How the calculation works

Each circuit. A circuit’s capacity is its voltage × breaker amps, × √3 (about 1.732) for three-phase, × the power factor (PF):

  • Breaker capacity = V × A (× √3) × PF
  • Usable at 80% = 0.8 × breaker capacity. In North America a breaker serving a continuous load is commonly limited to 80% of its rating (NEC 210.19 and 210.20), and colocation providers typically apply the same limit.
  • A+B pairs: with redundant feeds, either circuit must be able to carry the whole load if the other fails, so the pair’s total load has to stay within 80% of one circuit. With the load shared evenly between the feeds, that is at most 40% on each. If your equipment splits its load unevenly, or runs active/standby, check the total against 80% of one circuit rather than the reading on either feed.

For three-phase circuits the voltage is the line-to-line voltage (208 V on a 208Y/120 V system). The default power factor is 1.0, which makes kW equal kVA; enter your equipment’s power factor to see kW at that load.

The plan. Each circuit row has a count (circuits, or pairs for an A+B row), voltage, breaker size, phase and feeds. The calculator totals:

  • Breakered capacity: every breaker at 100%, B feeds included, because you order and usually pay for both
  • Usable with every feed at 80%: what the circuits carry with no redundancy kept
  • Usable with A+B redundancy: A+B pairs at 80% of one circuit each; on a plan that mixes in single circuits, those count at 80% and are shown separately, because they are not redundant

The racks. Device rows describe one typical rack: quantity × watts each. Critical load per rack is their sum, and the cage load is that × the number of racks. If your racks differ, run the calculator once per rack type. Circuits per rack = rack load ÷ usable capacity of one circuit of the first row’s type, rounded up; for an A+B first row the result is in pairs.

The contract. Critical load ÷ the metered kW commitment gives the share of the commitment used, and the commitment minus the load is the headroom. If the usable capacity of the circuits is below the commitment, the result says so. Stranded capacity is breakered capacity minus critical load. Pole positions follow a North American panel: three-phase circuits use 3, 208 V and 240 V single-phase use 2, and 120 V and 277 V single-phase (line-to-neutral) use 1. Other systems count differently (a 230 V line-to-neutral circuit on a 400/230 V panel, for example, is counted here as 2), so check the count against your provider’s panel schedule.

The 80% and 40% limits are planning practice, not electrical advice. Rules differ by country, code and provider; confirm the limits in your contract and with your provider.

Worked example

A cage of 6 racks, each with 16 servers at 450 W (measured) and 2 switches at 150 W, fed by 6 A+B pairs of 208 V 30 A three-phase circuits (one pair per rack). The contract has a 65 kW metered commitment and the panel has 44 pole positions. Power factor 1.0.

Circuits:

  • one circuit: 208 × 30 × 1.732 = 10.81 kW breakered; usable at 80% = 8.65 kW; each feed at 40% = 4.32 kW
  • 6 pairs = 12 circuits: breakered capacity 12 × 10.81 = 129.7 kW
  • every feed at 80%: 12 × 8.65 = 103.76 kW
  • usable with A+B redundancy: 6 × 8.65 = 51.88 kW

Load:

  • per rack: 16 × 450 + 2 × 150 = 7.5 kW, so 1 A+B pair per rack (7.5 kW ÷ 8.65 kW, rounded up)
  • 6 racks: 45 kW, which is 86.7% of the 51.88 kW redundant capacity, leaving 6.88 kW of headroom

Contract:

  • 45 kW is 69.2% of the 65 kW commitment, with 20 kW of headroom to it
  • but the circuits deliver only 51.88 kW with redundancy, 13.12 kW below the commitment: to grow to 65 kW with A+B kept, the cage needs more or larger circuits
  • poles: 12 three-phase circuits use 36 of 44 positions, leaving 8, room for 1 more A+B pair (6 positions); 44 positions hold at most 14 three-phase circuits
  • stranded capacity: 129.7 − 45 = 84.7 kW breakered that the racks do not draw