High-Efficiency AC Power Distribution for Data Centers

Overview

The argument starts with voltage. On a 20 A three-phase branch circuit, 120 V line-to-neutral gives 7.2 kW while 240 V gives 14.4 kW (power = volts x amps x 3, using line-to-neutral voltage). Higher voltage doubles branch capacity per amp, cuts copper and losses, and lets many oversized PDU isolation transformers be replaced by one transformer on the UPS bypass. In many data centres the collective PDU transformer capacity is 1.5 to 3 times the UPS capacity, which raises transformer losses.

The comparison runs on a 600 kW, 2N reference layout at 80% load with 104 racks at 4.6 kW per rack. Assumptions are 0.5 kW of electricity to reject 1 kW of heat, $0.12/kWh, and an average 112 ft sub-feed and 44 ft rack whip. Five places to perform the voltage conversion are scored on cost, losses, weight, footprint, fault current and breaker count, and configuration 5B, auto-transformers in distributed PDUs with a bypass isolation transformer, is recommended.

For 2N, total distribution loss is 17,750 W traditional versus 10,730 W proposed, a 40% saving. Material plus 10-year energy cost is $575,792 versus $420,811, a 27% saving. Configuration 5B cuts material cost 21%, energy cost 40%, 10-year total cost 27% and footprint 16%; for 1N the energy reduction is 47% and the total 36%. Moving IT power supplies from 120 to 240 V adds 1-3 percentage points of PSU efficiency, worth $4 to $31 per server per year plus about $2 to $16 in cooling.

The price is fault current. Traditional distribution offers 26.0 kA at the PDU/RPP input, 3.8 kA at the branch breaker and 3.2 kA at the rack; configurations 4A/4B/5B give 14 kA, 14 kA and 6.3 kA, above 10 kA, so higher-interrupting branch breakers may be needed. The paper also covers practical questions: IEC C14 and C20 connectors are rated only to 250 V, one reason 277 V IT distribution is unlikely, and 240 V circuits use C13/C19.

Key points

1. 20 A three-phase branch: 7.2 kW at 120 V line-to-neutral versus 14.4 kW at 240 V.
2. Collective PDU transformer capacity is often 1.5-3 times UPS capacity, adding transformer losses.
3. Moving IT supplies from 120 to 240 V adds 1-3 points of PSU efficiency: $4-31 per server per year plus about $2-16 in cooling.
4. 600 kW 2N: distribution loss 17,750 W traditional versus 10,730 W proposed, a 40% saving.
5. Material plus 10-year energy cost: $575,792 versus $420,811, a 27% saving.
6. 5B cuts 2N material cost 21%, energy 40%, 10-year total 27%, footprint 16%; at 1N energy falls 47% and total 36%.
7. Available fault current: traditional 26.0 kA (PDU/RPP input), 3.8 kA (branch), 3.2 kA (rack); 4A/4B/5B give 14, 14 and 6.3 kA.
8. IEC C14 and C20 are rated only to 250 V; 240 V circuits use C13/C19.

Using it in design

• Offer a 'North America 415/240 V' one-line template (480 V UPS, auto-transformer RPP/PDUs, isolation transformer on UPS bypass) next to the traditional 480 to 208/120 V.
• Calculate branch capacity as V_LN x A x 3 (derated per local code), and warn when the sum of PDU transformer kVA exceeds about 1.5x the UPS rating.
• Require breakers with adequate interrupting rating, or a current-limiting breaker upstream, when branch-level available fault current exceeds 10 kA.
• Keep rack PDU outlet types (C13/C19) and voltage rating in the catalog and flag devices needing 120 V or three-phase input.

Who should read it

Electrical designers of 480 V North American facilities and decision makers weighing the cost and efficiency case for 415/240 V.

Related standards

• NFPA 70
• IEC 60320-1

Publication details

Authors

Neil Rasmussen

Publisher

Schneider Electric

Year

2011

Document number

WP 128

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