How the calculation works

A circuit’s capacity follows from its voltage, breaker size and phase:

  • Single-phase: capacity = volts × amps × PF
  • Three-phase: capacity = volts × amps × √3 × PF, where volts is the line-to-line voltage and √3 ≈ 1.732

At a power factor (PF) of 1.0 the result in kW equals the apparent power in kVA, which is how circuits are usually quoted. The calculator then applies two planning limits:

  • 80% for continuous load. A breaker serving a load that runs for hours is commonly limited to 80% of its rating in North America (NEC 210.19 and 210.20), so usable capacity = 0.8 × capacity, and the current limit is 0.8 × the breaker amps.
  • 40% per feed for an A+B pair. Each circuit of a redundant pair must be able to carry the whole load alone, still at 80%, so the total load stays within 80% of one circuit: at most 40% on each feed when the load is shared evenly, as the calculator assumes. With an uneven split or active/standby power supplies, check the total, not the busier feed. The pair supports 80% of one circuit; ordering the B circuit doubles the breakered capacity but not the usable load.

It also shows the panel pole positions one circuit takes on a North American panel: 1 for 120 V or 277 V, 2 for 208 V or 240 V single-phase, 3 for three-phase. Other systems count differently; your provider’s panel schedule is the final word.

These limits are planning practice, not electrical advice: some equipment is rated for 100% continuous load, and rules differ by country and provider.

Worked example

A provider quotes an A+B pair of 208 V 30 A three-phase circuits for a cabinet, at a power factor of 1.0.

  • breaker capacity per circuit: 208 × 30 × 1.732 = 10.81 kW
  • usable at 80%: 0.8 × 10.81 = 8.65 kW, a current limit of 24 A per phase
  • each feed at 40%: 4.32 kW, or 12 A per phase on the PDU in normal operation
  • the pair supports 8.65 kW of load, from 21.62 kW of breakered capacity across both feeds

Compare an A+B pair of 208 V 30 A single-phase circuits:

  • breaker capacity per circuit: 208 × 30 = 6.24 kW; usable at 80% 4.99 kW
  • each feed at 40%: 2.5 kW (12 A); the pair supports 4.99 kW from 12.48 kW breakered

For the same breaker size, three-phase delivers √3 times the power, which is why denser cabinets are usually fed with three-phase circuits. To plan a whole cage of circuits against your racks and contract, use the colocation power calculator.