Example 1: Single-phase water heater
Given 5000 W, 230 V, PF = 1.00, and 125% design margin:
I = 5000 / (230 x 1.00) = 21.74 A
Idesign = 21.74 x 1.25 = 27.17 A
The next practical breaker size is 32 A.
Breaker Size Calculator
Use this online breaker size calculator to estimate electrical load current and select a practical MCB or circuit breaker size from power, voltage, phase, power factor, and design margin.
This is a planning estimate. Final breaker selection should also consider cable size, ambient temperature, coordination, fault level, and local code rules.
Introduction
A breaker size calculator helps you move from electrical load to a practical protective device rating. The main goal is to estimate the current drawn by the load, apply a design margin for continuous operation, and then choose the next standard breaker rating above that current.
This circuit breaker sizing calculator is useful for electrical panels, branch circuits, motors, heaters, and general power distribution design. It gives a fast estimate for both single-phase and three-phase systems so you can shortlist an appropriate breaker before checking cable ampacity, coordination, and local electrical code.
For practical electrical work, this helps users move quickly from load information to an initial breaker rating that can then be reviewed alongside conductor size, protection curves, and installation requirements.
Formula
Single phase current: I = P / (V x PF)
Three phase current: I = P / (sqrt(3) x V x PF)
Design current: Idesign = I x Margin
Three phase current: I = P / (sqrt(3) x V x PF)
Design current: Idesign = I x Margin
Variable Definitions
| Variable | Meaning | Typical Unit |
|---|---|---|
| P | Active load power | W |
| V | Supply voltage | V |
| PF | Power factor of the load | Unitless |
| I | Calculated load current | A |
| Idesign | Current after applying design margin | A |
Units
Enter load power in watts, voltage in volts, and power factor as a decimal between 0 and 1. The final breaker recommendation is shown in amperes.
If you are sizing a breaker for residential or industrial equipment, keep all values in matching units and use a realistic power factor for motors, compressors, and inductive loads.
Worked Examples
Example 2: Three-phase motor load
Given 15000 W, 400 V, PF = 0.90, and 125% design margin:
I = 15000 / (sqrt(3) x 400 x 0.90) = 24.06 A
Idesign = 24.06 x 1.25 = 30.08 A
A practical next standard breaker rating is 32 A.
Practical Breaker Sizing Notes
In real electrical design, breaker sizing is not based on load current alone. You should also check conductor ampacity, installation method, ambient temperature, motor starting current, short-circuit rating, and protective coordination with upstream and downstream devices. This online breaker size calculator gives a strong first estimate, but final selection should always be verified against the applicable electrical standard and the actual characteristics of the connected equipment.