Circuit breaker sizing equation
This calculator estimates the load current, applies a sizing factor, and rounds up to the next common breaker rating.
Ib = next standard size ≥ Ib,minwhere:
- I
- Calculated load current[A]
- P
- Load power converted to watts[W]
- V
- System voltage[V]
- PF
- Power factor for AC loads
- F
- Sizing factor, entered as a percentage and converted to a multiplier
- Ib,min
- Minimum calculated breaker current rating[A]
- Ib
- Recommended standard breaker current rating[A]
Current rating is only one part of breaker selection. Trip curve, short-circuit breaking capacity, cable protection, selectivity, and product standard must also be verified.
How to calculate circuit breaker size
Breaker current sizing begins by converting load power to current for DC, single-phase AC, or three-phase AC. The calculator multiplies load current by the selected sizing factor and rounds upward to the next listed standard current rating.
- Select the electrical system type.
- Enter load power, voltage, and AC power factor.
- Apply the project sizing or continuous-load factor.
- Round up to a standard breaker current rating.
- Verify cable protection, trip characteristic, breaking capacity, and selectivity.
Circuit breaker sizing example
A balanced three-phase 15 kW load at 400 V and 0.85 power factor draws about 25.5 A. Applying a 125% sizing factor gives a minimum rating of about 31.8 A, so the calculator selects the next listed size of 32 A.
I = 15,000 / (√3 × 400 × 0.85) = 25.5 A
Ib,min = 25.5 × 1.25 = 31.8 A → 32 A
| Input or result | Value |
|---|---|
| Load | 15 kW three-phase |
| Voltage | 400 V |
| Power factor | 0.85 |
| Sizing factor | 125% |
| Recommended current rating | 32 A |
Breaker current rating is not breaking capacity
The ampere rating describes normal-current and overload behavior. Breaking capacity describes the prospective short-circuit current the device can interrupt at a stated voltage and standard. Both must be adequate, and they cannot be inferred from each other.
| Breaker characteristic | Selection question |
|---|---|
| Rated current or setting | Can it carry the load and protect the conductor? |
| Trip curve or protection settings | Will it coordinate with starting current and downstream devices? |
| Breaking capacity | Can it interrupt the available fault current? |
| Poles and voltage | Does it match the circuit and isolation requirements? |
MCB vs. MCCB selection after sizing
Current alone does not decide whether a miniature circuit breaker or molded-case circuit breaker is appropriate. Required breaking capacity, adjustable protection, frame size, coordination, mounting, standard, and application determine the product family.
Coordinate breaker and cable sizing
The selected breaker must protect the downstream conductor while carrying the design load. Confirm the relationship between design current, breaker rating or setting, and corrected cable ampacity, then check available fault current.
Cable Size CalculatorShort Circuit Current CalculatorContact VIOX
Assumptions
- Standard rating list is used for preliminary selection
- Trip curve, breaking capacity, coordination, and local code are not selected automatically
Important Warnings
- Do not select a breaker from current alone. Verify cable ampacity, short-circuit current, trip curve, and equipment standard.
- Continuous-load rules differ by market and application.