Circuit Breaker Size Calculator: Plan Your Breaker Amp Rating

A circuit breaker protects a conductor and the equipment connected to it by opening when current exceeds its rated value. Getting the amp rating right matters: too small and the breaker nuisance-trips; too large and the wire can overheat before the breaker ever opens.

Important: This tool is a planning aid. Final breaker selection must comply with your local electrical code (NEC, BS 7671, AS/NZS 3000, or equivalent) and be carried out by, or verified by, a licensed electrician. Electrical work that does not meet code can result in fires, injury, or failed inspections.

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How breaker sizing works

The starting point is the load current — the maximum continuous line current the circuit will carry. From that you find the next standard breaker amp rating that is at least as large as your design current.

You can enter load current directly if you know it (from a nameplate, load calculation, or engineer’s drawing), or the tool can estimate it from real power (watts), voltage, number of phases, and power factor.

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The formulas

Single-phase current:

I = P ÷ (V × PF)

Three-phase current (line-to-line voltage):

I = P ÷ (√3 × V_L-L × PF)

Where P is real power in watts, V is the appropriate voltage, and PF is power factor (1.0 for purely resistive loads, 0.8–0.95 for motors and mixed loads).

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The 125% continuous load rule

A load is considered continuous if it is expected to run at maximum current for 3 or more hours (NEC Article 100 definition; similar concepts exist in IEC standards). For continuous loads, most codes require:

The breaker must be rated at least 125% of the continuous current.

Equivalently, the continuous current must not exceed 80% of the breaker’s rating. This extra headroom prevents the breaker from running hot and degrading over years of use.

Example: A lighting circuit draws 20 A continuously. Design current = 20 × 1.25 = 25 A. Next standard breaker ≥ 25 A is typically 30 A.

Enable the 125% toggle in the calculator to apply this factor automatically.

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Standard breaker sizes

Breakers come in discrete amp ratings — you cannot order a 22 A breaker. Common standard sizes (varies by region and manufacturer):

Small branch circuits: 6, 10, 13, 15, 16, 20, 25, 30, 32 A

Feeder / sub-panel: 40, 45, 50, 60, 63, 70, 80, 100, 125, 150 A

Main panels / distribution: 175, 200, 225, 250, 300, 400, 600, 800, 1000 A

The calculator finds the smallest standard size that is ≥ your design current.

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Quick reference (illustrative)

• 1,920 W, 240 V, 1φ, not continuous → 10–15 A
• 12,000 W, 240 V, 1φ, continuous load → 70 A
• 15 kW, 400 V, 3φ, not continuous → 32 A
• 30 A measured current (direct), not continuous → 30 A

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Real-world examples

240 V single-phase heater

6,000 W, PF 1.0:

I = 6,000 ÷ 240 = 25 A. Non-continuous → next standard ≥ 25 A is 30 A. Verify conductor size matches.

Continuous lighting load

18 A operating current, classified as continuous:

Design current = 18 × 1.25 = 22.5 A → next standard ≥ 22.5 A is 25 A (or 30 A in some regions).

Three-phase workshop equipment

10 kW load, 400 V L-L, PF 0.9:

I = 10,000 ÷ (√3 × 400 × 0.9) ≈ 16.0 A → next standard ≥ 16 A is 16 A or 20 A depending on availability and derating.

Motor branch circuits

Motors require special treatment under NEC Article 430 / IEC equivalent. The OCPD (overcurrent protective device) for a motor is typically sized at 125–250% of FLA (full-load amperes) to allow for starting inrush. Do not use a simple watt-to-amp calculation for motor sizing — use nameplate FLA and the applicable motor code table.

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What this tool does not do

• Conductor ampacity — wire gauge must be matched to breaker rating and installation conditions (conduit fill, ambient temperature, number of conductors)
• Terminal temperature limits — breaker terminals are often rated at 60 °C or 75 °C; the connected wire must match
• Motor inrush — starting current can be 6–10× running current and requires separate OCPD sizing rules
• Harmonic loads — variable frequency drives, UPS systems, and LED drivers draw non-sinusoidal current that can overheat conductors
• GFCI / AFCI requirements — ground-fault and arc-fault protection requirements depend on circuit location and local code amendments
• Derating — multiple conductors in conduit, high ambient temperatures, or bundled cables reduce allowable ampacity
• Coordination — upstream breakers must be sized to selectively isolate faults without tripping the whole panel

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FAQ

Why might the suggested size still be wrong?

Conductor ampacity, ambient temperature, conduit fill, terminal temperature limits, and local code minimums can all require a different size than simple current math suggests. The next standard size is a starting point, not a final answer.

120 V vs 240 V for single-phase — which do I use?

Use the voltage that matches how you measured or calculated the load current. A 240 V appliance uses 240; a 120 V outlet circuit uses 120. Three-phase always uses line-to-line voltage.

I entered both amps and watts — which wins?

If the load current field has a valid positive number, it takes priority and power fields are ignored until you clear it.

Does this choose GFCI or AFCI breakers?

No — this only suggests a thermal-magnetic amp rating for planning. GFCI, AFCI, and combination AFCI/GFCI requirements are determined by circuit location and code.

What about fuses?

Standard fuse ratings are similar, but fuse types (fast-blow, time-delay, current-limiting) and code rules differ significantly — especially for motors. Treat the suggested amperes as an order-of-magnitude check only.

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Try the Tooladex Circuit Breaker Size Calculator — enter your load current (or watts, voltage, and phase) to instantly find the next standard breaker size, with optional 125% continuous load factor.