Calculating Total Power Requirements for Data Centers

Overview

The paper defines a sequential worksheet for establishing a data centre's total electrical requirement. It starts from the IT nameplate load; that VA figure is derated to the watts actually drawn, future growth is added, and then UPS losses, battery recharge, lighting and cooling loads are layered on one by one.

The utility service is sized from total kW enlarged by a code margin. The generator is selected with load-type multipliers that depend on whether the UPS input is power-factor corrected. For legacy double-conversion UPSs with input filters the multiplier is much higher.

The worked 100 kW example shows that the rated electrical service can be about four times the steady-state critical load. In the 5,000 ft² example room, DX cooling takes 50% of electrical capacity, critical load 36%, UPS loss and charging 11% and lighting 3%.

The method rests on rule-of-thumb coefficients and does not replace a per-equipment load list; it dates from 2011, so it should be checked against current load profiles. The text gives 1,000 kW at 0.8 PF as 1,200 kVA, which arithmetically is 1,250 kVA, a discrepancy to watch for.

Key points

1. Actual IT power is roughly nameplate VA x 0.67.
2. Peak variation allowance: steady-state critical load x 1.05.
3. UPS inefficiency and battery charging allowance: critical load x 0.32 (88% typical UPS efficiency; recharge up to about 20% of rated UPS load).
4. Lighting: 2 W/ft² (21.5 W/m²) of data-centre floor.
5. Cooling electrical demand: 0.7 x electrical load for chilled water; 1.0 x for DX.
6. Utility service: total kW x 1.25 (NEC margin); Amps = kW x 1000 / (V x 1.73).
7. Generator: critical load x 1.3 (PF-corrected UPS) or x 3.0 (legacy double conversion with input filters) + cooling load x 1.5.
8. In the worked 100 kW example, rated electrical service is about 4x the steady-state critical load.
9. The paper states 1,000 kW at 0.8 PF as 1,200 kVA; the arithmetic gives 1,250 kVA.

Using it in design

• Use it as a quick-sizing calculator in novice mode: take IT kW, future kW, floor area, cooling type and UPS input type, and output service kW/A and generator kW, with the multipliers as editable defaults.
• Flag the generator multiplier (1.3 versus 3.0) as dependent on the UPS input THDi and power factor.
• Convert with kVA = kW/0.8 and do not carry over the paper's 1,200 kVA typo.
• In expert mode, replace these heuristics with per-equipment load lists.

Who should read it

Designers and project managers who need an early estimate of utility and generator size.

Related standards

• NEC Article 220/645
• IEC 60364-8-1
• ISO 8528-1

Publication details

Authors

Richard L. Sawyer

Publisher

Schneider Electric

Year

2011

Document number

WP 3

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