Why are continuous loads often multiplied by 125 percent?

Some electrical rules require branch-circuit and overcurrent-device sizing at 125 percent of a continuous load. The exact definition and requirement depend on the applicable code, equipment, and installation.

Can breaker size be selected without checking wire size?

No. The breaker must protect the conductors and equipment. Conductor material, insulation rating, installation method, ambient temperature, grouping, terminals, voltage drop, and local code must be checked separately.

How is current calculated from three-phase power?

For a balanced three-phase load using line-to-line voltage, current equals real power divided by the square root of three multiplied by line voltage, power factor, and efficiency.

Should motor breaker sizing use only running current?

No. Motors and other high-inrush loads require additional checks for starting current, overload protection, short-circuit protection, trip curve, duty, and manufacturer instructions.

Is the calculated breaker recommendation code compliant?

The result is an educational estimate, not a code-compliance determination. Final selection must be made using the applicable electrical standard, conductor ampacity, equipment data, fault level, coordination, and qualified professional judgement.

Electrical and Energy

Circuit Breaker Size Calculator

Estimate load current, design current, and the next common breaker rating for DC, single-phase AC, or balanced three-phase circuits—while keeping continuous-load and safety margins visible.

Current or power input Continuous-load factor Standard breaker ratings

Enter circuit details

Select the method that matches the information available.

Expected load current measured or calculated

Load-duration factor

Extra design margin optional

Standard rating series

Standard ratings vary by country, product family, voltage class, and breaker type. Always confirm manufacturer availability.

Electrical system

Real load power output/load power

Circuit voltage line-to-line for 3φ

Power factor AC only

Load efficiency η

Load-duration factor

Extra design margin

Standard rating series

Installed or planned breaker rating

Electrical system

Circuit voltage

Power factor AC only

Load efficiency

Load-duration factor

Extra design margin

What does a circuit breaker size calculator do?

This calculator estimates the electrical load current, applies a selectable load-duration factor and optional design margin, then identifies the next common breaker rating. It can start from a measured current, from real power and voltage, or from an existing breaker rating.

A circuit breaker is not chosen from amperes alone. Its job is to protect conductors and equipment against overcurrent and fault conditions. Final selection requires conductor ampacity, installation method, ambient temperature, grouping, terminal temperature ratings, fault-current capability, trip characteristic, load type, voltage, poles, and applicable regulations.

Load currentThe expected operating current before applying continuous-load or design allowances.

Design currentThe adjusted current used to compare the load with a standard protective-device rating.

Breaker ratingThe next listed rating is only a starting point and must not exceed protected conductor limits.

Circuit breaker sizing formulas

The power mode first estimates operating current. It then applies the selected duration and margin factors before choosing a common rating.

DC load currentI = P ÷ (V × η)

Single-phase ACI = P ÷ (V × PF × η)

Balanced three-phase ACI = P ÷ (√3 × VL × PF × η)

Design-current formula

Design current = load current × load-duration factor × margin factor. A 125% option is provided because it is commonly encountered for continuous-load sizing, but the correct rule depends on the governing standard and the equipment involved.

Power factor and efficiency are differentPower factor describes the relationship between real and apparent power in AC systems. Efficiency accounts for conversion losses. Both can increase the input current needed for a given output power.

How to use the calculator

Choose a methodStart with measured current, known power, or an existing breaker rating.

Enter system valuesFor power calculations, select DC, single-phase, or balanced three-phase.

Apply load factorsChoose the load-duration factor and add margin only when justified.

Verify the installationCheck conductors, terminals, fault rating, trip curve, and local rules before selection.

Example breaker calculations

Example	Operating current	Factor	Design current	Possible next rating*
18 A continuous load	18 A	125%	22.5 A	25 A
3.5 kW, 230 V, PF 0.90, η 95%	17.8 A	125%	22.2 A	25 A
15 kW, 400 V 3φ, PF 0.85, η 92%	27.7 A	125%	34.6 A	35 A
24 A non-continuous estimate	24 A	100%	24 A	25 A

*Illustrative common ratings only. Availability and permitted selection vary by device series, country, conductor, equipment, and applicable electrical standard.

Important checks before selecting a breaker

Conductor ampacityThe breaker must protect the cable after all applicable correction and derating factors.

Interrupting ratingThe breaker must safely interrupt the available prospective fault current at its installation point.

Trip characteristicMotors, transformers, capacitors, heaters, and electronic loads may need different trip behaviour.

Temperature and groupingAmbient heat, enclosure conditions, and grouped conductors can reduce usable conductor capacity.

Voltage and polesBreaker voltage, number of poles, AC/DC suitability, and system earthing must match the circuit.

Manufacturer dataUse equipment nameplates, installation instructions, and coordination tables where applicable.

Frequently asked questions

How do I calculate circuit breaker size from load current?

Multiply the expected load current by the selected load-duration factor and any justified design margin. Then compare that design current with available standard ratings while ensuring the breaker does not exceed the protected conductor's permitted ampacity.

Why does the calculator offer a 125% continuous-load factor?

A 125% factor is commonly used in some rules for loads expected to operate continuously. The legal definition of a continuous load and the required sizing method depend on the applicable electrical code, circuit type, and equipment.

Can I choose the next larger breaker automatically?

The next common rating is a useful estimate, but it is not automatically acceptable. The conductor, terminals, equipment protection requirements, special load rules, fault level, and local code can require a different rating or protection arrangement.

Does this calculator determine wire size?

No. Wire sizing requires conductor material, insulation type, installation method, ambient temperature, grouping, length, voltage drop, terminal ratings, fault withstand, and local rules. The displayed design current is only one input to that process.

How should I size a breaker for a motor?

Motor circuits often use separate rules for conductor sizing, overload protection, and short-circuit or ground-fault protection. Running current alone is not enough because starting current and acceleration time can cause nuisance tripping.

What voltage should I use for a three-phase calculation?

This calculator uses line-to-line voltage in the balanced three-phase formula. Enter the system line voltage and the total three-phase real power, not per-phase power.

Is a breaker rated at 100% of its current usable continuously?

Not always. Continuous loading rules depend on the breaker listing, enclosure, equipment, conductor, and governing electrical standard. Some assemblies are specifically rated for 100% continuous operation under stated conditions, while many applications use a lower continuous loading limit.

Electrical safety reminderThis calculator provides educational estimates only. Incorrect breaker selection can cause shock, fire, equipment damage, nuisance tripping, or failure to clear a fault. Final design and installation should be verified by a qualified electrical professional under the applicable standards.