Retrofitting Existing Power Systems for AI Clusters

Overview

The paper describes the power characteristics of AI clusters: racks well above 100 kW, synchronous load swings between idle and full load, and short transients above TDP. These characteristics create new problems for existing power systems designed for conventional IT loads. The text takes the challenges in turn and gives guidance on how existing facilities can be provisioned for these loads.

Numerically, cluster power can swing about 60% between idle and full load synchronously, and transients may exceed steady TDP by about 50%. Sizing to the electrical design point (EDP) adds 6-15% of power system capacity at roughly $0.18-0.46/W, which equals about 1-2.5% of AI cluster IT cost. A 7.4 MW Tier III liquid-cooled reference design (Reference Design 108) is used throughout.

On distribution, it shows why 120/208 V and standard 60/63 A rPDUs become impractical. At 120/208 V an 80 kW rack needs five 60 A circuits at 17.3 kW each (1N). The highest standard rPDUs give about 43.6 kW at 230 V 63 A and about 34.5 kW at 240 V 60 A; two of them give 87.2 / 69.0 kW. PDU transformers should be specified with 5-9% impedance, and a practical limit for a PDU/RPP is about 300 kW, or about three 100 kW racks each. In the reference design, 100%-rated 800 A breakers carry about 575 kW, and total capacity is 7,392 kW (Tier III, IEC, chilled water, liquid-cooled). A limit is that the paper rests on one reference design and on vendor rPDU products, so the tables should be adapted to the catalogue and verified for the site.

Key points

1. AI cluster power can swing about 60% between idle and full load synchronously; transients may exceed steady TDP by about 50%.
2. Sizing to peak EDP adds 6-15% capacity ($0.18-0.46/W; about 1-2.5% of AI cluster IT cost).
3. Highest standard rPDUs: 230 V 63 A about 43.6 kW; 240 V 60 A about 34.5 kW; two give 87.2 / 69.0 kW.
4. 240/415 V rPDU: 40 A 23.0 kW; 60 A 34.5; 100 A 57.5; 125 A 71.9; 150 A 86.3; 175 A 100.6 kW.
5. 230/400 V IEC rPDU: 32 A 22.2 kW; 63 A 43.6; 125 A 86.6; 150 A 103.9; 160 A 110.9 kW.
6. At 120/208 V an 80 kW rack needs five 60 A circuits at 17.3 kW each (1N).
7. PDU transformers should be specified with 5-9% impedance.
8. A PDU/RPP practical limit is about 300 kW (about three 100 kW racks).
9. Reference Design 108: 7,392 kW Tier III IEC chilled water liquid-cooled; 100%-rated 800 A breakers about 575 kW.

Using it in design

• In the AI rack template, default to 415/240 V (NA) or 400/230 V (IEC).
• Use the rPDU sizing table above as the catalogue.
• Add an aiDynamicLoad flag: add 6-15% headroom to UPS and switchgear sizing and check UPS step-load and peak capability.
• Warn when 120/208 V is chosen for racks above 20 kW.

Who should read it

Electrical designers and facility managers preparing existing sites for AI loads.

Related standards

• IEC 62040-3
• NEC Article 645
• IEC 60364
• UL 891

Publication details

Authors

Stuart Sheehan; Allegia Wiryawan

Publisher

Schneider Electric

Year

2026

Document number

WP 126

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