EngineerHub.orgDrawing No. EH–EE–005 // Electrical Engineering
Circuit Load Calculator
Reviewed August 2026
Estimate operating current, design current, and a recommended breaker size for a single- or three-phase circuit.
Drawing No. EH–EE–005 // Electrical Engineering
Reviewed August 2026
Estimate operating current, design current, and a recommended breaker size for a single- or three-phase circuit.
A 3.6 kW single-phase load at 230 V with a power factor of 0.95.
I = P / (V × pf) three-phase: I = P / (√3 × VL × pf)
I = 3600 / (230 × 0.95) = 16.5 A. Treated as a continuous load, the design current is 16.5 × 1.25 = 20.6 A, so a 25 A protective device is the first standard size above it.
The same 3.6 kW as a three-phase load at 400 V draws 3600/(√3 × 400 × 0.95) = 5.5 A per line — roughly a third of the single-phase current, which is why larger loads are taken three-phase.
Points that come up most often with this calculation.
Most wiring rules require protective devices and conductors for a load running three hours or more to be rated at 125 % of the load current, giving thermal headroom. Check the multiplier your adopted code actually specifies.
Current is inversely proportional to power factor. The same real power at pf 0.8 rather than 0.95 draws about 19 % more current, and cables and breakers must be sized for that current, not for the kilowatts.
No. It gives design current only. Cable size depends additionally on installation method, grouping, ambient temperature, permitted voltage drop and circuit length — use the cable sizing and voltage drop calculators alongside this one.
Protective devices come in standard sizes, so you take the next size up. The conductor must then be rated at or above that device, not merely above the load.
Include the inputs and assumptions used.