Register to start using the calculator!
One account unlocks every calculator on the site — try it free for 5 days, no card required.
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.Designing 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.
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 guide