Rack power planning starts with a simple question: how much equipment can this cabinet support while meeting the required failure scenario? The answer depends on usable electrical capacity, workload behavior, and the equipment that connects them.
Before comparing colocation proposals, build one power schedule that distinguishes normal demand, failover demand, and planned expansion. A larger circuit rating alone does not establish a larger supported deployment.
Separate kW, kVA, and usable capacity
Kilowatts, or kW, measure real power. Kilovolt-amperes, or kVA, measure apparent power. Power factor connects them: kW = kVA × power factor. At unity power factor, the numbers match; below unity, they do not. Both real-power and apparent-power limits matter when assessing equipment. Schneider Electric explains the distinction.
Keep three figures separate in the proposal: the circuit's nominal rating, its approved continuous operating limit, and the usable power included in the contract. Ask whether a quoted allocation applies to each feed, the pair, or the cabinet's total IT load. Also confirm where consumption is measured and what happens when demand approaches the agreed limit.
Calculate three-phase capacity with explicit assumptions
For a balanced three-phase load, apparent power in kVA is line-to-line voltage × line current × √3 ÷ 1,000. Use line-to-line voltage in this formula, not line-to-neutral voltage.
For example, 208 V × 60 A × √3 ÷ 1,000 is approximately 21.6 kVA at the stated current. If the approved allowable continuous current for that circuit is 48 A, or 80% of 60 A, the calculation becomes approximately 17.3 kVA. At power factor 1.0, that corresponds to 17.3 kW; at 0.95, approximately 16.4 kW.
The 80% assumption is conditional, not a universal rule. Equipment ratings, installation conditions, applicable requirements, and facility approval determine allowable operation. These calculations illustrate capacity; they do not authorize a load or establish contracted usable power. Have the facility and equipment requirements checked before finalizing the design.
Size A/B feeds for the required failure scenario
When either A or B must carry the whole critical load after the other feed fails, their capacities are not additive. Two feeds with 10 kW of approved usable capacity each support a maximum 10 kW critical load under that assumption, before any additional operating margin.
An 8 kW load might normally draw about 4 kW from each feed, then require approximately 8 kW from the surviving path. Actual sharing depends on the equipment. Check the surviving circuit, PDU, outlet banks, and device power supplies against that condition.
Multiple PSUs do not automatically guarantee full performance after losing half the inputs. Manufacturer behavior matters: NVIDIA's DGX H100/H200 guide describes a configuration that can continue with three powered PSUs at reduced performance. Confirm the exact redundancy mode, upstream independence, and treatment of equipment with only one power input.
Estimate workload demand beyond PSU nameplates
A PSU's output rating describes its capability; adding PSU ratings does not establish the server's expected AC input draw. Use manufacturer planning data for the actual configuration, then refine it with representative server telemetry and PDU measurements when available.
Capture busy periods and credible concurrent workloads, including startup or recovery conditions relevant to the deployment. An idle reading or monthly average is a weak basis for a capacity limit. Distinguish measured results from modeled estimates, and record which hardware, workload, and power settings produced them.
Keep operating margin separate from growth allowance. One addresses variation in the current deployment; the other reserves capacity for identified additions. Avoid applying an unexplained percentage to every number and accidentally counting the same allowance twice.
Check every circuit, phase, and connection
A rack total can look acceptable while one phase or outlet bank reaches its limit. Vertiv's rack PDU guidance explains how uneven circuit and phase loading can create overloads or leave capacity unusable elsewhere.
Review these items with the facility and deployment team:
- Supply voltage and phase configuration, including equipment input compatibility.
- Approved circuit current, PDU input rating, and internal branch limits.
- Outlet, connector, and cord types and ratings for each device.
- Phase and outlet-bank loading during normal operation and A/B failover.
- Metering coverage, alert thresholds, and ownership of capacity reviews.
Use the approved electrical design and manufacturer documentation for this review. Connection changes and failover validation belong in an agreed facility procedure.
Reserve growth with cooling and deployment in mind
Unused rack units do not establish expansion capacity. For each planned addition, identify the expected demand, power connections, and cooling requirements. Ask the facility to confirm support for the proposed rack density and airflow arrangement, including any specialized cooling the equipment requires.
Define the next expansion step before filling the current allocation: more approved power, another cabinet, or a different equipment layout. Bring that schedule into deployment planning, alongside delivery timing and acceptance criteria. This makes growth a specific project decision rather than an assumption attached to spare outlets.