Calculating a 24 V power supply for window actuators

In short: Add up the current draw of all 24 V actuators that run at the same time and add about 20 % reserve. The result is the minimum current of the power supply in amps. The VS-C24-7A control unit delivers up to 7 A and replaces a separate power supply in ventilation systems.

The formula

Power supply current = total current draw of all actuators running simultaneously × 1.2

You get the power in watts by multiplying the current by 24 V. The current draw is on the rating plate or in the actuator's datasheet – for Vent Systems in the downloads area of the product page.

Worked example

A stairwell has four fanlights. According to the datasheet each actuator draws 1.0 A (example value). All four run together.

If the actuators don't run at the same time, only the group that runs together counts. On the VS-C24-7A all connected actuators run together – for separate groups use one control unit per group.

Power supply or control unit?

• Pure power supply: provides 24 V only. To open and close you also need switches with polarity reversal or a control unit.
• VS-C24-7A control unit: converts 230 V to 24 V (max. 7 A) and drives the actuators open and closed by switch or radio. Rain and wind sensors can be connected. €60 net, in stock in Hamburg.
• SHEV systems: Here the approved smoke vent control panel powers the actuators. The VS-C24-7A is not a smoke vent control panel.

Don't forget the cable cross-section

At 24 V, noticeable voltage is lost over long cables. If too little reaches the actuator, it runs slower or stops. Estimate the voltage drop on a copper cable like this:

ΔU = 2 × cable length (m) × current (A) ÷ (56 × cross-section in mm²)

Example: 20 m cable, 3 A, 1.5 mm² gives about 1.4 V – just under 6 % of 24 V. With 2.5 mm² it is only about 0.9 V. The permissible voltage drop is in the actuator's datasheet; if in doubt, choose the larger cross-section.

What to look out for

• Only add up the actuators that really run at the same time.
• Inrush currents and ageing are reasons for the reserve – don't calculate too tightly.
• Mount the power supply or control unit dry and well ventilated.
• Never mix up 24 V and 230 V actuators: a 24 V actuator must not be connected to 230 V.
• The 230 V connection must be made by a qualified electrician.

Read more: Laying cables for window drives: cross-section, length and connection

Frequently asked questions

How many actuators can the VS-C24-7A handle?

Up to 7 actuators, depending on current draw. The total of all actuators must stay below 7 A.

Is a power supply with exactly the calculated total enough?

Better not. Plan for about 20 % reserve so the power supply delivers enough current reliably at start-up and after years of operation.

Where do I find my actuator's current draw?

On the rating plate or in the datasheet. For Vent Systems actuators it is in the downloads area of the product page.

Learn more: 24 V power supply in the glossary · Current consumption · Voltage drop · Cable cross-section · Planning a ventilation control · Controls in the shop.

See also in the glossary: Polarity reversal · Safety extra-low voltage (SELV)