This guide assumes you already know what an MCB is. If you need a primer, read our introductory pages first. Here we focus on real inrush behavior, curve selection, and troubleshooting in residential and light-commercial panels.
Curve B for purely resistive and small electronic loads; Curve C for most mixed circuits with moderate inrush; Curve D for high-inrush loads (motors, transformers, large LED drivers, welders).
Don’t solve nuisance trips by oversizing ampacity. Fix the curve first, then verify cable ampacity and upstream selectivity.
Use the Inrush Table below to map typical loads to a starting curve; confirm with actual nameplate and site tests.
MCBs protect against overload (thermal) and short-circuit (magnetic). The curve defines the instantaneous trip threshold (multiple of the rated current In). Choosing the right curve lets a breaker ride through brief inrush without compromising short-circuit protection.
MCBs protect against overload (thermal) and short-circuit (magnetic). The curve defines the instantaneous trip threshold (multiple of the rated current In). Choosing the right curve lets a breaker ride through brief inrush without compromising short-circuit protection.
MCBs protect against overload (thermal) and short-circuit (magnetic). The curve defines the instantaneous trip threshold (multiple of the rated current In). Choosing the right curve lets a breaker ride through brief inrush without compromising short-circuit protection.
MCBs protect against overload (thermal) and short-circuit (magnetic). The curve defines the instantaneous trip threshold (multiple of the rated current In). Choosing the right curve lets a breaker ride through brief inrush without compromising short-circuit protection.
| Load Type | Typical Inrush Multiplier (×In of steady current) | Notes |
|---|---|---|
| Incandescent/Resistive heater | 1.0–1.1 | Nearly no inrush; B curve is fine. |
| Small SMPS (phone chargers, routers) | 1.2–2.5 | Soft-start common; B or C curve. |
| LED drivers (domestic fixtures) | 3–8 | Aggregated inrush when many drivers start at once. |
| Refrigerator/Freezer compressors | 6–12 | Locked-rotor current; C or D curve depending on circuit diversity. |
| Heat pump/AC compressors | 6–12 (sometimes 14) | Consider C or D; check manufacturer LRA. |
| Single-phase induction motors (tools) | 6–12 | D curve for larger motors or frequent starts. |
| Transformers (magnetizing inrush) | 8–20 | Especially toroidal; often requires D curve. |
| Welders/Inverters | 10–20 | High peak; D curve typical. |
Use these as starting points; always verify with nameplate LRA/IFA, oscilloscope measurements, or manufacturer data.
| Circuit Context | Load Profile | Recommended Starting Curve | Why |
|---|---|---|---|
| General lighting (mixed LED + small SMPS) | Moderate aggregated inrush | C | Headroom for simultaneous LED inrush. |
| Purely resistive (heaters, toasters) | No inrush | B | Sensitive magnetic trip improves fault clearing. |
| Kitchen small-appliance ring | Mixed; some motors | C | Occasional inrush events. |
| Dedicated refrigerator/freezer | High LRA on start | C → D | Move to D if nuisance persists. |
| Workshop tool circuit | Motor starts, transformers | D | Frequent high peaks. |
| HVAC compressor circuit | High LRA, frequent cycling | C → D | Confirm manufacturer guidance and cable sizing. |
| Transformer-fed controls | High magnetizing inrush | D | Especially toroidal cores. |
1) Start with the matrix → 2) Validate with nameplate/measurements → 3) Confirm cable ampacity and upstream selectivity → 4) If trips persist, step curve up (B→C or C→D) before increasing amp rating.
Solving a curve problem with ampacity: Upsizing from 16 A to 25 A might stop trips—but you just reduced protection. Fix the curve first.
Ignoring aggregated inrush: Ten LED fixtures starting together behave differently than one.
No selectivity check: A stiffer downstream curve can pull upstream trips if coordination isn’t checked.
Long cable runs: Voltage drop and impedance can alter fault currents and trip dynamics. Re-calculate PSC at the device.
1. Identify the event: Trip occurs at startup or randomly?
2. Measure/estimate inrush: Use clamp meter with inrush capture or infer from LRA/nameplate.
3. Compare to curve threshold (e.g., Curve C magnetic ≈ 5–10×In).
4. If inrush > threshold → Step curve up (B→C / C→D).
5. If still tripping → Dedicated circuit for the problem load; check cable ampacity.
6. Verify upstream selectivity and RCD/RCBO coordination (some electronics leak at startup).
7. Persistent trips → inspect connections, neutral/earth faults, and supply issues (e.g., voltage sag).
12 fixtures, each driver 8 W, measured inrush ~6× nominal when cold.
10 A MCB on lighting radial. Curve B trips occasionally at switch-on.
Solution: Move to Curve C 10 A. Keep same conductor size. Nuisance trips eliminated.
1.5 kW single-phase motor, LRA ≈ 8× FLA.
Curve C 16 A trips at start 1/5 times.
Solution: Curve D 16 A + verify upstream selectivity and cable ampacity. Stable.
Q1. Will a D-curve always fix nuisance trips?
No. If the real issue is a genuine fault or undersized conductors, a D-curve masks symptoms. Diagnose before changing curves.
Q2. Can I mix B, C, and D on the same board?
Yes—designers mix curves by load profile. Just validate selectivity and short-circuit levels.
Q3. Is curve choice affected by RCD/RCBO?
Yes. Some loads (VFDs, SMPS) add leakage or high-frequency components. Ensure RCD type and MCB curve choices are compatible to avoid nuisance.
Johnson Lim is the General Manager of Changyou Technology and has over 10 years of experience in circuit protection technology and residential electrical safety. He is committed to developing and producing safer and smarter electrical products.
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