Specifying Data Center IT Pod Architectures

Overview

A pod is a row or pair of rows of racks sharing a PDU, containment, network and similar infrastructure. Its size is driven by three inputs: the electrical sub-feed available (voltage and breaker rating), the physical space (number of racks and row length) and the targeted average rack density. Table 1 gives available pod power: 400/230 V IEC 400 A = 275 kW, 250 A = 170 kW; 480/277 V 400 A = 260 kW, 250 A = 165 kW; 208/120 V 400 A = 150 kW, 250 A = 70 kW (ANSI values de-rated to 80% per NEC).

Common voltage and breaker combinations collapse into two standard classes: a low-power pod of about 150 kW (400 V/250 A, 480 V/250 A, 208 V/400 A) and a high-power pod of about 250 kW (400 V/400 A, 480 V/400 A). Airflow needed is 8,500-11,400 l/s (18,000-24,000 CFM) for 150 kW and 14,000-19,000 l/s (30,000-40,000 CFM) for 250 kW, assuming an 11-15 °C IT delta-T. Water flow at 5.6 °C delta-T is 7 L/s for 150 kW and 11.7 L/s for 250 kW.

Typical maximum continuous row length is 20-25 racks; longer rows are generally only seen in hyperscale facilities. A 250 kW pod is 40 racks (2 x 20) at 6 kW, 20 racks (2 x 10) at 12 kW or 12 racks (2 x 6) at 20 kW; a 150 kW pod is 24, 12 and 6 racks. Average density is pod power divided by rack count, while peak density is set by the rack branch circuit, so with good containment a 20 kW rack can sit beside a 2 kW rack. The authors advise under-estimating average density, because deploying below design density strands the more expensive power and cooling capacity while space is usually the cheapest resource.

Key points

1. Sub-feed power: 400/230 V 400 A 275 kW, 250 A 170 kW; 480/277 V 400 A 260 kW, 250 A 165 kW; 208/120 V 400 A 150 kW, 250 A 70 kW.
2. Two classes: about 150 kW low-power and about 250 kW high-power pods.
3. Airflow: 8,500-11,400 l/s for 150 kW; 14,000-19,000 l/s for 250 kW (11-15 °C delta-T).
4. Water flow at 5.6 °C delta-T: 7 L/s (150 kW) and 11.7 L/s (250 kW).
5. Typical maximum continuous row length is 20-25 racks.
6. 250 kW pod: 40 racks at 6 kW, 20 at 12 kW, 12 at 20 kW.
7. Peak rack density comes from the rack branch circuit, average from pod power over rack count.
8. Under-estimate average density; space is usually the cheapest resource.

Using it in design

• Make the pod a first-class object between room and rack, with feed power, input voltage, feeder breaker, feed redundancy, UPS location and rack count.
• Offer 150 kW and 250 kW pod templates and compute required airflow and 5.6 °C delta-T chilled-water flow from pod kW.
• Warn when row length exceeds 20-25 racks (egress check), when rack loads sum above the pod feed, or when a rack is fed from a PDU outside its pod.
• Apply 80% continuous de-rating in NEC jurisdictions.

Who should read it

Designers specifying rack rows, PDUs and cooling as repeatable pod units.

Related standards

• NFPA 70 (NEC)

Publication details

Authors

Robert Bunger; Patrick Donovan

Publisher

Schneider Electric

Year

2017

Document number

WP 260

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