Rack power formulas
P_rack [kW] = Σ (watts × qty) × load_factor ÷ 1000
S_rack [kVA] = P_rack ÷ PF
Single-phase: I = S × 1000 ÷ V_LN (V_LN = 230 V)
Three-phase: I_phase = S × 1000 ÷ (3 × V_LN) (= S ÷ (√3 × 400 V), balanced)
PDU usable = V_LN × phases × I_rated × 0.8
PDUs per feed = ceil( S_rack ÷ PDU usable )
Total PDUs = PDUs per feed × feeds (A or A+B)
A 32 A single-phase PDU at 230 V is ≈ 7.4 kVA nameplate and ≈ 5.9 kVA usable with the 80 % rule. The three-phase 32 A version is ≈ 22 kVA nameplate and ≈ 17.7 kVA usable — which is why dense racks (above 8 kW) are usually fed three-phase.
Rack density
- Up to 4 kW: low — network racks and light servers; room cooling usually suffices.
- 4–8 kW: medium — typical enterprise server rooms; aisle containment recommended.
- 8–15 kW: high — needs three-phase power and close-coupled (in-row) cooling.
- Above 15 kW: extreme — GPU/AI servers; usually liquid cooling or rear-door heat exchangers.
Assumptions
- Three-phase load is assumed balanced; in practice, distribute outlets to keep phases even.
- Inrush when many servers power on together is not modelled; smart PDUs offer sequenced power-up.
- Breaker and cable sizing must follow local electrical codes and be done by an electrical engineer.