Plan control panel: Divide circuits

Why the layout must be determined before ordering

A switch panel is the only component in the onboard electrical system that can hardly be changed after installation. It sits in a cutout, has a fixed number of slots, and labeling that was created at the time of manufacturing. Anything added later will be attached, and attached solutions are the beginning of a confusing onboard electrical system.

Planning, therefore, answers four questions, in this order: What loads are there? Which ones belong to a common circuit? What cross-section and fuse belong to each circuit? And how much space is left for what is yet to come?

Grouping loads

A circuit combines loads that can be switched and fused together. Three criteria determine the assignment.

Function. What is needed together belongs together. The interior lighting of a saloon is one circuit, not six.

Safety relevance. Bilge pump, navigation lights, and radio each get their own circuit. They must not fail because a fuse blows elsewhere. For the same reason, they should, if possible, not be placed behind a battery management system that can completely disconnect the service battery bank in case of a fault — the usual solution is to power them from the starter circuit.

Current and path. A large individual load — anchor winch, inverter, bow thruster — does not belong in the switch panel, but rather with its own cable and fuse directly to the battery or to a high-current distributor. Otherwise, its current would dominate the panel and its supply line.

NoteA common misconception is to group by room instead of by function. "Everything in the forward cabin" sounds neat but leads to the reading lamp being on the same fuse as the pressurized water pump — two loads with completely different current profiles.

Per circuit: Cross-section and fuse

Once the grouping is established, each circuit is calculated individually. The process is always the same:

1. Total current of the circuit. All loads that can run simultaneously, added up.
2. Distance. From the panel to the furthest load of the circuit, measured simply.
3. Cross-section. Based on the larger value of voltage drop and current carrying capacity.
4. Fuse. Above the continuous current, never above the current carrying capacity of the cable.

After that, the supply line to the panel is calculated. It does not carry the sum of all circuits, but the sum of what realistically runs simultaneously — but it is also the circuit where under-dimensioning is most unpleasantly noticeable because it affects all others. Generosity is worthwhile here.

The order of components

Between the battery and the panel there are two components, and their order is not arbitrary:

Battery → Main fuse → Main switch → Switch panel.

The fuse is placed first, as close as possible to the positive terminal, so that the short cable to the switch is also protected. If the switch were first, the thickest piece of cable with the highest short-circuit current would be unprotected.

In a DC electrical system, usually only the positive pole is switched and fused; the negative pole runs as a common return line to a central negative point. Two-pole disconnection is useful where stray currents and galvanic corrosion play a role — typically in systems with shore power.

Reserve

20 to 30 percent free circuits are the most cost-effective item in the entire planning. Something practically always gets added: USB sockets, additional lighting, a second fan, later a solar system with its own regulator.

The reserve must be twofold: free slots in the panel and capacity in the supply line. A panel with four free switches whose supply line is already at its limit has no reserve, only free slots.

Exceptions and special cases

AC in the same enclosure. Combined panels for 12 V and 230 V are common, but require separate wiring compartments, their own protective devices, and clear labeling. The AC side has its own requirements according to ISO 13297:2020.

Expanding an existing panel. If the cutout is to remain, a replacement panel of the same size is usually the cleaner way than an add-on. Before ordering, the cutout dimensions, installation depth, and the location of the cable entries should be recorded — the installation depth is the value most often forgotten and most often prevents installation.

Systems with monitoring. If a battery or tank monitor is integrated into the panel, it needs its own, continuous supply that does not drop with the main switch — otherwise, the amp-hour meter loses its reference point every time it is switched off.

Typical mistakes

• Grouped by rooms instead of by function. Loads with completely different current profiles end up on one fuse.
• Safety-relevant loads in common circuits. They fail too if there's a fault elsewhere.
• High-power consumers routed through the panel. Anchor winch and inverter belong directly to the battery.
• Supply line too small. It limits everything downstream.
• No reserve. Or only free switches without reserve in the supply line.
• Main switch before the main fuse. Leaves the most dangerous cable section unprotected.
• Installation depth not measured. The most common reason why a replacement panel doesn't fit.
• Labeling conceived only after installation. It belongs in the planning, because for manufactured panels, it's part of the order.

Clarified before ordering

Onboard voltage and whether 230 V should be in the same panel. Number of circuits including reserve. Rated current per circuit and from that, the fuse sizes. Total current and cross-section of the supply line. Cutout dimensions, installation depth, and location of cable entries. Whether a monitor should be integrated and where it gets its continuous supply from. And the exact wording of the labeling for each circuit.

Anyone who has these points clear before ordering will receive a panel that fits — and will only have to update the list for the next conversion instead of creating it from scratch.