Circuit Breaker Calculator

Step-by-Step Guide

1. 1Enter the total load: Type the load in watts or kilowatts using the unit selector. Use the equipment's actual rated power, not an average — sizing for peak load is what keeps the breaker from nuisance tripping under normal conditions.
2. 2Select supply voltage: Choose from 120V, 230V, 240V, 400V, or 415V depending on your electrical system. US systems typically use 120V or 240V; EU and much of Asia use 230V single-phase or 400V three-phase.
3. 3Choose single-phase or three-phase: Select the phase type matching your electrical supply. Three-phase circuits use the √3 factor in the current formula and draw less current than single-phase for the same power, which is why heavy industrial and commercial loads use it.
4. 4Set load type: Mark the load as Continuous if it runs 3 or more hours at a time — lighting, HVAC, EV chargers — or Non-Continuous for intermittent loads like power tools. Continuous loads get the NEC 125% safety factor applied automatically.
5. 5Enter power factor: Use 1.0 for resistive loads like heaters and incandescent lighting, or 0.8–0.9 for motors and other inductive equipment. A lower power factor increases the calculated current for the same real power.
6. 6Read the recommended breaker size: The calculator returns the calculated current, the adjusted current after the continuous load factor, the recommended standard breaker size, and a wire gauge suggestion. Export the result or save it to history for your project records.

What This Tool Provides

• ✓Recommended breaker size from the standard ladder
• ✓Calculated and continuous-load-adjusted current
• ✓Safety margin percentage
• ✓Wire gauge recommendation based on adjusted current
• ✓Single-phase and three-phase support
• ✓Five voltage presets (120V–415V)
• ✓6 built-in common load presets
• ✓Calculation history (last 20 entries)
• ✓Export results as a text report
• ✓Copy result to clipboard
• ✓100% browser-based — no data sent to server
• ✓No registration required

Pro Tips

• 💡Mark a load as continuous whenever it's expected to run 3 hours or longer at a stretch — lighting, HVAC, water heaters, and EV chargers all qualify. Skipping the 125% factor on these loads produces a breaker that runs at its full thermal limit for hours, accelerating wear and increasing nuisance trip risk.
• 💡Always cross-check the recommended breaker against the wire gauge actually installed, not just the load. A 20A breaker protecting 14 AWG wire (rated for 15A) is a code violation regardless of what the load calculation says — breaker and conductor must be coordinated together.
• 💡For motors and other inductive loads, use a power factor of 0.8 to 0.9 rather than defaulting to 1.0. Using 1.0 for a motor circuit understates the true current draw, which can lead to an undersized breaker that trips under normal running conditions.
• 💡When a load calculates to more than 80A on a single-phase system, seriously evaluate whether three-phase power is available or practical. The same power delivered at three-phase draws meaningfully less current, often allowing a smaller breaker and lighter conductors.
• 💡Size for future growth on shared circuits like sub-panels and workshop feeders. Choosing one breaker size larger than the immediate calculated need, within safe limits, saves a re-pull later if you add equipment — but never do this by ignoring the calculated minimum.
• 💡Use the wire gauge recommendation as a starting point, not a final answer. It's based on ampacity alone — for cable runs longer than about 15 meters, also run a voltage drop calculation, since a run that's fine for ampacity can still need a larger wire to keep voltage drop in range.

Common Mistakes to Avoid

• ✕Don't install a larger breaker than recommended just because it's what's on hand. An oversized breaker won't trip in time to protect the wire during an overcurrent fault, allowing insulation to overheat well past its safe operating temperature.
• ✕Don't forget the continuous load factor on loads that clearly run for hours, like HVAC compressors or lighting banks. Skipping it because the non-continuous number 'looks close enough' produces a breaker sized 20% too small for sustained operation.
• ✕Don't use single-phase formulas for three-phase equipment or vice versa. Applying the wrong formula changes the calculated current substantially — three-phase current is roughly 42% lower than the equivalent single-phase current for the same power.
• ✕Don't ignore the low-safety-margin case. A breaker recommendation with less than 10% headroom above the adjusted current is prone to nuisance tripping from normal load variation; consider stepping up one standard size for circuits that see load fluctuation.
• ✕Don't skip a separate voltage drop check on long cable runs. This calculator sizes the breaker to the load current only — a circuit with a correctly sized breaker can still need a larger wire gauge than ampacity alone suggests once cable length is factored in.