Breaker Size Calculator

Written by Great Medico

Updated September 25, 2026 •2 min read

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When you need a quick breaker size check, the useful number is the one you can reproduce. Circuit sizing becomes much clearer when the ideal equation is separated from practical constraints such as ripple, temperature, tolerances, surge, and component ratings. The goal is to make the equation, inputs, and result easy to sanity-check rather than hide them behind a black box.

What this calculator does

This calculator focuses on recommended breaker size from Voltage, Quantity, Wattage. It is designed for quick estimation and hand-checking, with the visible fields defining the scope of the model rather than implying a larger simulation.

How to use it

Start with Voltage, Quantity, Wattage. Let the unit controls handle supported conversions instead of converting values mentally. Set Current type to match the solve path you want. If the page offers both inputs and derived fields, fill the quantities you know and leave result fields for the calculator.

How the calculation works

Total connected wattage is the sum of quantity × wattage for up to five appliance rows. Load current is P/(kV), the page multiplies that by 1.25, then chooses the next value from its built-in standard breaker list.

Example

Suppose the appliance rows total 1,200 W on a 120 V single-phase supply. The page calculates 10 A load current, applies 125% to get 12.5 A, then selects 13 A as the next value in its built-in list. That is only the calculator’s planning rule, not a universal code prescription.

How to interpret the result

The recommended breaker size belongs to the ideal relationship shown above. Keep voltage, current, frequency, impedance, duty cycle, and component units consistent, then compare the result with real component limits rather than using it as the limit itself.

Limitations and notes

The 125% multiplier and built-in breaker list are only a planning shortcut. Real breaker and conductor sizing depends on continuous-load rules, conductor ampacity, temperature ratings, motor/HVAC provisions, local electrical code, and equipment instructions. Appliance type presets do not substitute a wattage automatically.