Breaker Size Calculator
Screen the next standard OCPD frame from design load current and a duty margin (125% continuous / 150% motor). Example: 32 A @ 125% → 40 A. Use the calculator below—not coordination, AIC, or NEC-table replacement.
Calculator
Quick: design amps and duty class. Advanced: safety factor encodes continuous vs motor starting assumptions before you pick a frame from the standard ladder.
About this calculator
Turns load current and a safety factor into the next standard breaker frame for planning before coordination and interrupting ratings. For cable and feeder checks in the same workflow lane, use the protection calculators hub.
Calculation Results
Calculation Results
Safety Factor: 125% (continuous load) (× 1.25)
Calculated Current: 40.00 A
Recommended Breaker Size: 40 A (next standard size above 40.0 A)
Engineering disclaimer
This calculator provides simplified breaker sizing for preliminary engineering only. Final protection device selection must follow applicable standards (IEC/NEC), manufacturer data, short-circuit studies, and coordination checks by a qualified engineer.
Standard breaker frame — quick reference
Common IEC/NEC-style frames used in this calculator’s ladder. Multiply design load by your duty margin, then pick the next standard rating at or above the product.
| Load (A) | Margin | Product (A) | Next frame (A) |
|---|---|---|---|
| 32 | 125% continuous | 40 | 40 |
| 63 | 125% continuous | 78.8 | 80 |
| 90 | 125% continuous | 112.5 | 125 |
| 48 | 150% motor | 72 | 80 |
How to Calculate Breaker Size (Formula & Explanation)
Formula: Breaker Size (A) = Load Current (A) × Safety Factor → next standard frame on the ladder (15, 20, 30, 40, 50, 60, 80, 100, 125, 150, 175, 200, 225, 250, 300, 400, 600 A).
Use higher safety factors for continuous or motor loads to account for thermal loading and inrush. Final selection must also consider short-circuit rating, coordination with upstream devices, conductor ampacity, and local code requirements.
Guides: Fuse vs Breaker Sizing · Motor Starting & Protection · Coordination & SCCR · Power Calculators Hub
Typical Examples & Frame Ladder
- 32 A continuous panel load × 125% → 40 A frame.
- 90 A feeder × 125% → 125 A standard frame.
- Motor branch with 150% factor → initial frame before coordination study.
Standard breaker frame ladder
Gray bars: common frames; blue bar: recommended frame; dashed line: load current; amber line: after safety margin.
Screening only—not a trip curve, coordination study, or code table replacement.
Frequently Asked Questions
What is a breaker size calculator used for?
It is used to estimate a suitable breaker current rating from load current and safety factor during preliminary electrical design.
How do I calculate breaker size from load current?
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.
What is the standard breaker sizing formula?
The formula is Breaker Size (A) = Load Current (A) × Safety Factor, then round up to the next standard breaker rating.
Can you show a breaker size example?
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.
Can I replace a 20 amp breaker with a 30 amp breaker?
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.
Why do continuous loads often use 125%?
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.
When should I use 150% for motor loads?
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.
Does voltage affect breaker size?
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.
Is this calculator enough for final design?
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.
When should I use a fuse instead of a circuit breaker?
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.
How do selective coordination and SCCR fit after breaker sizing?
Amp screening here is step one only. Use the Protection Coordination & SCCR Guide with Short-Circuit Planning before final device selection.
What is the difference between MCB, MCCB, and fuse sizing?
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.