AC vs. DC Power Distribution for Data Centers

Overview

The options compared are North American 480/208 V AC with PDU transformers, 400/230 V AC, 48 V DC, 380 V DC and a hybrid 575 V DC system. At 50% load the product of UPS, distribution and IT power-supply efficiency comes out at 83.56% for 480 to 208 V AC, 86.39% for 400/230 V AC, 80.74% for 48 V DC, 87.64% for 380 V DC and 80.74% for hybrid 575 V DC. The paper attributes many published DC advantages to outdated AC efficiency assumptions.

It grants that 380 V DC may offer about a 1% efficiency edge over the best AC system at full load, about 1.5% less rack heat and about 10% copper saving (9.5 kW per wire against 8.625 kW for 400/230 V AC with 50 A wiring). Wiring conduction losses are insignificant in real data centres because load diversity keeps circuits well below rating. DC UPS cost is typically 10% to 20% lower than AC UPS, but 48 V DC distribution needs 10X or more copper weight and cost and is impractical above 20 kW per cabinet.

Safety and compliance are weighed too: 380 V DC would not be legal in Japan, where the regulatory limit is 300 V, and North America has no arc-flash regulations for commercial DC. In hosting sites with much telecom equipment, the 48 V DC requirement may reach 10% of the AC requirement, and point-of-use battery-less rack rectifiers are recommended. The conclusion is that the North American 480 V AC system with PDU transformers should be viewed as inefficient and obsolete for new designs.

Key points

1. Overall efficiency at 50% load (UPS x distribution x IT PSU): 480 to 208 V AC 83.56%; 400/230 V AC 86.39%; 48 V DC 80.74%; 380 V DC 87.64%; hybrid 575 V DC 80.74%.
2. Wiring conduction losses are insignificant in real data centres because load diversity keeps circuits well below rating, though wire sizing must still meet the worst case.
3. 380 V DC may offer about 1% efficiency over the best AC system at full load, about 1.5% less rack heat and about 10% copper saving (9.5 kW per wire vs 8.625 kW).
4. DC UPS cost is typically 10-20% below AC UPS, but 48 V DC distribution needs 10X or more copper weight and cost and is impractical above 20 kW per cabinet.
5. 380 V DC would be illegal in Japan, where the limit is 300 V; North America has no arc-flash regulation for commercial DC.
6. In hosting sites with much telecom equipment the 48 V DC need may reach 10% of the AC need; point-of-use battery-less rack rectifiers are recommended.
7. The paper calls the North American 480 V AC system with PDU transformers inefficient and obsolete for new designs.

Using it in design

• In the power-chain efficiency calculator multiply UPS, transformer/PDU, distribution and PSU efficiencies at the actual load fraction rather than nameplate, and offer architecture presets for the five options with Table 1 values as defaults.
• In North American projects, suggest 415/240 V distribution to eliminate PDU transformers.
• Warn when 48 V DC busway is proposed above 20 kW per cabinet, or when DC distribution exceeds a local regulatory voltage limit (for example 300 V in Japan).
• Give PDU catalogue entries a hasTransformer flag and a part-load efficiency curve.

Who should read it

Electrical designers choosing the distribution voltage and UPS architecture of a new facility.

Related standards

• IEC 60364
• NFPA 70

Publication details

Authors

Neil Rasmussen

Publisher

Schneider Electric

Year

2011

Document number

WP 63

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