Breaker Size Calculator
Enter your load details to find the correct circuit breaker size. The calculator supports DC, single-phase AC, and three-phase AC systems, applies the NEC 125% continuous-load rule (or the correct factor for motors, air conditioners, and welders), and rounds the result up to the nearest standard ANSI/UL breaker rating. Switch between watt-based and amp-based input in one click.
How to size a circuit breaker
A circuit breaker protects the wiring in a circuit by interrupting current flow when that current exceeds a safe level. Sizing one correctly requires two things: knowing the full-load current the circuit must carry, and applying the appropriate NEC safety factor for the type of load. The basic rule for continuous loads (those running for three hours or more, such as lighting and HVAC) is stated in NEC 210.20(A): the breaker must be rated at no less than 125% of the continuous load current. This prevents the breaker from running near its thermal limit indefinitely, which shortens its life and risks nuisance tripping. For resistive loads that cycle on and off, 100% of the design current is acceptable.
DC, single-phase AC, and three-phase AC circuits
The formula for design current depends on the circuit type. For a DC circuit and a single-phase AC resistive load, the current is simply power divided by voltage (I = P / V). When the load has a power factor below 1.0 (motors, fluorescent ballasts, variable-speed drives), you divide by both voltage and the power factor: I = P / (V x PF). For a three-phase AC circuit, the line current is I = P / (1.732 x V x PF), where 1.732 is the square root of 3 and V is the line-to-line voltage. Three-phase distribution is common in commercial buildings and industrial facilities because it delivers more power for the same conductor size.
Motor, air conditioner, and welder circuits
Motors draw several times their running current when they start, so the NEC treats them differently from standard loads. NEC 430.52 allows a motor branch-circuit breaker to be sized at up to 250% of the motor's full-load amps to ride through startup inrush without tripping. The overload protection in the motor controller (starter heaters or electronic overload relay) provides the sustained overload protection that the breaker does not. Air conditioners fall under NEC Article 440: the branch-circuit breaker may be sized at 175% of the compressor's rated-load amps. Welders are governed by NEC 630.11 and use a duty-cycle-based factor that accounts for their intermittent operation.
Wire gauge and ampacity
A breaker protects the wire, not just the load. The wire gauge must be rated to carry at least as much current as the breaker's ampere rating. NEC Table 310.12 lists the ampacity of common copper conductors at 60 and 75 degrees Celsius. At 75 C (the more common rating), 14 AWG handles 15 A, 12 AWG handles 20 A, 10 AWG handles 30 A, and 8 AWG handles 50 A. Installing a larger breaker than the wire can handle is the leading cause of electrical fires in DIY wiring. Always match or exceed the breaker's rating with the wire ampacity, and use a licensed electrician if you are unsure.
Standard breaker sizes and matched copper wire gauge (NEC 310.12)
| Breaker size (A) | Min. copper wire (AWG/kcmil) | Typical use |
|---|---|---|
| 15 | 14 AWG | Lighting, outlets |
| 20 | 12 AWG | Kitchen outlets, small appliances |
| 30 | 10 AWG | Dryers, water heaters (240V) |
| 40 | 8 AWG | Electric ranges (240V) |
| 50 | 6 AWG | Hot tubs, large air conditioners |
| 60 | 6 AWG | EV chargers (Level 2), large loads |
| 100 | 1 AWG | Main panel feeds, subpanels |
| 150 | 2/0 AWG | Subpanel feeds |
| 200 | 4/0 AWG | Residential main panel |
Minimum copper conductor sizes at 60/75 degrees C for common ANSI/UL breaker ratings. Always verify with your local electrical code.
Frequently asked questions
What is the 125% rule for circuit breakers?
NEC 210.20(A) states that when a circuit supplies a continuous load (one that operates at a steady level for 3 hours or more), the breaker must be rated at no less than 125% of that load's current. This 80/20 rule ensures the breaker operates well below its thermal limit, preventing premature tripping and extending the breaker's service life. For example, a 10-amp continuous load requires a breaker rated at least 12.5 amps, so you would install the next standard size up: a 15-amp breaker.
How do I calculate amps from watts?
For a DC circuit or a single-phase AC resistive load, amps = watts / volts. For a single-phase AC load with a power factor below 1.0 (such as a motor or variable-speed drive), amps = watts / (volts x power factor). For a three-phase circuit, amps = watts / (1.732 x volts x power factor). The power factor is listed on the equipment nameplate or in the manufacturer's specifications.
Why does the breaker need to be larger than the load current?
A circuit breaker has a thermal element that trips when it reaches a certain temperature. Running a breaker at exactly 100% of its rating for hours generates enough heat to weaken the thermal element over time, causing nuisance trips or, in the worst case, a breaker that fails to trip at all. The NEC 125% rule builds in a margin so the breaker runs cool and reliably.
Can I use a larger breaker than the calculated size?
You can go one standard size up from the calculated minimum if the exact size is unavailable, but there are limits. The wire ampacity sets the ceiling: you must never install a breaker larger than what the wire can safely carry. Going up a breaker size while keeping the same wire is the most common cause of house fires from overloaded circuits. If the load truly requires a larger breaker, upgrade the wire at the same time.
What wire gauge should I use with a 20-amp breaker?
A 20-amp breaker requires a minimum of 12 AWG copper wire (or 10 AWG aluminum) at the standard 75-degree-C rating per NEC Table 310.12. Using 14 AWG with a 20-amp breaker is a code violation and a fire hazard, because 14 AWG copper is rated for only 15 amps.
How is a three-phase circuit different from single-phase?
Single-phase power, the type used in most US homes, delivers power through two conductors 180 degrees out of phase, or through one hot wire and a neutral. Three-phase power uses three conductors 120 degrees apart and delivers more power for the same current. The line current in a balanced three-phase system is I = P / (1.732 x V x PF), which is lower than the single-phase current for the same wattage, meaning smaller wire and breakers are needed.
What is a power factor and when does it matter?
Power factor (PF) is the ratio of real power (watts) to apparent power (volt-amps). Resistive loads like heaters and incandescent lights have a PF of 1.0. Inductive loads like motors, transformers, and fluorescent ballasts draw more current than their watt rating alone suggests, so they have a PF below 1.0 (typically 0.80 to 0.95). If you ignore a low power factor and size a breaker only on watts, the breaker will be undersized for the actual current, which can cause nuisance tripping.