Use this free breaker calculator to estimate the right circuit breaker size from watts or amps. It supports single-phase and three-phase systems, continuous-load adjustment, and design margin so you can pick the next standard breaker rating confidently.
Important: This calculator is an estimate tool. Final design must comply with your local electrical code, equipment nameplate data, conductor ampacity, temperature correction, and installation method.
A breaker calculator is a tool used to estimate the correct circuit breaker rating for an electrical load. It helps convert load information—such as watts, voltage, phase type, and power factor—into current, then applies safety and design adjustments to recommend a standard breaker size. In practical terms, this means you can quickly estimate whether a 20A, 30A, 60A, or larger breaker is appropriate for a specific circuit.
Choosing the right breaker is not just about preventing nuisance tripping. The primary purpose of a breaker is protection: it interrupts dangerous overcurrent conditions before conductors overheat or equipment is damaged. A proper breaker selection process should always include conductor sizing, insulation temperature ratings, ambient conditions, and the specific code rules where the installation is located.
The calculator on this page follows a practical sizing workflow used by many electricians, technicians, and designers during initial planning:
Single-phase current: I = P / (V × PF)
Three-phase current: I = P / (√3 × V × PF)
Design current: Idesign = I × Continuous Factor × (1 + Margin)
Once the design current is known, the breaker rating should be selected from available standard sizes at or above that current.
Single-phase and three-phase systems carry power differently. For equal power and voltage classes, three-phase systems generally draw less current per phase, which often results in different breaker and conductor requirements.
For example, a 15 kW load at 400 V three-phase with PF 0.9 draws significantly less line current than an equivalent single-phase arrangement. This is why entering the correct phase type in a breaker calculator is critical.
When in doubt, check equipment nameplates and one-line diagrams. Many commercial motors, compressors, and HVAC systems are three-phase, while most residential branch circuits are single-phase.
Continuous loads can run for long periods and produce sustained thermal stress in wiring and overcurrent devices. Applying the 125% factor is a common requirement under many electrical standards to ensure circuits remain safe under prolonged operation.
If your load is non-continuous, applying 125% may be unnecessary; if it is continuous and you skip this factor, you may choose an undersized breaker that trips frequently or operates too close to thermal limits. Always verify what counts as continuous load in your local code and project specification.
Breakers are manufactured in discrete ratings. If your calculated design current is 42A, you usually do not choose a 42A breaker—you select the next available standard size, such as 45A or 50A depending on product availability, code allowances, and conductor ampacity.
| Calculated Design Current | Typical Selection Approach | Example Standard Rating |
|---|---|---|
| Up to 20A | Choose next standard rating not below design current | 20A |
| 20.1A to 30A | Move to next available size | 25A or 30A |
| 30.1A to 50A | Evaluate conductor and equipment limits | 35A, 40A, 45A, or 50A |
| 50A to 100A+ | Coordinate with cable size, fault level, and application type | 60A, 80A, 100A, etc. |
Breaker selection is never isolated from wire size. A larger breaker does not automatically mean a better or safer installation. The breaker must protect the installed conductor and match equipment requirements.
Suppose a heater is rated 4,500 W at 230 V and PF is close to 1.0. Current is 4,500 / (230 × 1.0) ≈ 19.57 A. If treated as continuous: 19.57 × 1.25 = 24.46 A. Add 10% margin: 26.91 A. Next standard breaker is typically 30A.
For a 12,000 W load at 400 V, PF 0.9: current ≈ 12,000 / (1.732 × 400 × 0.9) ≈ 19.25 A. If continuous and adding 15% margin: 19.25 × 1.25 × 1.15 ≈ 27.66 A. Standard selection generally moves to 30A.
If equipment nameplate shows 47 A and it is continuous with 0% extra margin: 47 × 1.25 = 58.75 A. Next standard breaker is 60A. If your conductor cannot support this selection under installation conditions, redesign is required.
Residential: branch circuits, water heaters, electric ranges, mini-split systems, EV charger planning.
Commercial: lighting panels, HVAC feeders, kitchen equipment, office floor distribution.
Industrial: motor branch circuits, process equipment, control panels, machinery feeders.
The same foundational principles apply across all sectors, but equipment characteristics and code details become more strict as system complexity increases.
Use this breaker calculator as a planning and educational tool. Before installation, confirm compliance with applicable local regulations and manufacturer instructions. Validate conductor size, insulation temperature rating, correction factors, interrupting rating (AIC), panel compatibility, and coordination with upstream/downstream protection devices.
Not automatically. A larger breaker can compromise conductor protection if wire size is not increased accordingly. Frequent tripping should be diagnosed, not bypassed.
No. It estimates breaker rating only. Wire size must be selected separately based on ampacity tables, temperature rating, installation method, and local code.
If code and equipment allow, that exact standard size may be acceptable. Still verify continuous-load requirements and conductor compatibility.
When calculating current from real power (watts), yes. If current is directly known from nameplate or measurement, PF is not needed for the current conversion step.