Residential breakers in North America are often arc-fault or standard thermal-magnetic—curve letters B/C/D are more visible on IEC-style devices and industrial panels. Globally, the idea is the same: instantaneous trip multiplier must clear inrush without nuisance while thermal protection respects run amps.
Curve classes in plain language
| Class | Magnetic band (typical) | Good for |
|---|---|---|
| B | ~3–5× rated I | Resistive: heaters, lighting (steady) |
| C | ~5–10× rated I | Mix loads, compressors, small motors |
| D | ~10–20× rated I | High inrush: large motors, transformers, welders |
A 20 A Type C might allow ~100–150 A briefly; Type B might trip at ~80 A. Same handle rating, different spike tolerance.
Workshop and home-lab planning
Before drywall closes or machines land:
- Inventory loads — watts, voltage, motor vs. resistive
- Assign inrush factors — 6× default motors; 3× for VFD with soft-start
- Run AC Inrush Current per branch candidate
- Document breaker + curve on panel schedule
- Verify wire ampacity for run amps × 125%, not peak
Example branch sketch
| Load | P | V | Factor | Peak (calc) | Breaker pick |
|---|---|---|---|---|---|
| Table saw | 1800 W | 120 | 6× | ~90 A | 20 A Type C |
| Air compressor | 2400 W | 240 | 7× | ~70 A | 20 A Type C/D |
| LED bench supply | 400 W | 120 | 3× | ~10 A | 15 A Type B |
Numbers are illustrative—your nameplate wins.
Common mistakes
- Same breaker for motor and sensitive electronics — inrush noise on shared branch
- #14 wire on 20 A breaker because trips stopped after upsizing breaker only
- No coordination between main and subfeed—sub trips first under combined starts
Hybrid and overseas installs
EU 230 V single-phase shops: I_run = P/V drops vs. 120 V, but peaks still drive curve choice. UK garage feeders often specify Type C for socket radial with motor loads.
Breaker selection is front-loaded design. Pick the curve when you still have the one-line diagram—not after the third nuisance trip ruins a batch run.