Choosing the Optimal Data Center Power Density

Overview

The study costs a non-redundant 1 MW hall at average densities from 1 to 60 kW/rack. Up to about 5 kW/rack the savings come from fewer racks and less floor space; beyond that, wider and deeper racks and higher-amperage rack PDUs eat into the gain. The model assumes 125 CFM/kW (3.54 m3/min/kW) of IT airflow and space at $1614/m2, with standard racks below 8 kW, wide racks at 8-20 kW and wide-and-deep racks above 20 kW.

The recommended design point is an average of 5-8 kW/rack with a peak of 11-11.5 kW/rack. Below 5 kW is a cost penalty, above 8 kW returns diminish, and above 15 kW design complexity outweighs savings. The peak figure follows from limiting rack PDUs to 16 A or 20 A; PDUs of 25 A (30 A in North America) or more need branch-rated breakers in the rack.

The price of oversizing for uncertainty depends heavily on scope: for a peak-to-average ratio of 2 it costs under 1% of the project at rack level, 4% at pod level and 20% or more at room level. Designing rack distribution to an 11.5 kW peak costs $0.08/W. For example, planning 1000 kW at 20 kW/rack for 50 racks but deploying at 5 kW/rack strands 750 kW of power and cooling.

The paper reflects a 2014 enterprise IT context, when average density was 3-5 kW/rack. The numeric optimum should not be applied to AI-era densities, though the method and the stranded-capacity logic remain valid.

Key points

1. Recommended average density is 5-8 kW/rack, with a peak of 11-11.5 kW/rack.
2. Below 5 kW/rack is a cost penalty, above 8 kW returns diminish and above 15 kW complexity outweighs savings.
3. Rack PDU breakpoints: 208 V 3-phase 20/30/40 A = 5.8/8.6/11.5 kW; 230 V 3-phase 16/25/32 A = 11.0/17.3/22.1 kW.
4. Oversizing premium at peak-to-average 2: under 1% at rack level, 4% at pod, 20% or more at room.
5. Uncontained perimeter cooling struggles above about 7 kW/rack; row coolers show a cost penalty from 30 kW/rack.
6. IT devices typically draw under 50% of nameplate power-supply rating.
7. Example: 20 kW/rack planned but 5 kW/rack deployed strands 750 kW of 1000 kW.

Using it in design

• Capture peak kW/rack and the peak-to-average ratio as well as the average, and express density in W/rack instead of W/m2.
• Size rack PDUs from the voltage x phase x breaker table and warn when 25 A (30 A NA) or larger units imply in-rack branch-rated breakers.
• Compare planned density x rack positions against installed power and cooling and flag the stranded kW.
• Label the numeric optimum as legacy enterprise guidance and do not apply it to AI or HPC halls.

Who should read it

Design engineers and owners deciding rack density and capacity planning.

Related standards

• NFPA 70 (NEC)
• IEC 60364
• IEC 60309
• ASHRAE TC 9.9 Thermal Guidelines

Publication details

Authors

Kevin Brown; Wendy Torell; Victor Avelar

Publisher

Schneider Electric

Year

2014

Document number

WP 156

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