Breaker Size Calculator

Find the load current (or convert watts/kW at your voltage and phase in the calculator), multiply by 1.25 for continuous loads (or 1.50 as a motor starting screen), then pick the next standard frame at or above that product. Example: 32 A × 125% → 40 A. Always match wire ampacity to the breaker—see the Cable Size Calculator for mm² screening.

A breaker size calculator (also called a circuit breaker calculator or breaker sizing calculator) estimates OCPD frame from load current × duty margin (125% continuous / 150% motor start), then picks the next standard rating. Example: 32 A × 125% → 40 A frame. Use after kVA to amps or factory load gives design amps—or enter kW directly above.

Yes for preliminary amp rating. A circuit breaker calculator and a breaker sizing calculator both answer “what size breaker do I need?” from load current (or watts) and duty margin. Trip curves, AIC/SCCR, and selective coordination are separate steps after this screen.

Typical screens (always verify nameplate): electric dryer often ~24 A continuous → 30 A; 32 A EV continuous → 40 A; 90 A feeder × 125% → 125 A. Use the Quick examples buttons, then confirm conductors in the wire chart and cable sizing tool.

It is used to estimate a suitable breaker current rating from load current and safety factor during preliminary electrical design.

First calculate the load current, then multiply by an appropriate safety factor (for example 125% for continuous load). Finally, select the next higher standard breaker size. This calculator automates those steps for you.

The formula is Breaker Size (A) = Load Current (A) × Safety Factor, then round up to the next standard breaker rating.

Example: 80 A continuous load × 1.25 = 100 A, so select at least a 100 A standard breaker, then verify with coordination and code checks.

Usually no—not without checking the whole circuit. A 30 A breaker is only valid if the conductors, terminal ratings, and connected load are approved for 30 A continuous service under your code (NEC, IEC 60364, or local rules). Swapping a tripping 20 A breaker for a 30 A unit on the same wire often violates ampacity, reduces protection, and can create overheating. If the 20 A trips, diagnose overload, inrush, or a weak breaker first; if the load truly grew, upgrade wire size and equipment together, then pick the breaker from the calculator ladder.

For continuous loads (typically 3 hours or more), US panel practice keeps the load at about 80% of the breaker rating—the same as sizing the OCPD at 125% of continuous current. Example: 32 A × 1.25 → 40 A frame. See 80% rule vs 125% above.

Continuous loads heat the breaker for long periods. A 125% factor helps prevent nuisance tripping and keeps the breaker operating within its thermal limits, following typical code practice for continuous current (inverse of the 80% rule).

NEC 430.52 caps motor branch-circuit short-circuit / ground-fault devices as a percentage of FLA (often up to ~250% for inverse-time breakers—check the table for your motor/device). This tool’s 150% option is a conservative starting frame for inrush, not a full 430.52 selection. See 430.52 screen and the motor protection guide.

Motors have inrush current that can be several times the running current. For many applications, using 150% as a starting point helps accommodate inrush while coordination and protection studies refine the exact rating under NEC 430.52 / manufacturer MOCP.

Use NEC 450.3 percentages of primary/secondary rated current after you know transformer kVA and voltages—not only the generic continuous 125% path. Start with the Transformer Size Calculator, then apply 450.3. See 450.3 notes above.

FLA (full-load amps) is the motor’s rated running current at nameplate HP/kW. LRA (locked-rotor amps) reflects starting inrush and can be 5–8× FLA. This calculator uses FLA × duty margin for frame screening—use 150% as a starting point, then refine with the motor starting & protection guide and SCCR checks in the SCCR calculator.

Breaker current rating (A) is based on load current, but its voltage rating (for example 230 V, 400 V) must be suitable for the system. This tool focuses on current sizing; voltage and breaking capacity must be checked separately.

No. It is a preliminary sizing tool. Final designs must include short-circuit calculations, coordination studies, cable sizing, ambient conditions, and must comply with IEC/NEC or local codes.

Use the same amp screening path here first. Fuses suit switchgear and MCC buckets where predictable I²t and spare links are acceptable; breakers suit panelboards where reset matters. See the Fuse vs Breaker Sizing Guide for decision tables and a 63 A feeder example.

Amp screening here is step one only. Use the Protection Coordination & SCCR Guide with Short-Circuit Planning before final device selection.

MCB and MCCB are breaker frames; fuses are one-shot links. At screening stage, all use load current × margin → next standard amp rating. Trip curves, fuse class, and SCCR come later in coordination studies.