Circuit Breaker Size Calculator

Find the load current and apply 1.25 only when the adopted continuous-load rule applies, then view the next reference frame. Example: 32 A × 125% → 40 A. Special loads require a separate code/OEM path. Always verify conductor ampacity, terminal limits, fault-current rating and coordination.

This calculator shows a general-load reference frame from current and an applicable duty factor. It does not replace special-load OCPD tables, manufacturer limits, conductor ampacity, fault-current or coordination checks. Example: 32 A continuous × 125% → a 40 A reference frame.

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).

Motor protection follows a dedicated motor/OCPD code path and manufacturer limits. This generic calculator does not select the motor breaker. See the motor boundary above and the motor protection guide.

No. Motor short-circuit/ground-fault and overload protection use dedicated rules that depend on motor and protective-device type, adopted code edition and manufacturer limits. Use the motor protection workflow instead of a generic multiplier.

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 is running current and LRA represents locked-rotor starting current. Their protection requirements cannot be reduced to one generic multiplier. Use the motor starting & protection guide, manufacturer data and SCCR checks.

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.

Select 3-phase, enter line voltage and load as amps or kW (with PF). The tool derives line current if needed, multiplies by your duty margin, and returns the next standard frame—same ladder as single-phase. Example: after kVA to amps gives 90 A line current, × 125% → 125 A frame. Verify conductors in the Cable Size Calculator.

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.