How to size KNX power supplies for your project

A KNX TP line needs its own power supply. This guide shows how to choose the supply rating from the devices on the line, with a worked example and the cable rules to check.

1. What the KNX power supply does

On KNX TP (twisted pair), the bus cable carries both the telegrams and the power for the bus devices. Each line segment is powered by its own KNX power supply, which delivers a safety extra-low voltage (SELV) of about 30 V DC through a choke. The choke is what lets data and power share the same pair of wires, so an ordinary DC supply cannot replace a KNX power supply.

2. Add up the bus current of every device

Every bus device takes a current from the bus. The value is printed in the datasheet, usually as "current consumption from the bus" in mA. Make a list of all devices on the line and add their values.

Small devices such as push-button interfaces and binary inputs often take only a few mA.
Touch panels, displays, presence detectors and devices with LEDs or radio can take much more.
Devices with their own auxiliary supply may take very little from the bus. Read the datasheet instead of assuming.

If you do not have every datasheet yet, plan with 10 mA per device as a rough placeholder, then replace it with the real values before ordering.

3. Keep a reserve

Do not run a supply at its full rating. A reserve of about 20% leaves room for devices added later and for differences between datasheet values and real consumption.

Required rating = total bus current × 1.2

4. Pick the next standard rating

KNX power supplies are commonly made with ratings of 160 mA, 320 mA and 640 mA, and some manufacturers offer 1280 mA units. Choose the first rating above your required value.

Rating	Typical use
160 mA	Small lines or a decentralized supply for a few devices
320 mA	Small to medium lines
640 mA	The usual choice for a full line of standard devices
1280 mA	Lines with many high-consumption devices, where the manufacturer's line rules allow it

5. Worked example

A villa floor has one KNX line with these devices (example values, use your own datasheets):

12 push-button interfaces at 5 mA = 60 mA
8 presence detectors at 15 mA = 120 mA
6 switch and dimming actuators at 10 mA = 60 mA
2 room thermostats with display at 20 mA = 40 mA

Total bus current: 280 mA. With a 20% reserve: 280 × 1.2 = 336 mA. A 320 mA supply is too small, so this line needs a 640 mA supply.

6. Check the line rules too

Current is only one limit. Before you finalize the design, also check the KNX TP topology rules:

A standard line segment takes up to 64 bus devices. More devices need a new line with its own power supply and a line coupler.
Total bus cable length in one line segment: up to 1,000 m.
Distance between the power supply and any device: up to 350 m.
Distance between any two devices: up to 700 m.
If you need two power supplies in one segment, follow the installation rules in the manufacturer's documentation for that supply.

Long cable runs to the far end of a line also lower the voltage at the last devices. Placing the supply near the middle of the line, rather than at one end, helps.

7. KNX power supplies on KNX MORE

Plan the whole project for free

List your rooms and devices in the free KNX project planner and send the request to the KNX MORE team.