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; valid estimates can be saved to a LoadCairn Pro project.
Live planning estimate
Planning-limited capacity
Estimates for planning only, not a substitute for a licensed engineer’s design or manufacturer data.
- 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%
- Remaining PDU capacity (planning guidance)
- 2,782 W
Calculation details
Inputs used
- Rack real-power load
- 5000 W
- PDU voltage (line-to-line)
- 208 V
- Rated current per line
- 30 A
- Selected phase
- balanced three-phase
- Power factor
- 0.9
- Planning cap
- 80%
Formula and live derivation
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) Utilization (%) = rack load (W) ÷ capacity (W) × 100; remaining planning headroom (W) = planning capacity − rack load
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
5000 W ÷ 9,727 W = 51.4% nominal utilization
5000 W ÷ 7,782 W = 64.3% planning-cap utilization
7,782 W − 5000 W = 2,782 W planning headroom
Calculations retain full precision; displayed values are rounded for readability.
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.
Sources
- Schneider Electric Electrical Installation Guide — Definition of reactive power
Defines apparent and active power, power factor, and the single- and three-phase equations. Its three-phase equation is for balanced loads and uses phase-to-phase voltage with line current.
- Schneider Electric — APC NetShelter Rack PDU model APDU10250SM
A model-specific manufacturer listing showing how a PDU’s phase, voltage, current, and power ratings depend on the selected product. This example is not a device recommendation; check the documentation and nameplate for your own PDU.
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
Limitations
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.