Power supply sizing for KNX bus systems

Every KNX device draws bus current to power its bus coupling unit. The total of every device's draw on a line, plus headroom, determines the PSU rating you need.

Common ratings: 320 mA, 640 mA, and 960 mA. A 320 mA PSU supports about 60 typical devices. A 640 mA PSU is the standard choice for a fully loaded line of 64 devices. 960 mA PSUs exist for extended lines or for lines with unusually current-hungry devices.

The calculation is straightforward. List every device on the line. Look up the bus current draw for each (typically 5 to 12 mA each, listed in the data sheet). Sum them. Add 30 percent headroom for telegram bursts and future expansion. Round up to the next available PSU size.

A worked example. 50 devices averaging 8 mA each = 400 mA. Add 30 percent = 520 mA. Choose a 640 mA PSU.

Be careful with high-draw devices. Some touch panels, especially older ones, can draw 30 to 60 mA. They look like one device on the count but eat a much bigger share of the budget. Always check the datasheet rather than estimating.

Where to install. The PSU goes on the line it powers, with the integrated choke connected to the bus. Some installers add a second PSU in parallel to extend the line; if you do this, follow the vendor instructions exactly and confirm the second PSU is rated for parallel operation.

If you suspect a PSU is undersized in a running project, look at the bus voltage at the far end of the line under load. KNX nominal is 30 V DC, and the bus typically runs at 28 to 30 V. A drop below 21 V under load means the PSU cannot keep up. Replacing the PSU often fixes years of intermittent issues.