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Overcurrent Protection Sizing
Circuit and cable parameters
Overcurrent protective device
Short-circuit check (optional)
Enter the short-circuit currents to check breaking capacity, instantaneous tripping and thermal protection of the cable. Leave empty to skip.Overcurrent protection sizing online
The calculator selects and verifies the overcurrent protection of a circuit — an MCB circuit breaker or a gG fuse — in accordance with the PN-HD 60364-4-43 standard. It is a natural complement to cable cross-section sizing: after selecting the cable you select the matching protection.
The tool checks the overload condition (Ib ≤ In ≤ Iz, I2 ≤ 1,45·Iz), and once the short-circuit currents are entered, also the breaking capacity, instantaneous tripping and thermal protection of the cable. It additionally selects a residual current device RCD/RCBO.
How to size protection in 3 steps
Enter the design current Ib and the cable current capacity Iz (or import them from a saved cable cross-section sizing).
Choose the device type and characteristic — the calculator selects the smallest rated current In that meets the overload protection conditions.
Optionally enter the short-circuit currents and enable RCD selection to obtain a full circuit verification.
What the calculator computes
The calculator performs a complete set of protection sizing checks:
- Overload protection: selection of In for an MCB circuit breaker or gG fuse per Ib ≤ In ≤ Iz and I2 ≤ 1,45·Iz.
- Short-circuit check: breaking capacity Icn ≥ Ik max, instantaneous tripping Ik min ≥ Ia and permissible thermal time t_dop.
- RCD/RCBO selection: residual current IΔn (30/300 mA), type (A, AC, F, B) and rated current In(RCD) ≥ In of the protection.
- Comparison of adjacent devices from the rated range, with an indication of compliance with the conditions.
Important design notes
The calculator does not replace a full check of automatic disconnection with the fault loop impedance (Zs ≤ U0/Ia), discrimination or selection of the RCD type per the load manufacturer's requirements (inverters, EV chargers, heat pumps usually require type F or B). Provide the short-circuit currents Ik from a separate calculation. For a full thermal verification use the device manufacturer's I²t data.
Related calculators
Plan a complete electrical circuit using the other tools in the electrical module:
Guide: selecting overcurrent protection
Want to understand where the conditions Ib≤In≤Iz and I2≤1.45·Iz come from, how the B, C and D characteristics differ and when an RCD is required? Read our detailed guide.
Read the protection selection guideDesigning a cable line? Build the route's longitudinal profile and a CAD-ready DXF.
Paste the route coordinates — Altivo builds the longitudinal profile with elevations, and the DXF export drops straight into AutoCAD. No install, no dongle, in the browser.