Rack Power
Calculate total power draw and headroom for equipment installed in a server rack.
About Rack Power
Total rack power draw in watts converts to current draw (amps) via Ohm's law (amps = watts / volts), but the number of circuits needed isn't simply total amps divided by breaker rating — electrical codes such as the NEC require continuous loads to be derated to 80% of a breaker's rating to prevent nuisance trips and heat buildup from sustained near-capacity operation. Sizing circuits at the full derated capacity avoids both under-provisioning (risking trips under normal load) and unnecessary over-provisioning of PDU/circuit infrastructure.
Formulas
Total current draw
amps = watts / voltage
Circuits needed
circuits = ceil(total_watts / (breaker_amps × derating% × voltage))
Worked example
With the default values below, Rack Power returns Circuits Needed (at derated capacity) of 1. Change any input above and every figure here is recalculated in your browser.
Inputs used
- Total Device Power Drawminimum 0 W
- 4,000 W
- Circuit Voltageone of 120V (NA standard), 208V (NA 3-phase derived), 230V (International standard)
- 208V (NA 3-phase derived)
- Circuit Breaker Ratingminimum 1 A
- 30 A
- Continuous Load Deratingaccepts 50–100
- 80
Results
- Circuits Needed (at derated capacity)
- 1
- Total Current Draw
- 19.23 A
- Usable Power per Circuit (derated)
- 4,992 W
- Usable Current per Circuit (derated)
- 24.00 A
Step by step
- 1Values used
- 2Total current draw
- 3Circuits needed
- 4Circuits Needed (at derated capacity)= 1
- 5Total Current Draw= 19.23 A
- 6Usable Power per Circuit (derated)= 4,992 W
- 7Usable Current per Circuit (derated)= 24.00 A
Assumptions and limits
- The result is derived only from the 4 inputs listed above. Anything not among them is not part of the calculation.
- In particular, this calculator does not take inflation, tax or compounding frequency into account.
- Figures are rounded for display to 2 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.
How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.
Frequently Asked Questions
Why does electrical code require an 80% derating for continuous loads?
Circuit breakers and conductors heat up under sustained load, and running at 100% of rated capacity continuously (as opposed to a brief peak) increases the risk of nuisance tripping and long-term thermal stress — most electrical codes (e.g. NEC 210.19 and 210.20 in the US) require continuous loads to be sized at no more than 80% of the breaker/conductor rating, which is why data center power planning almost always references this 80% figure.
Should I use nameplate power ratings or measured power draw for this calculation?
Nameplate ratings represent worst-case maximum draw and are typically used for conservative circuit sizing and safety compliance, while measured/typical power draw (from actual utilization under real workloads) is more useful for day-to-day capacity and cost planning — for circuit and breaker sizing specifically, err toward nameplate or a well-tested peak measurement to avoid under-provisioning.
Why use 208V instead of 120V for high-density racks?
Higher voltage delivers more power through the same current, so a 208V circuit at a given amperage carries roughly 73% more power than a 120V circuit at the same amperage — this lets high-density racks (common in modern data centers) draw substantial power without requiring excessive numbers of circuits or larger, more expensive conductors.