KNX Bill of Materials: A Practical Project Checklist
Create a KNX bill of materials with a worked channel-count example, bus-current budget, accessory checklist, spare-capacity rules, and procurement checks.
A KNX bill of materials is a purchasing schedule tied to a design. It should explain not only how many devices to buy, but why those quantities are sufficient and which assumptions could change them.
The quickest way to produce an expensive omission is to start with “one actuator per room.” Rooms do not consume actuator channels; separately controlled loads do. Some need switching, some need dimming, and some should never be connected to the room automation at all.
Build the BOM from three inputs: a function schedule, an electrical load schedule, and a proposed topology. Keep installation materials and engineering services visible even if another contractor supplies them. Otherwise a low hardware total can look like a complete project price.
Use rows that can survive a product substitution
A line saying “KNX actuator, quantity 3” cannot be checked. It needs enough information to compare the proposed replacement against the original design.
| BOM field | What to record | Why it matters |
|---|---|---|
| Reference and location | Unique tag, panel, floor, or room type | Connects purchasing to drawings and commissioning |
| Function and quantity basis | Loads served, channels used, units installed | Makes the count auditable |
| Electrical requirement | Voltage, load type, inrush/duty constraints | Prevents selection by nominal amperes alone |
| Product identity | Manufacturer, order code, hardware/application version where relevant | Distinguishes similar-looking variants |
| Interfaces and supplies | KNX medium, auxiliary voltage, Ethernet, licensed options | Exposes dependent purchases |
| Physical allowance | DIN modules or dimensions, terminal/access requirements | Makes cabinet and wall-box coordination possible |
| Procurement status | Proposed, approved, ordered, delivered, installed | Prevents an unapproved alternate becoming the as-built record |
Link each row to a datasheet and an approved submittal. Keep the quantity calculation in the schedule or in a referenced sheet; it should not live only in the estimator’s memory.
Worked example: count outputs before selecting devices
Consider a hypothetical small office with eight switched lighting circuits, four dimmed lighting circuits, six independently operated blinds, and two fan coils. This is a counting exercise, not a construction-ready design.
| Function | Required controlled units | Illustrative selection | Important qualification |
|---|---|---|---|
| Switched lighting | 8 circuits | Two 6-channel switching actuators | 12 channels installed; 4 unused, subject to load compatibility |
| Dimmable lighting | 4 circuits | One 4-channel dimmer | Only if every driver/load is compatible and capacity is adequate |
| Blinds | 6 motors | Two 4-blind actuators | Eight motor channels, not eight unrelated relay outputs |
| Fan coils | 2 units | Two suitable FCU controllers | Depends on valve, fan, piping, and control interface |
Do not add all those units and order a single generic relay bank. Blind motors need interlocked direction control through a suitable device. A dimming circuit may require a gateway and compatible drivers instead of a phase-cut dimmer. An FCU’s fan speeds are not simply extra lighting channels.
This example’s four unused switching channels are available only where their wiring, electrical grouping, and ratings make them useful. They are not a site-wide reserve that can magically serve another distribution board. For the underlying allocation method, see how many channels a KNX actuator needs.
Calculate two power budgets, not one
Add the KNX bus-current requirements of the selected devices on each supply segment. Separately calculate auxiliary supplies for displays, gateways, or other equipment that requires them. A device connected to KNX is not necessarily powered entirely by KNX.
The following values are assumptions for illustrating the arithmetic, not typical ratings to use for procurement:
| Assumed device group | Quantity | Assumed bus current each | Subtotal |
|---|---|---|---|
| Wall controls | 12 | 10 mA | 120 mA |
| Presence sensors | 8 | 12 mA | 96 mA |
| Actuator/controller devices | 7 | 10 mA | 70 mA |
| IP interface | 1 | 15 mA | 15 mA |
| Total | 28 | — | 301 mA |
With a project-chosen 25% planning allowance, the budget becomes 301 × 1.25 = 376.25 mA. That exceeds a 320 mA supply’s nominal rating. A larger approved supply may be appropriate, but its selection still requires the topology, cable limits, operating conditions, and manufacturer’s instructions to be checked.
The 25% allowance is a design choice, not a KNX rule. Device-count limits and telegram traffic are separate checks; spare current alone does not establish spare line capacity. ABB’s SV/S power-supply technical data is an example of the product-level documentation needed to confirm the actual supply characteristics. See also KNX power-supply selection.
For auxiliary power, record the voltage, maximum load, startup demand if relevant, cable drop, protection, and whether the supply must remain available during maintenance. Do not substitute an unchoked auxiliary output for a KNX bus output without explicit manufacturer approval.
The easily missed lines are often not KNX devices
| Package | Items to account for | Typical owner to confirm |
|---|---|---|
| Bus installation | Approved cable, terminals, labels, joints, test allowance | Electrical contractor |
| Cabinet | Enclosure, rail, ducts, terminals, protection, ventilation allowance | Panel builder and electrical designer |
| Wall equipment | Correct back boxes, mounting frames, finish samples, adapters | Electrical and interiors teams |
| Network | Switch ports, patch leads, VLAN configuration, power backup where required | IT/network contractor |
| Integration | Gateway, protocol option, point/license allowance, connector work | Integrator and third-party supplier |
| Service | Spare units, programming interface, replacement instructions | Owner and integrator |
| Handover | Labels, test records, ETS backup, approved drawings | Commissioning lead |
Count cable from routes, not from the distance between symbols on a room plan. Include the actual riser path and approved termination allowances. Keep electrical protection design with the qualified designer; a controls BOM does not establish conductor sizes or protective-device coordination.
Panel space also needs its own reconciliation. A row of devices can fit on the DIN rail while leaving no usable termination space. Compare the BOM with the cabinet layout, not just the enclosure’s advertised module count.
Specify spare capacity by resource
“20% spare” is incomplete. Ask: spare what, and where?
For outputs, identify unused compatible channels. For panels, reserve usable physical and wiring space. For the bus, reserve current and review traffic/address constraints. For gateways, check remaining points, concurrent clients, and licenses. For maintenance, distinguish a spare device on a shelf from installed spare capacity.
A shelf spare may need an application file, supported firmware, and secure commissioning materials to be useful. Document who maintains those resources and who is allowed to access them. Buying a spare without a replacement procedure can leave the operator with a sealed box and the same outage.
Freeze a baseline, then price changes against it
At tender stage, label unresolved choices. At approved-design stage, fix order codes and quantity bases. Before ordering, reconcile every revision against the current drawings and load list. At handover, replace estimated locations and quantities with installed records.
A substitution review should answer four questions: does the replacement meet the electrical duty, expose the required KNX functions, fit physically, and preserve the commissioning workflow? A cheaper unit with different feedback objects can consume the saving in rework.
Record additions and deletions explicitly. If the client adds a curtain, the change may involve an actuator channel, cable, control page, scene membership, and acceptance testing. The BOM is where those dependencies become visible.
Is ETS part of the BOM?
Record who provides the required engineering software and licenses, even if they are a service cost rather than an installed asset. Verify the current license terms and project needs with KNX before purchasing; do not assume every project needs its own separate license.
Should labor be mixed into the material schedule?
Keep it in a clearly separated service section or linked commercial schedule. Programming, integration, mock-up work, testing, and training should remain visible in the total scope.
When is the BOM ready for quotation?
When a second engineer can trace each major quantity to a function or drawing, identify the unresolved assumptions, and distinguish included work from exclusions. A supplier can then quote the project scope without guessing what “complete KNX system” means.