Count the Modules Before You Cut the Hole in the Wall

Most of the arguments about home automation happen at the wrong level. Which protocol, which brand, which app. Those are real questions, but they are not usually what stops a project.

What stops projects is a hole in a wall that was sized for eighteen circuit breakers.

The distribution board is the least glamorous decision on a build and one of the few that is genuinely hard to reverse. Once the niche is cut, the board is flush-mounted, the wall is plastered and the passage is painted, the number of DIN modules you have is the number you are going to have. Everything the house can do electrically has to fit in that space.

So it is worth knowing what actually goes in there.

KNX gear lives on the same rail as your breakers

A KNX installation is not a box hidden in the roof. The parts that switch and dim your lights are DIN rail modules that sit in a board alongside the protection gear, and they are not small.

Take a Hager TXA606D, which is a fairly ordinary switching actuator. Six outputs at 16A, 230V, with the bus coupling unit built in. Hager's own technical data lists it at four modules wide and 90mm high. Six controllable circuits, four module positions.

It can also be configured as a three-channel blind and shutter actuator instead, which is useful, but it does not get any narrower when you do that.

Dimming is worse per circuit. An Ekinex EK-GA1-TP universal dimmer handles two dimmed groups at 300W maximum per channel, and Ekinex list it as four modular units at 18mm each. Two circuits, four modules. That is one module position per dimmed light group, plus overhead.

Then there is the bus supply. A Hager TXA112 is a 640mA KNX power supply with an integrated choke, and it is another four modules. It does no switching at all. It just keeps the bus alive.

None of these numbers are unusual. I am using Hager and Ekinex here because that is what I work with, but the picture is the same across the manufacturers. Actuator channels take space.

What that adds up to on a normal house

Run the arithmetic on a modest four bedroom house where somebody has decided they want proper lighting control.

Say twenty four switched circuits. Not twenty four light fittings, twenty four independently controllable groups, which on a house with a decent lighting design is not extravagant. Add four dimmed groups on top of that, because the lounge and the main bedroom deserve it, plus a bus supply to keep the whole thing alive.

There are two honest ways to build that and they land in different places.

Build it from 6-fold modules and you need four TXA606D actuators for the switched circuits, at sixteen modules. Two Ekinex dimmers, eight modules. The supply, four modules. Twenty eight module positions.

Build it from high density modules and it gets tighter. A Hager TYM620D switches twenty circuits by itself, and Hager's own datasheet puts it at ten width units. Add a 6-fold for the remaining four circuits, the two dimmers and the supply, and you land around twenty six.

So the good answer saves you two modules. That is the point I want to make. Even doing it the efficient way, with the densest actuator Hager sells for this job, you have used most of a board and you have not yet installed a single circuit breaker, earth leakage unit, surge protection device, isolator or IP interface. Nothing that a normal SANS-compliant board needs is in there yet.

Now look at what gets specified on site. The common residential boards in this market are twelve way, eighteen way, twenty four way built as two rows of twelve, and thirty six way built as three rows of twelve. Thirty six module positions in total on the biggest of those.

Twenty six of thirty six on the efficient version, before protection. That is the whole problem in one line.

The board is not optional and it is not free to move

The obvious response is to stop putting everything in the board. Fair enough. That is a real design choice with a name, and the manufacturers document it properly.

ABB's lighting application manual puts it about as plainly as anyone. Its planning section says the first question is whether switch actuators are installed centrally or on a distributed basis. Central installation, all loads wired back to one location, is "more transparent, fewer components are required and the cost per channel is lower," but it "requires much more wiring effort."

Distributed installation puts small actuators near the loads instead. Less load cable. But ABB is honest about the cost of that. Distributed installation "significantly increases the cost per channel," the system "can quickly lose on transparency," and the devices themselves may end up difficult to reach. More devices also means more programming, and the manual notes that extra power supplies and couplers may be needed.

Their conclusion, and mine after enough sites, is that a mix works best. And note the reason they give for going distributed in the first place. Their words: distributed installation may not be possible due to a lack of space in a residential building.

Read that the other way round. Where the board is too small, distribution stops being a design preference and becomes a workaround. You pay more per channel, the system gets harder to service, and someone in five years is opening a ceiling void to find a device.

I would rather have that as a choice than a consequence.

The decision that happens at first fix

There is a second half to this, and it is about cable, not space.

Conventional wiring loops light points together and drops down to a switch. The intelligence is the physical switch position. Twelve lights on a circuit, four switch drops, done. It is efficient and it is why houses have been wired that way for decades.

KNX inverts it. Every group you want to control independently comes home to the actuator as its own switched live. The switch on the wall carries bus, not load. So the number of separately controllable circuits in the house is fixed by how many cables were pulled back to the board during first fix, not by anything that happens in software later.

This is the part that surprises people. Adding a light to an existing KNX house is easy if the circuit already terminates at the board. If it does not, you are chasing walls.

So the two constraints stack. Cable count decides what can be controlled. Module count decides whether the hardware to control it fits. Both are set before the ceilings close, and neither is on a standard electrical layout drawing unless someone asks for it.

The load shedding angle nobody plans for

Eskom has been quiet for a long stretch now, and I have written before that this does not make resilience irrelevant, it just moves the risk to municipal distribution. City Power outages arrive without a schedule.

Here is where the board layout matters for that.

If your KNX supply, actuators and gateway are grouped together, feeding that group from an inverter or UPS circuit is a straightforward piece of wiring. Lights, gate, alarm interface and control logic stay up. If they are scattered across a board that is already jammed, or worse, distributed into six ceiling voids on unknown circuits, splitting essential from non essential loads later becomes an expensive mess.

A separate automation sub-board is often the cleanest answer. It costs a bit more at build, it keeps the protection board tidy for whoever inspects it, and it gives you one obvious thing to back up.

What I would put in the brief

None of this needs a full automation design at concept stage. It needs four lines on a drawing and one conversation.

Ask for a module count, not a way count. "Thirty six way board" tells you nothing about whether the automation fits. Ask the electrical designer and the integrator for the estimated module total including actuators, supply and interfaces, then size the enclosure with headroom on top of that. Boards fill up. They always fill up.

Decide central versus distributed deliberately. Write down which it is and why. If it is distributed, make sure someone records where every device physically sits, because in three years that information is worth more than the device.

Fix the circuit schedule before first fix. Every independently controllable lighting group needs its own home run. That list is a design decision and it belongs to whoever is doing the lighting design, not to the electrician on the day.

Put the automation on its own sub-board and its own supply path. Even if there is no inverter yet, wire as though there will be.

Give the board somewhere sensible to live. It needs cable access from the whole house, ventilation, and enough space in front of it that a person can work on it. A board buried behind a hinged door in a scullery cupboard is a board nobody maintains properly.

Why I keep coming back to this

I get called to houses where the electrical work is finished, the client has decided they want automation, and the honest answer is that the cheap version of what they want is no longer available. Not because KNX cannot do it. Because there is nowhere to put the hardware and no cable to the places they want to control.

The fix at that stage is a second board, a lot of chasing, and a conversation about money that nobody enjoys.

The same house, with a bigger enclosure and a properly written circuit schedule at first fix, would have cost a fraction more during construction and left every option open. Including the option of doing nothing about automation for five years and then changing their mind.

Space and cable are cheap while the walls are open. What they buy you is the right to decide later.

If you are at drawing stage on a build and want to talk through what the board and the circuit schedule should look like before the electrician starts, that is worth half an hour early rather than a redesign later. No sales pitch. Just a conversation about what the project needs.

Wayne KNX Logic wayne@knxlogic.co.za | 082 564 3982 | www.knxlogic.co.za

Sources

Output module 6x 16A (C)/230V~, KNX TXA606D, Hager product technical properties. Six outputs, 16A, four modules wide, 90mm high, integral bus coupling unit, IP20. https://hager.com/intl-en/products/information/txa606d-output-6-fold-16a-c-load-adapted-ne

2-fold universal dimmer EK-GA1-TP, Ekinex product page. Two outputs, 300W maximum and 10W minimum controlled power per channel, four modular units at 18mm each, DIN rail EN 60715. https://www.ekinex.com/en/23/2-fold-universal-dimmer.html

Product datasheet, TYM620D, Hager, dated 10 February 2026. Output module 20x 16A/230V, ten width units, 90mm high, 65mm deep, manual switch, IP20. https://assets.hager.com/step-content/P/PDS_1607842800IDEN/Document/std.lang.all/PDS_TYM620D_00ID_en-GB.pdf

Power supply 30V DC 640 mA, TXA112, Hager. KNX bus power supply with integrated choke, listed at four modules. https://hager.com/intl-en/products/information/txa112-power-supply-30v-dc-640-ma

ABB i-bus KNX Application Manual, Lighting (2CDC 500 051 M0203). Central versus distributed installation planning, cost per channel, transparency, wiring effort, and the note that distributed installation may not be possible due to lack of space in a residential building.

South African residential distribution board configurations, 12 module, 15 module, 18 module, 24 module (2x12) and 36 module (3x12), per local supplier catalogues including CBI Low Voltage and ElectroMechanica.