Electrical Power Distribution and Efficiency
What this domain tests
This domain covers the path from utility to server and the losses along it: transformers, switchgear, UPS systems, PDUs, and busway.
Core concepts:
- The distribution chain: utility transformer, main switchgear, UPS, PDU, rack PDU, server power supply. Each stage has a loss, and losses compound.
- UPS efficiency and topology: double-conversion UPS is efficient at high load but drops sharply at light load; eco mode and line-interactive designs trade protection for efficiency. Efficiency curves are load-dependent - always check the curve, not the headline number.
- Redundancy penalties: 2N systems run at lower per-module load, moving operation toward less efficient points on the curve.
- Power factor and harmonics: non-linear IT loads create harmonics; power factor correction and proper grounding matter.
- Distribution losses: transformer no-load and load losses, busway vs. cable, voltage drop, and conductor sizing.
- Stranded capacity: over-provisioned UPS modules and underutilized PDUs waste both capital and energy.
Traps to watch:
- Quoting UPS efficiency at full load when the system runs at 30% load.
- Ignoring that 2N redundancy lowers load factor per module and therefore efficiency.
- Forgetting transformer losses when computing facility overhead.
- Assuming eco mode is always acceptable; it may violate the owner's protection requirements.
Worked example
A UPS rated 96% at full load may be 90% at 30% load. For a 300 kW IT load on a 1,000 kW UPS, losses at 90% efficiency are 300/0.90 − 300 = 33 kW. At 96% they would be 12.5 kW - a 20 kW difference that shows up directly in PUE.