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LoadCairn / Rack power

Rack power & PDU capacity calculator

Compare an expected rack load with a PDU’s estimated real-power capacity. See the calculation and change each assumption in your browser; values are not saved, and no account is required.

PDU and rack inputs

Starting values are illustrative and editable: 5,000 W, 208 V line-to-line, 30 A per line, balanced three-phase, 0.90 power factor, and an 80% planning cap. They do not describe a specific PDU.

Enter expected rack real power. A zero-watt load is valid.

For three phase, enter line-to-line voltage; for single phase, enter voltage across the load.

Use the PDU nameplate current per line, not a sum across lines.

Use a measured or manufacturer-specified value when available.

An editable planning assumption, not a universal code rule or device guarantee.

Electrical phase

Three-phase uses line-to-line voltage and current per line, and assumes balanced loading. For single phase, use voltage across the load and its line current.

Unsaved · Calculated in your browser

Live planning estimate

Planning-limited capacity

7,782 WRack load is within the selected planning cap. Rounded for display; calculations retain full precision.
Rack load
5,000 W
Apparent capacity
10,808 VA
Nominal real-power capacity
9,727 W
Nominal utilization
51.4%
Planning-cap utilization
64.3%
Planning headroom
2,782 W

Formula and live derivation

Apparent capacity (VA) = V × A                    [single phase]
Apparent capacity (VA) = √3 × V(line-to-line) × A(line) [balanced three phase]
Nominal real-power capacity (W) = apparent capacity × power factor
Planning capacity (W) = nominal real-power capacity × (planning cap % ÷ 100)

balanced three-phase: √3 × 208 V × 30 A = 10,808 VA

10,808 VA × 0.9 PF = 9,727 W nominal

9,727 W × 80% = 7,782 W planning capacity

Worked example

The editable starting example uses a 5,000 W load and a hypothetical balanced three-phase PDU input of 208 V line-to-line, 30 A per line, and 0.90 power factor. The 80% planning cap is an assumption for illustration, not a rating or universal electrical rule.

√3 × 208 V × 30 A = 10,808 VA

10,808 VA × 0.90 = 9,727 W nominal

9,727 W × 80% = 7,782 W planning capacity

5,000 W ÷ 9,727 W = 51.4% nominal utilization

5,000 W ÷ 7,782 W = 64.3% planning-cap utilization

7,782 W − 5,000 W = 2,782 W planning headroom

Assumptions and limits

Assumptions

  • The three-phase estimate assumes balanced loading across the phases. The single-phase estimate uses one line voltage and one line current.
  • The entered power factor is applied to apparent capacity to estimate real-power capacity. Use a value appropriate to the equipment and measurement conditions.
  • The entered planning cap is applied to nominal real-power capacity; 80% is only the editable starting assumption shown here.

Not evaluated

  • Does not check individual phase loading, branch circuits, breakers, receptacles, or outlet limits.
  • Does not evaluate A/B-feed redundancy, transfer behavior, inrush or transient load, voltage drop, conductor sizing, or protective-device coordination.
  • Does not apply environmental or installation derating, validate equipment compatibility, or determine local-code requirements.
  • Does not confirm the PDU input rating, usable outlet capacity, or a device-specific continuous-load limit. Read the selected unit’s current manufacturer documentation and nameplate.
  • Power factor can vary with load and waveform. Nonlinear loads, harmonics, and unbalanced systems need equipment-specific analysis.

Reference points

Check your actual PDU documentation

These references explain the simplified equation and show why equipment ratings must come from the exact product documentation.