Calculate total load in watts/amps, find the right breaker size, and check if your circuit can handle the demand.
This tool has three modes. The Multiple Appliances tab lets you add every device on a circuit to see the total load and recommended breaker. The Single Circuit tab converts between watts, amps, and kVA for one load. The Breaker Sizing tab takes a known amperage and returns the correct breaker and wire gauge per NEC guidelines. Enter your voltage, fill in the load details, and click Calculate.
Choosing the wrong circuit breaker is one of the most common — and most dangerous — electrical mistakes homeowners and contractors make. An undersized breaker trips constantly and creates fire hazards from repeated overheating. An oversized breaker won't trip when it should, leaving wires unprotected during an overload or fault.
Consider a typical home kitchen circuit running at 120V. A coffee maker (1,200W), a microwave (1,100W), and a toaster (900W) running simultaneously draw about 26.7 amps — well beyond the 15-amp or even 20-amp breaker that many kitchens have. The NEC (National Electrical Code) requires circuits serving continuous loads be sized at 125% of the calculated load, meaning that 26.7A scenario actually needs a 34-amp minimum design, pointing to dedicated 20A circuits for each major appliance.
For workshops, electricians typically size for motor loads at 180% of full-load current, per NEC Article 430. A 10-amp table saw motor should be on a 20-amp breaker minimum. These calculations are critical for permits, insurance, and safety inspections.
The core formulas used in this tool:
The power factor (PF) accounts for reactive loads like motors, transformers, and fluorescent lighting that draw more current than their wattage implies. Purely resistive loads (heaters, incandescent bulbs) have PF = 1.0. Motors typically run 0.80–0.90. When unsure, 0.9 is a safe estimate for mixed loads.
The NEC (National Electrical Code) requires that continuous loads (operating 3 hours or more) not exceed 80% of a circuit breaker's rated capacity. This means a 20A breaker should carry no more than 16A of continuous load. This is why the standard safety margin in this calculator defaults to 125% — the reciprocal of 80%.
Divide watts by voltage (and multiply by power factor if it's not 1.0). For example, a 1,500W space heater on 120V: 1,500 ÷ 120 = 12.5 amps. Apply the 125% NEC margin: 12.5 × 1.25 = 15.6A, which rounds up to a 20-amp breaker. This calculator does all of this automatically.
In the US, standard residential wire sizing: 15A breaker → 14 AWG, 20A → 12 AWG, 30A → 10 AWG, 40A–50A → 8 AWG, 60A → 6 AWG, 100A → 4 AWG or 3 AWG. This tool shows recommended wire gauge with every breaker result. Always verify with a licensed electrician for your specific installation.
Single-pole breakers (one slot in the panel) handle 120V circuits. Double-pole breakers (two slots) handle 240V circuits like dryers, ranges, water heaters, and EV chargers. If your voltage is 240V, you need a double-pole breaker. This calculator notes which type is appropriate based on the voltage you select.