Eight channels are sufficient for a compact expansion. They are not enough for a grown-up on-board electrical system with separately operable light circuits, multiple pumps, drives, and special functions. In that case, multiple units are combined — in series or parallel, depending on what is scarce.
First clarify: What is scarce?
Before asking about the arrangement, one must ask about the bottleneck. There are two, and they require different solutions:
- The number of channels is scarce. Twelve circuits, eight outputs — the most common case. Here, simply more outputs are needed, not more current.
- The total current is scarce. Eight circuits suffice, but the sum of concurrently running consumers exceeds 90 A. Here, more current capacity is needed.
In practice, the first case is significantly more common. Whoever confuses the two solves the wrong problem: Two units whose channels are barely loaded together do not solve a current problem — and a larger supply line does not create an additional output.
Sensible distribution
If two units are set, the distribution of circuits is not a mere formality. Three criteria have proven effective:
By area. Everything that belongs together spatially goes to the same unit. This noticeably shortens cable runs if the units are located in different places.
By importance. Critical circuits — lighting in escape routes, ventilation, safety-relevant consumers — not all on the same unit. If one fails, a part of the on-board electrical system remains operable.
By simultaneity. Distribute consumers that typically run together across different units. This keeps the load per unit more even, instead of pushing one to its limit while the other is idle.
NoteThe three criteria occasionally contradict each other. In that case, importance takes precedence over simultaneity, and simultaneity over area — short cables are convenient, but redundancy in case of failure is more valuable.
Recalculate the supply line
The point most frequently overlooked when expanding: Two units can draw more current together than one. The supply line from the battery and its fuse are designed for the original case and must be re-evaluated.
Crucial again is the realistic simultaneity case across both units — not the sum of the nominal currents. But this sum increases, and with it, the cross-section that the common supply line needs.
Two approaches are feasible: a common, appropriately larger supply line with distribution close to the units — or two separate supply lines, each independently fused. The second approach costs more cable and provides genuine separation: a fault in one branch does not shut down the other.
Documentation becomes mandatory
For one unit, the occupancy table is helpful. For two, it is necessary. The reason is simple: there are now two "Channel 3s," and the question of which one is meant cannot be read from the device.
The table therefore needs one more column: Unit, Channel, Consumer, Current Limit, Function. The units themselves are given a clear designation that is also physically on the housing — not "top" and "bottom," because that changes with the next modification.
Test after expansion
- Supply line and main fuse checked for the new overall case.
- Both units clearly labeled, physically and in the configuration.
- Occupancy table extended and printed, stored in the technical area.
- Critical circuits distributed, not bundled on one unit.
- Failure test performed: What still works if one unit is de-energized?
Typical errors
- Solved the wrong problem. Missing channels and missing current are two different bottlenecks.
- Left the supply line unchanged. The bottleneck then shifts from the unit to the cable before it.
- All important circuits on one unit. Wastes the redundancy that the second unit provides.
- Units named by installation location. "Top" and "bottom" will not survive the next modification.
- Never tested for failure. Only the test shows whether the distribution is suitable.
