Circuit breaker sizing equation
This calculator estimates the load current, applies a sizing factor, and rounds up to the next common breaker rating.
I = P / VI = P / (V × PF)I = P / (√3 × V × PF)Ib,min = I × FIb = 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.
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.
FAQ
Why does the calculator round up?
Protective devices are sold in standard current ratings. The tool rounds the calculated minimum to the next common rating.
Does this choose MCB or MCCB?
No. It estimates current rating only. The product family depends on breaking capacity, setting range, application, and standard.