Hong Life CalculatorsUnderstanding the circuit breaker size calculator — concept and principle
This calculator estimates the minimum circuit breaker rating for a load under the US National Electrical Code rule for continuous and non-continuous loads, then rounds up to the next standard breaker size. It handles DC, single-phase and three-phase supplies.
How it is calculated
- Load current: DC I = P ÷ V; single-phase I = P ÷ (V × PF); three-phase I = P ÷ (√3 × V × PF).
- Minimum rating = 125% of the continuous load + 100% of the non-continuous load (a continuous load runs at full current for 3 hours or more).
- Round up to the next standard rating of NEC 240.6(A): 15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100, 110, 125, 150, 175, 200 A and so on.
- Breaker loading = load current ÷ breaker rating.
The principle
A thermal-magnetic breaker heats up with current. Standard breakers in an enclosure are not rated to carry their full nameplate current continuously, so the code requires a 25% margin for loads that run for hours. Requiring 125% of the continuous current is the same as loading a breaker to no more than 80%. The breaker protects the wire, so the wire must also be rated for the chosen breaker.
Worked example
Default inputs: 3,600 W, single-phase, 240 V, power factor 1, continuous load.
- Load current = 3,600 ÷ (240 × 1) = 15 A.
- Minimum rating = 15 × 1.25 = 18.75 A.
- Next standard size = 20 A.
- Breaker loading = 15 ÷ 20 = 75%.
- Also check: the wire must be rated for the breaker (in this case a conductor rated for 20 A).
| Voltage (V) | Standard breaker size (A) |
|---|---|
| 60 | 80 |
| 120 | 40 |
| 180 | 25 |
| 240 | 20 |
| 300 | 15 |
| 360 | 15 |
| 480 | 15 |
How to read the result
The result is the smallest standard breaker allowed for the load; a larger breaker is not "safer" unless the wire is upsized too, because the breaker must protect the conductor. Motors, heat pumps, air conditioners, welders and other special equipment follow their own rules and nameplate data (for example a maximum overcurrent protection rating).
Common mistakes
- Forgetting the 125% factor for continuous loads.
- Choosing a breaker larger than the wire's ampacity.
- Ignoring power factor for motor loads.
- Applying NEC rules in countries that use IEC or KEC standards with different rating series.
Frequently asked questions
What size breaker do I need for 15 A continuous?
Under the NEC, 125% × 15 = 18.75 A, so a 20 A breaker.
Why only 80% loading?
Standard breakers are not rated to carry 100% of their rating continuously; the 125% rule keeps them at 80% or less.
Can I use a bigger breaker to stop nuisance trips?
Not unless the wire and equipment are rated for it; an oversized breaker may not protect the wire from overheating.
Sources: NFPA 70, National Electrical Code, sections 210.20(A) and 240.6(A) · 한국전기설비규정(KEC), 저압 전기설비 과전류 보호