Why 1.25 times the current?

That margin stops a fuse blowing on normal peaks and at higher ambient temperature. Loads with a large inrush may need a higher factor or a slow blow type.

What if the cable is too thin for the fuse?

Then use a thicker cable. Lowering the fuse below the load does not help: it will blow in normal use.

Does a fuse belong between two parallel batteries?

Between parallel batteries it is unusual; the branches to the loads are what needs protecting. The check in the editor therefore does not require one there.

Does this apply to 230V breakers?

The principle is the same, but at 230V separate rules apply to discrimination, characteristics and earth leakage protection.

Calculator

Fuse size calculator for 12V and 24V

Enter the power or current of the load and the cross-section of the cable. You get the next common fuse rating and whether your cable can carry it.

Free, no account needed
12V, 24V, 48V and 230V
Result while you type
Carries over into your diagram

System voltage

Calculate with

Load power WConductor size

Recommended fuse

70 A

Current: 50 A
Minimum needed: 62.5 A
Cable can carry: 120 A

Calculated with 25 % headroom above the continuous current; a high inrush current may require a slow-blow type.

The result is guidance based on your input. Manufacturer instructions and local regulations always take precedence; you remain responsible for the installation.

A fuse protects the cable

That is the core of it: the fuse should blow before the cable overheats. So the rating sits above the continuous current of the load — often 25 % above, to survive normal peaks — and below what the cable can carry. If that does not fit, the cable is too thin rather than the fuse too small.

As close to the source as possible

Every branch from the battery should be protected at the battery; the cable before the fuse is unprotected.

Pick the right type

Blade fuses for small loads, MIDI and MEGA or ANL for main currents; watch the interrupt rating with lithium.

Include inrush current

Motors, fridges and inverters briefly draw far more than their continuous current; use the peak from the datasheet.

From calculation to complete diagram

Designing a complete electrical system? In WireElectric you place fuse holders, busbars and loads in the diagram, and the app checks per circuit whether the protection matches the cable and the load.

You can start drawing without an account.

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WireElectric editor showing a 12V campervan diagram: LiFePO4 battery, MEGA fuse, busbars, MultiPlus-II and SmartSolar MPPT with cables between the terminals.

Questions about fuses

Why 1.25 times the current?: That margin stops a fuse blowing on normal peaks and at higher ambient temperature. Loads with a large inrush may need a higher factor or a slow blow type.
What if the cable is too thin for the fuse?: Then use a thicker cable. Lowering the fuse below the load does not help: it will blow in normal use.
Does a fuse belong between two parallel batteries?: Between parallel batteries it is unusual; the branches to the loads are what needs protecting. The check in the editor therefore does not require one there.
Does this apply to 230V breakers?: The principle is the same, but at 230V separate rules apply to discrimination, characteristics and earth leakage protection.

Other calculators

Every calculator answers one question about your installation. WireElectric then runs the same calculations for every connection in your complete diagram.