AC Versus DC Power Distribution: Issues to Consider When Comparing Options for Improving Data Center Energy Efficiency
Eaton · Robert W. Johnson
Eaton WP153001 compares end-to-end power-chain efficiency, from source to 12 VDC inside the server, for North American AC architectures and 380 VDC distribution, and concludes that AC with a high-efficiency UPS matches or beats DC.
On this page
Overview
The paper walks from a legacy 480 V/208 V chain through newer components, high-efficiency UPS modes and PDU elimination with 480/277 V or 400/230 V distribution, and compares these with narrow- and wide-range DC buses. Chain efficiencies: legacy AC 61%; newer components 83%; high-efficiency UPS mode 87.5%; no PDU with 480/277 V or 400/230 V 89%; DC narrow-range 86%; DC wide-range 84%.
Practical constraints are listed too. Few server PSUs accept 277 V input, which makes 480/277 V impractical; 400/230 V is the realistic option. High-efficiency UPSs can achieve mode transfer times of 0-4 ms. Redundant A/B feeds halve the load per source and lower the converters' operating efficiency.
DC issues include extra poles needed for breaking above 250 VDC, limited discrimination, arc-fault fire risk, undefined arc-flash energy, and PSU holdup that must exceed fault clearing time.
The paper also proposes auto-zigzag transformers to derive 415/240 V from 480 V delta, with about 1.5% impedance against about 5.75% for a typical isolation transformer.
Key points
- Chain efficiency: legacy AC 61%; newer components 83%; high-efficiency UPS mode 87.5%; no PDU with 480/277 V or 400/230 V 89%; DC narrow-range 86%; DC wide-range 84%.
- High-efficiency UPSs can achieve mode transfer times of 0-4 ms.
- Few server PSUs accept 277 V, making 480/277 V impractical; 400/230 V is the realistic option.
- Redundant A/B feeds halve load per source and lower the converters' operating efficiency.
- DC issues: extra poles above 250 VDC, limited discrimination, arc-fault fire risk, undefined arc-flash energy, PSU holdup that must exceed fault clearing time.
- An auto-zigzag transformer for 480 V delta to 415/240 V has about 1.5% impedance versus about 5.75% for a typical isolation transformer.
Using it in design
- Provide a power-chain efficiency calculator by stage (transformer, UPS mode, PDU, PSU) including the effect of the A/B load split.
- Offer voltage options per region (208/120, 415/240, 400/230, 480/277, 380 VDC) with a PSU compatibility check.
- For DC buses above 250 V require DC-rated multi-pole protection and a holdup-versus-clearing-time check.
- Flag 277 V distribution when the IT PSUs are not rated for it.
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Who should read it
Designers comparing AC and DC distribution on energy efficiency.
Related standards
- NEC Article 450
- NEC 210.9
- NEC 250.36
- ETSI EN 300 132-3
Publication details
- Authors
- Robert W. Johnson
- Publisher
- Eaton
- Year
- 2012
- Document number
- WP153001EN
- 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
Robert W. Johnson (2012). AC Versus DC Power Distribution: Issues to Consider When Comparing Options for Improving Data Center Energy Efficiency. Eaton. WP153001EN. https://web.archive.org/web/20240707022753/https://www.eaton.com/content/dam/eaton/markets/data-center/AC-Versus-DC-Power-Distribution.pdf
Events on this subject
- Electricity Eurasia 2027
- ELECO 2026 Elektrik-Elektronik ve Biyomedikal Mühendisliği Konferansı
- The smarter E Europe (Intersolar Europe, ees Europe, Power2Drive, EM-Power)
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