Rack Power Draw Calculator
Size the electrical supply for a server rack and check how hard it leans on its circuit. Give the rack's total IT load, the supply voltage and the breaker rating, and the tool returns the current the rack draws, how much of the circuit that uses, and the headroom to the 80% continuous-load limit that data-centre and electrical best practice recommends.
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Before you rely on this: First-pass guide only. Verify safety-critical or regulated work against the relevant standards, your project requirements and a qualified professional.
How to use this calculator
- Enter the total IT load of the rack in watts — add up the measured or nameplate power of every server, switch and appliance in it.
- Enter the supply voltage (default 230 V) and the circuit / breaker capacity in amps (default 16 A).
- Read the current draw, the circuit utilisation and the headroom, and check the verdict for whether you are inside the 80% limit.
How it works
Rack current is the total IT load divided by the supply voltage — a single-phase estimate. Circuit utilisation expresses that current as a percentage of the breaker's rated capacity. Because a continuous load must not exceed 80% of a breaker's rating, the tool computes the 80% derated limit (0.8 × capacity) and reports the remaining headroom. Above 80% the rack is flagged for review; above 100% it is an overload that will trip the breaker.
Worked example
Worked example. A rack pulls 3000 W from a 230 V, 16 A circuit. Current = 3000 ÷ 230 = 13.043 A, which is 13.043 ÷ 16 × 100 = 81.52% of the circuit. The 80% limit is 12.8 A, so the rack is 0.24 A over the derated limit and should be reviewed before adding any more equipment.
Common mistakes
- Sizing to the breaker's full rating instead of 80% — a continuous IT load must be derated, so a 16 A circuit is really good for about 12.8 A.
- Using nameplate power for every device, which sums to far more than the real draw; measured or typical figures give a more useful number.
- Ignoring redundancy — a rack fed from A and B supplies must still survive on one feed, so each feed has to carry the whole load within its own 80% limit.
Frequently asked questions
Why the 80% rule?
Breakers are rated for continuous duty at 80% of their trip rating; running closer to 100% for hours risks nuisance trips and heat. Keeping continuous IT load at or below 80% builds in that margin.
My rack is three-phase — is this still valid?
This is a single-phase estimate (load ÷ voltage). For a three-phase rack the per-phase current is lower and involves a √3 factor and phase balancing, so treat this as a conservative single-phase check and size three-phase feeds separately.
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