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This limits losses from the utility feed all the way to the server rack. This includes UPS losses, transformers, and PDU inefficiencies. If your UPS is 5+ years old, you might already be failing this requirement.
The is more than just a document—it is the roadmap to designing efficient, cost-effective, and compliant data centers for the next decade. Whether you are a mechanical engineer selecting cooling systems, an electrical engineer sizing UPSs, or a facility manager trying to reduce PUE, this standard is your definitive guide. ashrae 90.4 pdf