Electrical Tool
Coordinate a gG fuse or MCB with the conductor per IEC 60364-4-43 overload rules and get the smallest standard rating that satisfies both protection conditions.
Quick Answer
Enter the design current IB and (ideally) the conductor capacity IZ. The calculator returns the smallest standard rating In with IB ≤ In ≤ IZ and the conventional tripping current I₂ ≤ 1.45·IZ per IEC 60364-4-43.
Overload Protection
Use practical planning values. This screens overload coordination only — short-circuit protection, disconnection time, selectivity and manufacturer data are separate checks for a qualified person.
Protective device
Circuit
Overload protection requires IB ≤ In ≤ IZ andI₂ ≤ 1.45 · IZ, where I₂ is the conventional operating current (gG: 1.6·In for ≥16 A; MCB: 1.45·In).
Formula / Method
It scans the standard rating series (IEC 60269 gG or IEC 60898 MCB) and returns the smallest rating that is at least the design current and still protects the conductor: IB ≤ In ≤ IZand I₂ ≤ 1.45·IZ.
In = min standard rating with IB ≤ In ≤ IZ and k₂·In ≤ 1.45·IZ
Inputs
Use the protective-device type, the design current IB and — to check both conditions — the conductor capacity IZ from the Cable Ampacity calculator.
Uses
Useful for IEC/DACH circuit design: picking a gG fuse or MCB that coordinates with the cable, sanity-checking an existing board, and documenting the overload basis before final design.
FAQ
The conventional operating current I₂ differs: a gG fuse uses ~1.6·In (for ratings ≥ 16 A), an MCB uses 1.45·In. So a gG fuse needs more conductor headroom to satisfy I₂ ≤ 1.45·IZ. Physical and breaking-capacity differences are a separate matter.
No — this checks overload coordination only. Short-circuit protection (I²t / breaking capacity) and fault-loop disconnection time (the 0.4 s / 5 s limits) are separate checks that depend on the prospective fault current and the earthing system.
Only if both conditions still hold. Condition 2 (I₂ ≤ 1.45·IZ) is usually the limit for gG fuses — going one size up can violate it even when In ≤ IZ still looks fine.
Learn More
Project
ITCOREOPS can support overload and short-circuit coordination, cable sizing, rack power, structured cabling, documentation and operational handover for technical rooms and data centers.
Disclaimer
This is a planning screen, not a substitute for design by a qualified person (Elektrofachkraft). Short-circuit protection, disconnection time, selectivity and manufacturer data are separate; the applicable standard edition and local network operator govern.
Workflow
Use this after sizing the cable (Cable Ampacity) to confirm the gG fuse or MCB protects the conductor against overload per IEC 60364-4-43.
Support
For protective-device coordination, cable sizing, rack power, structured cabling and operational infrastructure support, visit ITCOREOPS.
Feedback
Report display problems, calculation issues or suggestions for improving this calculator.
Worked example
Inputs: gG fuse, IB = 38 A, IZ = 54 A (e.g. 10 mm² Cu). Smallest standard ≥ IB → In = 40 A.
Condition 1: 38 ≤ 40 ≤ 54 ✓.
Condition 2: I₂ = 1.6 × 40 = 64 A ≤ 1.45 × 54 = 78.3 A ✓.
Result: 40 A gG.
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