WattThe / Data center / Electrical sizing
Turn a kW load into three-phase amperage, apply the NEC continuous-load rule, and get first-pass UPS and generator sizing.
Three-phase current follows one formula: amps = watts ÷ (√3 × line voltage × power factor). At 480 V and 0.95 PF, a 500 kW load draws about 633 A. But the number you design conductors and breakers around is larger: the National Electrical Code treats data center loads as continuous (three hours or more), and NEC 210.20/215.3 require overcurrent devices to be sized so the continuous load doesn't exceed 80% of the rating — equivalently, dividing your calculated amps by 0.8.
UPS systems are rated in kVA, so the kW load is divided by power factor before headroom is added; a 20–30% margin covers load growth, battery aging, and the fact that running a UPS near 100% shortens its life and eliminates redundancy. Standby generators are sized in kW with similar headroom, and for large GPU halls the step-load behavior matters — modern accelerators can swing hundreds of kW in milliseconds, which stresses generator governors and is a real topic to raise with your genset vendor.
Treat this as a first-pass estimate for feasibility and interconnection conversations. Actual one-line design — conductor sizing, fault current, selective coordination, transformer impedance — is stamped-engineer territory.
For continuous loads (running 3+ hours), the NEC requires breakers and conductors to be rated at least 125% of the load — the same as saying the load can only occupy 80% of the circuit rating. A 633 A continuous load therefore needs at least a 791 A rated circuit.
kVA is apparent power — the current the equipment actually must carry — while kW is real power delivered. They are related by power factor (kW = kVA × PF), so a 500 kW load at 0.95 PF requires about 526 kVA of UPS capacity before headroom.
20–30% above peak load is a common planning figure. It covers motor starting and step loads, altitude and temperature derating, and future growth. Large AI clusters also need attention to fast load swings, which can exceed what a minimally sized genset tolerates.
Estimates are for planning and education, not engineering design or financial advice. Verify rates with your utility and confirm electrical work with a licensed engineer or electrician.
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