Breaker Size Calculator
Calculate the correct circuit breaker size for DC, AC single-phase, and AC three-phase circuits using total wattage, voltage, and power factor with a 125% safety derating factor per NEC guidelines.
About This Calculator
The Breaker Size Calculator helps electricians, homeowners, and DIY enthusiasts determine the correct circuit breaker rating for any electrical circuit. Whether you're wiring a new room, installing a microwave, adding a dishwasher, or setting up industrial three-phase equipment, choosing the right breaker size is critical for safety and code compliance.
This calculator supports DC, AC single-phase, and AC three-phase systems. Simply select your current type, enter the system voltage (120V for US households, 230V for EU, UK, India, and Australia), add the power factor for AC circuits, and input the wattage of up to three appliances on the circuit. The calculator computes the total load current using the standard electrical formulas: I = W/V for DC, I = W/(V × PF) for AC single-phase, and I = W/(V × PF × √3) for AC three-phase. It then applies the NEC 125% safety derating factor (multiplying the load current by 1.25) and selects the nearest standard breaker size.
Standard breaker sizes range from 15A to 400A, covering everything from residential lighting circuits to heavy industrial equipment. The 125% safety margin ensures your breaker won't nuisance-trip under normal operating conditions and provides adequate protection for temporary inrush currents and voltage fluctuations.
Regional Notes
India: Household voltage is 230V AC single-phase at 50 Hz. Common breaker sizes are 6A, 10A, 16A, 20A, 25A, and 32A for residential use, with 63A main switches. The Indian Electricity Rules mandate proper breaker sizing for all installations.
US: Household voltage is 120V/240V AC single-phase at 60 Hz. NEC Article 210.20 requires breakers to be sized at 125% of continuous load. Common residential breakers are 15A and 20A for general circuits, with 30A–50A for appliances.
UK: Household voltage is 230V AC single-phase at 50 Hz. BS 7671 (IET Wiring Regulations) governs breaker sizing. Consumer units use MCBs (Miniature Circuit Breakers) in standard ratings matching the BS EN 60898 series.
Note: This calculator is for estimation purposes only. Always consult a licensed electrician and follow local electrical codes when installing or replacing circuit breakers.
Frequently Asked Questions
How do I calculate the right breaker size for my circuit?
To calculate breaker size, first determine the total load current of all appliances on the circuit. For DC: I = W/V. For AC single-phase: I = W/(V × PF). For AC three-phase: I = W/(V × PF × √3). Then multiply the total load current by 1.25 (125% safety factor per NEC) and select the next standard breaker size. Standard breaker sizes include 15A, 20A, 25A, 30A, 35A, 40A, 45A, 50A, 60A, 70A, 80A, 90A, 100A, 110A, 125A, 150A, 175A, 200A, 225A, 250A, 300A, 350A, and 400A.
What is the 125% rule for breaker sizing?
The 125% rule (NEC 210.20) requires a circuit breaker to be rated at least 125% of the continuous load. This means you multiply the total load current by 1.25 and choose the next standard breaker size. For example, a 16A continuous load requires a 20A breaker (16 × 1.25 = 20A). This safety margin prevents nuisance tripping and allows the breaker to handle temporary inrush currents without overheating.
What size breaker do I need for a microwave?
A typical 1000W microwave on a 120V circuit draws about 8.3A (1000/120). Applying the 125% safety factor gives 10.4A, so a 15A breaker is sufficient. On a 230V system, the same microwave draws about 4.3A, and a 15A breaker is more than adequate. Always check your microwave nameplate for exact wattage and consult a licensed electrician for installation.
What size breaker does a dishwasher need?
A dishwasher typically requires a 15A or 20A dedicated circuit breaker depending on its wattage. For a 1800W dishwasher on 230V AC single-phase with a 0.88 power factor, the load current is about 8.9A. With the 125% safety factor, the recommended breaker size is 15A. In the US (120V), the same dishwasher draws about 17A, requiring a 20A or 25A breaker. Always check local electrical codes.
Can I replace a breaker with a bigger one?
You should not replace a breaker with a larger one unless the wiring is also upgraded to handle the higher current. The breaker protects the wire — if you install a 30A breaker on 14 AWG wire (rated for 15A), the wire can overheat and cause a fire before the breaker trips. Always ensure the wire ampacity matches or exceeds the breaker rating. Consult a licensed electrician before making any changes to your electrical panel.
What are standard breaker sizes?
Standard residential and commercial circuit breaker sizes include: 15A, 20A, 25A, 30A, 35A, 40A, 45A, 50A, 60A, 70A, 80A, 90A, 100A, 110A, 125A, 150A, 175A, 200A, 225A, 250A, 300A, 350A, and 400A. The most common residential branch circuit breakers are 15A and 20A for general lighting and receptacle circuits, with 30A, 40A, and 50A used for major appliances like ranges, dryers, and air conditioners.
What is the difference between DC and AC breaker sizing?
For DC circuits, the load current is simply I = W/V. For AC circuits, power factor (PF) must be considered because not all apparent power converts to real work. AC single-phase uses I = W/(V × PF). AC three-phase includes an additional √3 factor because power is distributed across three phases: I = W/(V × PF × √3). DC breakers are also rated differently from AC breakers due to arc extinction characteristics — always use the correct type for your application.
How many watts can a 15 amp breaker handle?
On a 120V circuit, a 15A breaker can handle up to 1800W (15 × 120) total load. However, for continuous loads, NEC recommends limiting to 80% of the breaker rating, which is 1440W. On a 230V circuit, a 15A breaker can handle up to 3450W (15 × 230), or 2760W continuous. On a 400V three-phase system, a 15A breaker can handle up to 10,392W (15 × 400 × √3).