Breaker Size Calculator
Plan your electrical engineering project with our free breaker size calculator. Get precise measurements, material lists, and budgets.
Formula
Amps = Watts / (Volts x PF); Breaker = Amps x 1.25 (continuous) or x 2.50 (motor)
Calculate the actual current draw by dividing watts by voltage times power factor. For continuous loads (3+ hours), multiply by 1.25 per NEC 210.20. For motor loads, multiply by 2.50 per NEC 430.52. Select the next standard breaker size that meets or exceeds the calculated value. Verify the wire gauge is rated for the selected breaker size.
Worked Examples
Example 1: Kitchen Appliance Circuit
Problem: Size a breaker for a 1,800-watt continuous kitchen load on a 120V circuit.
Solution: Current = 1,800 / 120 = 15.0 amps\nContinuous: 15.0 x 1.25 = 18.75 amps\nNext standard breaker = 20 amps\nMinimum wire = 12 AWG
Result: 20-amp breaker with 12 AWG wire
Example 2: 240V Electric Dryer
Problem: Size a breaker for a 5,400-watt electric dryer on a 240V circuit.
Solution: Current = 5,400 / 240 = 22.5 amps\nContinuous: 22.5 x 1.25 = 28.13 amps\nNext standard breaker = 30 amps\nMinimum wire = 10 AWG
Result: 30-amp breaker with 10 AWG wire
Frequently Asked Questions
How do you size a circuit breaker for a load?
To size a circuit breaker, first calculate the load current by dividing watts by voltage (and power factor for AC loads). For continuous loads that run for 3 hours or more, the NEC requires the breaker to be rated at 125 percent of the load current. For example, a 1,920-watt continuous load on a 120-volt circuit draws 16 amps, so the minimum breaker size is 16 x 1.25 = 20 amps. Select the next standard breaker size that equals or exceeds this value. The wire size must also be rated for the breaker amperage per NEC Table 310.16.
Why do you multiply by 1.25 for breaker sizing?
The 1.25 multiplier (125 percent rule) comes from NEC 210.20(A) and applies to continuous loads, which are loads expected to operate for 3 hours or more. This safety factor ensures the breaker and conductors operate well below their maximum ratings during sustained operation, preventing heat buildup that could damage insulation or cause nuisance tripping. Without this margin, conductors and breakers running at 100 percent capacity for extended periods would generate excessive heat, reducing their lifespan and creating fire hazards. Some special breakers rated for 100 percent continuous load can eliminate this requirement.
What size wire do I need for a 20-amp breaker?
A 20-amp circuit breaker requires a minimum of 12 AWG copper wire per NEC Table 310.16. Using 12 AWG wire on a 20-amp breaker is the standard residential wiring configuration for kitchen, bathroom, laundry, and general-purpose receptacle circuits. Never use 14 AWG wire on a 20-amp breaker because 14 AWG is only rated for 15 amps and would overheat before the breaker trips. You can always use a larger wire gauge (lower number) than required. For example, 10 AWG wire is acceptable on a 20-amp breaker but not required unless the wire run is very long.
How do you size a breaker for a motor?
Motor circuit breakers are sized differently from standard loads per NEC Article 430. For standard inverse-time circuit breakers, the maximum size is 250 percent of the motor full-load current. If 250 percent does not correspond to a standard breaker size, you may round up to the next standard size. For example, a motor drawing 10 amps would need a breaker no larger than 10 x 2.50 = 25 amps. Dual-element fuses allow only 175 percent, and instantaneous-trip breakers allow up to 800 percent for specific motor types. The wire size for motor circuits is based on 125 percent of the full-load current, not the breaker size.
Can I put a bigger breaker on my circuit?
No, you should never install a larger breaker than the wire can safely handle. The breaker protects the wire from overheating and causing a fire, so the breaker must match the wire ampacity rating. For 14 AWG wire, the maximum breaker is 15 amps. For 12 AWG, the maximum is 20 amps. For 10 AWG, the maximum is 30 amps. Installing an oversized breaker means the wire could overheat and melt its insulation before the breaker ever trips, creating a serious fire hazard. If you need more capacity, you must run new, larger wire to support the bigger breaker.
How do I size an HVAC system for a building?
HVAC sizing uses Manual J calculations considering square footage, insulation, window area, climate zone, and occupancy. A rough estimate is 1 ton of cooling per 400-600 square feet. Oversized systems short-cycle and waste energy; undersized systems cannot maintain comfort.