Circuit Breaker Size Calculator
Convert a single-phase or three-phase real-power load into calculated current, apply a continuous-load planning multiplier, move to the next included common ampere rating, and screen that rating against entered conductor ampacity, equipment maximum overcurrent protection, and interrupting capacity.
Not an installation approval. Circuit-specific NEC articles, equipment nameplates, conductor rules, listing instructions, fault-current studies, local adoption, and the authority having jurisdiction can change the permitted overcurrent device.
Breaker planning result
The 40 A planning rating is at or below both entered ampacity limits, and the entered interrupting rating exceeds available fault current. Complete the governing NEC and equipment checks.
The result follows a four-part coordination sequence
Convert power to current
Single-phase current uses real input power divided by voltage and power factor. Three-phase current also divides by the square root of three. Efficiency converts output load power to required electrical input before the current is calculated.
Apply the load treatment
The continuous scenario multiplies calculated current by 125 percent. The noncontinuous scenario leaves it at 100 percent. Real circuits can combine load types or have article-specific rules, so one switch cannot classify every installation.
Move to a rating
The calculator selects the first ampere value at or above design current from its included planning ladder. That is a candidate for review, not permission to use the next size up under every conductor, load, or equipment rule.
Check the entered ceilings
The candidate is compared with conductor ampacity and the equipment maximum OCPD. Its interrupting rating is also compared with available fault current. A failed comparison is a stop signal, while a pass only means these entered values are arithmetically coordinated.
Ampere rating is not interrupting rating
A 40 A breaker’s ampere rating describes its load-current function. A 10 kA interrupting rating describes the fault current it is designed to interrupt under specified conditions. The numbers answer different questions and cannot substitute for one another.
Trip behavior is not one point
Thermal-magnetic and electronic breakers have time-current behavior, tolerances, instantaneous regions, temperature effects, and application constraints. This calculator does not draw a trip curve or prove coordination with upstream and downstream devices.
Worked example: a 7.2 kW, 240 V single-phase load
The default scenario assumes 7.2 kW of real load power, 240 V, unity power factor, and 100 percent efficiency. Dividing 7,200 W by 240 V gives 30.00 A. Selecting the continuous treatment multiplies 30 A by 1.25, producing 37.50 A design current.
The first included common rating at or above 37.50 A is 40 A. That leaves 2.50 A between the planning rating and the design current, so the design current uses 93.75 percent of the rating. The entered conductor ampacity and equipment maximum OCPD are both 50 A, leaving 10 A between each ceiling and the 40 A candidate.
Available fault current is entered as 5 kA, while the proposed breaker interrupting rating is 10 kA. The simple comparison shows 5 kA of numerical margin. It does not account for series ratings, current-limiting effects, motor contribution, marked combinations, or the complete fault-current study. Those are system-level decisions.
A real 7.2 kW appliance may provide a minimum circuit ampacity and maximum overcurrent-protection value on its nameplate, or it may fall under an article with different rules. In that case, the marked or governing method takes precedence over this generic scenario.
What the included rating ladder means
The calculator searches this planning sequence: 15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100, 110, 125, 150, 175, 200, 225, 250, 300, 350, 400, 450, 500, 600, 700, 800, 1,000, 1,200, 1,600, and 2,000 A. It is a convenience list for scenario comparison, not a complete reproduction of every standard rating, device family, frame, trip unit, fuse, molded-case, power-breaker, or manufacturer offering.
When a calculated design current falls between two ladder values, the higher value is shown so the planning device is not below the modeled design current. That arithmetic does not confirm the connected conductor can be protected at that value. The separate conductor input makes the conflict visible, but the entered ampacity must itself come from a valid complete conductor calculation.
Four independent limits to verify
| Limit | What it protects or describes | Required project evidence |
|---|---|---|
| Calculated load and continuous duty | Establishes the current the circuit is expected to carry under the governing load calculation. | NEC article, demand factors, nameplate values, duty cycle, phases, voltage, power factor, and efficiency. |
| Conductor ampacity | Defines the current the conductor can carry after applicable table selection, temperature correction, adjustment, and terminal limits. | Material, insulation, size, installation method, ambient temperature, grouping, termination ratings, and local rules. |
| Equipment maximum OCPD | Limits the protective device permitted for listed or labeled equipment. | Nameplate, installation instructions, listing conditions, manufacturer data, and applicable equipment article. |
| Interrupting rating | Describes the device’s ability to interrupt fault current at its application voltage and conditions. | Available fault-current study, system voltage, device rating, marked series combination where used, and contribution from relevant sources. |
OSHA’s construction wiring rule requires conductors and equipment to be protected from overcurrent according to their ability to conduct current safely and requires sufficient conductor ampacity for the load. That safety principle is why the breaker, conductor, and equipment cannot be sized as isolated parts.
Loads that need a specific method
Motors, HVAC, transformers, and welders
Motor branch circuits separate conductor sizing, overload protection, and short-circuit/ground-fault protection. HVAC equipment often supplies minimum circuit ampacity and maximum overcurrent protection on the nameplate. Transformers and welders have dedicated provisions. A generic 125 percent multiplier cannot replace those methods.
EV charging, energy storage, and renewable systems
Electric-vehicle supply equipment can be a continuous load and has dedicated requirements. Photovoltaic, battery, inverter, and bidirectional systems involve source circuits, continuous currents, conductor corrections, equipment ratings, and rapid-shutdown or disconnect provisions. Follow the governing article and listed instructions.
Each result should remain traceable to its own assumptions.
A safer breaker-selection workflow
- Identify the governing load article. Determine whether the circuit is general, continuous, motor, HVAC, transformer, EV, renewable, or another specific application.
- Calculate load current correctly. Use nameplate and code methods, including phase, voltage, efficiency, power factor, demand, and duty where applicable.
- Size and verify conductors. Apply ampacity, correction, adjustment, terminal, small-conductor, voltage-drop, and installation rules.
- Apply equipment limits. Observe marked minimum circuit ampacity, maximum OCPD, fuse-only or breaker-only instructions, and permitted device types.
- Determine available fault current. Include utility/source data and contributions appropriate to the system study.
- Choose a listed device. Confirm voltage, poles, frequency, interrupting rating, enclosure, environmental rating, terminals, conductor range, and application.
- Coordinate and document. Evaluate selective coordination, ground-fault protection, arc-energy implications, labeling, permits, and local inspection requirements.
Frequently asked questions
Should a continuous load use 125 percent?
The calculator offers 125 percent as a general continuous-load planning scenario, but the complete NEC calculation can combine continuous and noncontinuous portions or follow equipment-specific rules. Confirm the load classification and governing section rather than labeling every long-running device the same way.
Can I put a 40 A breaker on any 40 A load?
No. The calculated load treatment may require more than 40 A, and conductor ampacity, terminal ratings, equipment maximum protection, circuit type, receptacles, and other rules may control. Interrupting rating must also be adequate for available fault current.
Why does power factor affect current?
Real power is only part of apparent power in an AC load with power factor below one. For the same real kW and voltage, lower power factor requires more RMS current. The calculator divides by power factor, so current increases as the entered value decreases.
What is the difference between kAIC and ampere rating?
Ampere rating relates to normal load and overcurrent operation. Interrupting rating, often expressed in kA, addresses the maximum prospective fault current the device can interrupt under stated conditions. A high ampere rating does not guarantee adequate interrupting capacity, and vice versa.
Does this calculator model breaker trip curves?
No. It chooses an ampere rating from a planning ladder and compares entered limits. It does not model thermal memory, magnetic pickup, instantaneous trip, electronic settings, tolerances, ambient effects, inrush, coordination, or manufacturer-specific time-current curves.
What if the suggested rating exceeds the equipment maximum?
The calculator flags the conflict. Do not simply use the larger breaker or reduce the input to make the flag disappear. Recheck the load method, equipment instructions, circuit architecture, and conductor design with a qualified professional. The equipment may require a different circuit or device.
References
- National Fire Protection Association, NFPA LiNK available publications: 2026 NFPA 70, National Electrical Code
- National Fire Protection Association, Errata No. 70-26-1 for the 2026 edition of NFPA 70
- Occupational Safety and Health Administration, 29 CFR 1926.404, Wiring Design and Protection
Educational planning only. Work on electrical systems can cause fire, shock, arc flash, injury, or death. Use the locally adopted code, permits, listed equipment instructions, qualified professionals, and required safe-work practices.