Microsoft GPT-4 cluster is a large GPU cluster operated by Microsoft, OpenAI, located in the United States, in the private sector. It came online around April 2022. At that moment it ranked number 1 among known clusters.
It is built on 25,000 NVIDIA A100. That amounts to about 7.9K H100e of AI compute. The cluster draws roughly 21.29 MW. Its hardware is estimated at $563M.
Epoch AI rates the certainty of this record as likely. Training finished August 2022. Training lasted ~3 months, meaning that it was operational late May at the latest