 + uncertainty), and expected operating power as that value times the managed power ratio.
• Warn when the specified average density exceeds 15 kW per rack without a liquid-cooling plan.
• Warn when expected utilization will be low, since PUE will suffer; do not over-specify density.

Who should read it

Planners and engineers setting the design basis and capacity target of a new data centre.

Related standards

• TIA-942
• ASHRAE TC9.9 Thermal Guidelines

Publication details

Authors

Neil Rasmussen

Publisher

Schneider Electric

Year

2013

Document number

WP 155

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