The question "how many circuits do I need" is almost always answered too narrowly. And the question "with what rated current" is almost always answered according to the consumer instead of the cable. Both are errors that leave a panel too small and improperly fused after two years.
What justifies a dedicated circuit
A dedicated circuit is not a luxury, but the means by which a fault can be isolated. Four criteria each argue for separation:
- Independent operation. What is to be switched separately needs its own circuit. This is the most common and simplest criterion.
- Importance. Consumers that should still run even if others fail — navigation, lighting in walkways, ventilation — do not belong with comfort consumers.
- Current magnitude. A large consumer forces the entire circuit to a high rated current — and thus to a thick cross-section for all connected consumers.
- Probability of failure. What is outside, gets wet, or is moved is more likely to fail. Keeping such consumers separate prevents their failure from darkening half the interior.
How many circuits it actually turns out to be
The reliable method is a list, not a gut feeling. Each consumer gets a line, followed by the question of whether it can be switched together with another. What remains is the number.
Reserve is added to this number. One to two free circuits are the difference between "adding a consumer" and "replacing the panel". Experience shows that they are occupied after the first season — not because of poor planning, but because needs become apparent during operation that no one could have identified beforehand.
NoteCircuits on an existing panel cannot be increased. Reserve is therefore the only flexibility you can take with you when buying — everything else will later cost a new panel.
Determining the rated current
Circuit breakers are available in several gradations — the usual standard value is 10 A; smaller and larger rated currents are also available. The selection follows a rule older than any panel:
The fuse protects the cable, not the device.
This leads to the following sequence: first determine the operating current of the consumer, then its inrush current, then determine the required cross-section via length and permissible voltage drop, then its current carrying capacity — and the rated current lies between the inrush current and the carrying capacity.
If no suitable value is found there, the cross-section is too small. Increasing the rated current to avoid tripping reverses the protection exactly.
Sensibly grouping consumers
Where several consumers are to be connected to one circuit, two rules apply.
Firstly: The rated current is based on the sum in the simultaneity case, the cross-section on the rated current. Four lights at 1 A each on a 10 A circuit are unproblematic; four pumps at 4 A each are not.
Secondly: Consumers connected to the same circuit fail together. This is acceptable for lighting in the same room, but not for navigation and a coffee machine.
Labeling is part of the planning
Labeling is created not after installation, but with the list. Those who add it later label according to what they remember — and that is not accurate everywhere.
Meaningful designations are those that a third person understands: "Saloon lighting" instead of "Light 2". And the assignment also belongs on a sheet in the technical area, with rated current and cross-section per circuit — this is the document you will need first for any subsequent troubleshooting.
Typical errors
- Too few circuits. The error that is most expensive to correct retroactively.
- Rated current chosen according to the consumer. The cable is decisive.
- Important and unimportant consumers mixed. A fault in a comfort consumer then shuts down essential systems.
- No reserve. One to two free circuits are the only flexibility you can buy.
- Labeling added later. It is then only approximately correct — and approximately does not help with troubleshooting.
