Electrical
Fuse & Breaker Sizing Reference
The IEC 60364-4-43 overload-protection conditions, gG fuse vs MCB conventional tripping current, MCB curve types and the standard rating series. Use alongside Fuse & Breaker Sizing.
Last updated: June 2026
Terms
- IECInternational Electrotechnical Commission
- The international body that writes electrical safety standards used across most of the world outside North America. When a chart cites an IEC number, it's pointing to the official rulebook behind a calculation.
- MCBMiniature Circuit Breaker
- The switch in a fuse box that automatically trips (switches off) if a circuit draws too much current, protecting the wiring from overheating. It can be reset and reused, unlike a traditional fuse.
- gGgG fuse (general purpose)
- A standard type of fuse rated to protect general-purpose cables and equipment from both overload and short-circuit — the common fuse type in an industrial or commercial fuse box.
The Two Overload Conditions (IEC 60364-4-43)
Condition 1: IB ≤ In ≤ IZ
Design current ≤ device rating ≤ conductor capacity — the device must carry the expected load without itself becoming the limiting factor, and must not exceed what the cable can safely carry continuously.
Condition 2: I₂ ≤ 1.45 · IZ
The conventional operating current (the current that guarantees the device trips within the standard's specified time) must not exceed 145% of conductor capacity — this is what actually protects the cable against sustained overload, not the nominal rating alone.
Conventional Tripping Current (I₂) by Device Type
| Device | I₂ | Practical effect |
|---|---|---|
| gG fuse (≥ 16 A) | 1.6 × In | Needs more conductor headroom — condition 2 is usually the binding constraint for gG fuses |
| MCB (IEC 60898) | 1.45 × In | Tighter margin needed between rating and conductor capacity than gG |
Because gG's I₂ factor (1.6) is higher than the 1.45 limit itself, a gG fuse needs extra conductor capacity margin beyond just I_n ≤ I_Z — an MCB with the same nominal rating can sometimes work where a gG fuse of that rating fails condition 2.
MCB Curve Types (IEC 60898)
The curve type sets the instantaneous (short-circuit) tripping multiple — it doesn't change the overload (I₂) behaviour above, only how fast the breaker trips on a high-current fault or inrush.
| Curve | Instantaneous trip | Typical use |
|---|---|---|
| B | 3 – 5 × In | Resistive loads, long cable runs, residential final circuits — sensitive to nuisance tripping from inrush |
| C | 5 – 10 × In | General-purpose — small motors, fluorescent lighting, moderate inrush loads. Most common industrial default |
| D | 10 – 20 × In | High-inrush loads — transformers, large motors, X-ray equipment, welding |
| K | 8 – 12 × In | Motor circuits requiring tighter coordination than C, less common |
| Z | 2 – 3 × In | Highly sensitive electronic/semiconductor circuits — lowest trip threshold |
Standard Rating Series
| Series | Common ratings (A) |
|---|---|
| IEC 60898 MCB | 6, 10, 13, 16, 20, 25, 32, 40, 50, 63 |
| IEC 60269 gG fuse | 6, 10, 16, 20, 25, 32, 40, 50, 63, 80, 100, 125, 160, 200, 250 |
The calculator scans the relevant series and returns the smallest rating that satisfies both conditions — manually picking "the next size up" without re-checking condition 2 can produce a non-compliant combination.
Frequently Asked Questions
gG fuse vs MCB — what actually changes?
The conventional operating current I₂ differs: a gG fuse uses ~1.6×In (for ratings ≥16 A), an MCB uses 1.45×In. Since condition 2 caps I₂ at 1.45×IZ, a gG fuse effectively needs more conductor headroom than an MCB of the same nominal rating to satisfy the same overload condition.
Can I just pick the next standard rating up?
Only if both conditions still hold at the new rating. Condition 2 (I₂ ≤ 1.45·IZ) is usually the binding limit for gG fuses — going up a rating step can violate it even though condition 1 (IB ≤ In ≤ IZ) still looks fine. Always re-check both conditions, don't just bump the rating.
Does the curve type (B/C/D) affect cable protection?
No — overload protection (the IB/In/IZ/I₂ conditions on this page) is independent of curve type. Curve type only governs instantaneous tripping on short-circuit/high-inrush events. Choosing B/C/D is a separate decision driven by load inrush characteristics and short-circuit coordination, not overload protection.
How does this relate to the Cable Ampacity Calculator?
Size the cable first with the Cable Ampacity Calculator to establish IZ, then use that IZ value here to coordinate the protective device. Sizing the device before the cable risks picking a device rating the conductor can't actually support.