Rack Power Distribution: Just the End of the Power Chain, or Key Strategic Advantage? (Mission Critical Rack Power Distribution Whitepaper)

Overview

The paper opens with density categories: citing AFCOM DCISE-001 (2014), many racks run 2-4 kW, medium density is 5-9 kW and 10-25 kW is no longer unusual. It then applies the 80 % rule: a rack needing 20 A at 208 V requires a 30 A circuit (80 % rule = 24 A), about 5 kVA. The example shows how the rule inflates circuit sizing at rack level.

The author's most concrete argument concerns A/B feeds. A 7 A + 7 A snapshot looks safe, but peaks push the sum above the 16 A threshold (80 % of 20 A), and losing one feed would leave about 18 A on the survivor. Modern servers may idle at 20-25 % of maximum power, so peak current can be 4-5 times idle. Planning on average values is therefore misleading.

On cost, decommissioning a single 1U server is said to save about USD 500 a year in energy, USD 500 in OS licence and USD 1,500 in hardware maintenance. Mission-critical power and cooling is cited at USD 12,500 (Tier II) to USD 25,000 (Tier IV) per kW, i.e. USD 125k-250k for a 10 kW rack. ASHRAE A3/A4 classes allow 40 C and 45 C inlets and hot-aisle exhaust may reach 50 C, so PDUs rated to 60 C are emerging. The paper is sponsored, so its strategic framing of the PDU favours the vendor.

Key points

1. Densities: many racks 2-4 kW, medium 5-9 kW, 10-25 kW no longer unusual (AFCOM DCISE-001, 2014).
2. A rack needing 20 A at 208 V requires a 30 A circuit (80 % rule = 24 A), about 5 kVA.
3. A/B example: 7 A + 7 A snapshot looks safe, but peaks exceed the 16 A threshold (80 % of 20 A) and one feed lost leaves about 18 A on the survivor.
4. Modern servers may idle at 20-25 % of max power, so peak current can be 4-5x idle.
5. Decommissioning one 1U server saves about USD 500/yr energy, USD 500 OS licence and USD 1,500 hardware maintenance.
6. ASHRAE A3/A4 inlets of 40 C and 45 C; exhaust may reach 50 C, so 60 C-rated PDUs are emerging.
7. Mission-critical power and cooling cost cited at USD 12,500 (Tier II) to 25,000 (Tier IV) per kW; USD 125k-250k for a 10 kW rack.

Using it in design

• For A/B rack feeds, check that the sum of both feeds' peak load stays below 80 % of one branch breaker (failover check) and warn when only average load is entered.
• Keep a PDU temperature rating in the catalogue and warn if hot-aisle or rear-mounted PDUs are rated below the design exhaust temperature.
• Offer rack density presets based on the DCISE-001 categories.

Who should read it

Electrical designers evaluating A/B rack circuits and PDU temperature ratings, and colocation operators.

Related standards

• NEC 80 % continuous load rule
• The Green Grid PUE
• ASHRAE TC 9.9 (2011/2012)
• AFCOM DCISE-001

Publication details

Authors

Julius Neudorfer

Publisher

Legrand

Year

2015

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