Data Center Rack Power and PDU Sizing: A/B Capacity Planning

Start with the rack load schedule

Record rack quantity, maximum design IT kW per rack, power factor, cord arrangement, growth and circuit quantity. State whether the kW basis is measured demand, an OEM estimate or nameplate data. Mixed-density halls need a rack-by-rack schedule rather than one average that hides high-density rows. Cooling demand and UPS losses belong in upstream facility sizing; they are not included in the rack-output calculation below.

Each redundant path needs the surviving load

For independently sized A/B paths, either path must carry the full design IT load if the other fails. Do not divide required path capacity by two. Balanced normal sharing is only a planning assumption; actual power-supply behaviour can produce unequal loading. A and B rack cords do not double the installed IT kW, but they do require independently adequate distribution capacity and separate circuit inventories.

Planning equations and a worked example

Design kW = rack count × kW per rack × (1 + growth allowance / 100). Required load kVA per surviving path = design kW / power factor. Planning capacity kVA per path = required load kVA / permitted post-failure loading fraction. Balanced three-phase load current = load kVA × 1000 / (√3 × line-to-line voltage).

For 20 racks at 10 kW, 20% future load allowance and 0.95 power factor, design IT demand is 240 kW and the surviving-path load is approximately 252.6 kVA. If the project adopts an 80% maximum post-failure loading criterion, planning capacity is approximately 315.8 kVA on each path. At 415 V this corresponds to approximately 439.3 A planning capacity. These are declared example assumptions, not a universal 80% rule or a selected breaker rating.

Branch counts and phase allocation

One circuit per rack per path gives 20 A circuits plus 20 B circuits in this example. Two circuits per rack per path gives 40 on each path. Actual quantities depend on rack PDUs, connectors, single- or three-phase supplies, equipment cords and the expansion plan. Allocate single-phase circuits across phases using expected loads, not only equal circuit counts. Spare ways and spare electrical capacity are different: an empty breaker position does not prove upstream capacity is available.

What the calculation cannot select

The balanced current estimate is not a primary breaker selection. Final frame size, trip settings, continuous-current suitability, cable ampacity and derating need the equipment schedule and local requirements. UPS inverter fault contribution and bypass-source fault level require protection coordination. Neutral sizing requires phase and harmonic analysis; no fixed neutral multiplier is produced. A transformer, if required in a floor PDU, needs its own efficiency, harmonic and thermal review.

Use the planner and consultant worksheet

The rack power and PDU planner applies the equations with editable assumptions and calculates circuit counts. Sign in to use the engineering calculator. The same page includes a consultant procurement worksheet download covering Division 26 coordination, source states, assembly verification, transfer, metering, cooling recovery and handover. It is a discussion template, not a certified design, approved tender or proprietary drawing.

Send the right inputs for a panel proposal

  • Rack schedule, design kW, growth and power-factor basis
  • Approved SLD with A/B source and maintenance states
  • UPS OEM fault-current data in inverter and bypass modes
  • Circuit counts, supply phases, connectors and cable entry
  • Prospective fault level, separation and environmental requirements
  • Branch metering and DCIM points schedule

See PDU vs RPP, branch circuit monitoring and data center panel supply.

Technical references

Site-specific load and distribution design must be reviewed by the responsible electrical engineer.

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