Circuit Breaker Size Calculator
What size breaker do I need? Enter any load — watts, VA, amps, or horsepower — at 120–600 V single- or three-phase, and get the NEC 240.6 standard breaker size with the 125% continuous-load rule plus a companion wire recommendation.
How it works
The calculator converts your load to current, then applies the continuous-load factor and picks the smallest standard breaker (NEC 240.6) at or above the result:
- Amps: used directly. Watts, 1φ: I = P ÷ V (resistive load). Watts, 3φ: I = P ÷ (√3 × V × PF). VA, 1φ: I = S ÷ V. VA, 3φ: I = S ÷ (√3 × V). HP: I = HP × 746 ÷ V (DC), or ÷ (V × eff × PF) for 1φ, ÷ (√3 × V × eff × PF) for 3φ.
- Continuous loads (3+ hours: water heaters, lighting, EV chargers) are sized at 125% per NEC 210.19.
- The breaker is the next NEC 240.6 standard size ≥ the effective current (15, 20, 25, 30, …, 6000 A).
- The companion wire is the smallest copper/aluminum size from NEC Table 310.16 (60/75°C) whose ampacity covers the effective current, honoring the NEC 240.4(D) small-conductor caps (14 AWG → 15 A, 12 AWG → 20 A, 10 AWG → 30 A).
Worked examples
4,500 W water heater, 240 V single-phase (continuous)
I = 4,500 ÷ 240 = 18.75 A; continuous × 125% = 23.44 A → 25 A breaker, 10 AWG copper (60°C, 30 A). Note: 12 AWG (25 A ampacity) is skipped because NEC 240.4(D) caps it at a 20 A breaker. As a non-continuous load the same heater needs only a 20 A breaker with 12 AWG copper.
10 HP motor, 460 V three-phase (90% eff, 0.85 PF)
I = 10 × 746 ÷ (√3 × 460 × 0.90 × 0.85) = 12.24 A; × 125% = 15.30 A → 20 A breaker, 12 AWG copper (60°C). NEC Article 430 allows motor branch-circuit breakers up to 250% of FLA for starting inrush — this tool applies the general 125% rule.
15 kVA three-phase load, 208 V (non-continuous)
I = 15,000 ÷ (√3 × 208) = 41.64 A → 45 A breaker, 8 AWG copper (75°C, 50 A). If it were continuous, 41.64 × 1.25 = 52.04 A would push the answer to a 60 A breaker.
Related tools (different jobs)
- 100 Amp Wire Size Calculator — starts from a known current in amps and sizes the wire per NEC Table 310.16 ampacity (60/75/90°C columns, conduit derating). Use it when the headline question is “what size wire”, not “what size breaker”.
- HP to Amps Calculator — motor horsepower ↔ amps conversion with NEC motor FLA tables.
- DC Wire Size Calculator — direct-current wire sizing.
- Electrical Unit Converter — volts/amps/ohms/watts plus a quick ampacity reference.
This tool is the dedicated general breaker sizer: load → current → 125% continuous rule → NEC 240.6 standard breaker, 120–600 V, any load type.
Reference tables
Breaker sizes: NEC 240.6(A). Ampacity: NEC Table 310.16, not more than 3 current-carrying conductors, 30°C ambient. The 90°C column is omitted here — per NEC 110.14(C) most terminations limit you to 60/75°C anyway.
FAQ
What size breaker do I need for my load?
Enter the load (watts, VA, amps, or HP), the voltage, and the phase, then mark whether it is continuous. The calculator computes the load current, multiplies by 125% for continuous loads (NEC 210.19), and picks the smallest NEC 240.6 standard breaker at or above that current. Example: a 4,500 W water heater on 240 V single-phase draws 18.75 A; as a continuous load that becomes 23.44 A, so the answer is a 25 A breaker with 10 AWG copper wire (60°C column).
What is a continuous load, and why the 125% factor?
A continuous load runs for 3 hours or more (NEC Article 100) — water heaters, lighting, EV chargers. NEC 210.19 requires branch-circuit conductors and breakers to be sized at 125% of a continuous load so nothing runs at its thermal limit for hours. The same 4,500 W / 240 V load is 18.75 A non-continuous (20 A breaker, 12 AWG copper) but 23.44 A continuous (25 A breaker, 10 AWG copper) — one checkbox changes the whole answer.
What size breaker for a 10 HP motor on 460 V three-phase?
With typical 90% efficiency and 0.85 power factor: current = 10 × 746 ÷ (√3 × 460 V × 0.90 × 0.85) = 12.24 A. At 125% for continuous duty that is 15.30 A, so the general-rule answer is a 20 A breaker with 12 AWG copper (60°C). Note: NEC Article 430 lets motor branch-circuit breakers be sized larger (up to 250% of full-load current for inverse-time breakers) to survive starting inrush — the companion wire is still sized at 125%.
How is this different from the 100 Amp Wire Size calculator?
That tool starts from a known current in amps and sizes the wire per NEC Table 310.16 ampacity (copper/aluminum, 60/75/90°C columns, conduit derating) — the breaker is only a side note. This tool starts one step earlier: it computes the load current from watts, VA, HP, or amps across 120–600 V single- or three-phase, applies the continuous-load 125%, and its headline answer is the NEC 240.6 breaker size, with the wire as the companion recommendation.
What size breaker for a 15 kVA three-phase load at 208 V?
Current = 15,000 VA ÷ (√3 × 208 V) = 41.64 A. Non-continuous, so no 125% factor — the next NEC 240.6 standard size is a 45 A breaker, paired with 8 AWG copper (50 A at 75°C). If the load were continuous, 41.64 A × 1.25 = 52.04 A would push the answer to a 60 A breaker.
Can I size and install a breaker myself, or do I need an electrician?
This calculator is a planning reference, not an installation permit. Breaker panels carry lethal voltage, and the Authority Having Jurisdiction (AHJ) — your local inspector — has the final word on what is allowed, including local amendments, AFCI/GFCI requirements, and panel-specific breaker compatibility. Use the result to plan and to check quotes, then have a licensed electrician do or verify the work.
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