Updated: 08.11.2026
A window drive is only as reliable as its supply cable. If the cable is too thin or too long, too little voltage reaches the drive. It then moves slowly, does not close fully or stops. This mainly affects 24 V drives, because at low voltage even small losses matter.
In brief: At 24 V, current draw and cable length determine the cross-section. Calculate the voltage drop and if in doubt choose the next larger cross-section. Connect the drive’s supply cable in a junction box with strain relief.
Calculating voltage drop simply
For copper cables, as an approximation: voltage drop in volts = 2 × length in m × current in A ÷ (56 × cross-section in mm²). The length is the one-way distance from controller to drive; the factor 2 accounts for the outgoing and return conductor.
Example: a 24 V drive drawing 1 A, 20 m away, 1.5 mm² cable. Voltage drop = 2 × 20 × 1 ÷ (56 × 1.5) ≈ 0.48 V, i.e. about 2%. That is not critical. With several drives on one cable, the currents add up.
The basics are explained in the glossary entry Cable cross-section. How large the power supply must be is shown in the guide Calculate 24 V power supply.
Installation and connection
- Do not extend the drive’s supply cable; connect it to the feed cable in a junction box.
- Leave a cable loop at the sash so that the cable is not under tension when the window opens.
- Route the cable so that it cannot be trapped between sash and frame.
- With 230 V, connection work must be carried out by a qualified electrician.
Typical mistakes
| Mistake | Consequence |
|---|---|
| Cross-section too small on a long cable | Drive moves slowly or stops |
| Plus and minus swapped (24 V) | Drive moves in the wrong direction |
| No slack for cable movement | Cable breaks after a while |
Frequently asked questions
Which cable should I use for a 24 V window drive?
For short distances and one drive, 1.5 mm² is often sufficient. For long cables or several drives, calculate the voltage drop and choose 2.5 mm² or more if necessary.
May I shorten the supply cable?
Whether shortening is permitted is stated in the drive’s instructions. If in doubt, do not shorten it but neatly place the excess length in the junction box.
Technically reviewed by Dietmar Heiken, Sales, Vent Systems HDI GmbH, Hamburg – around 20 years of industry experience. Questions? Phone +49 40 50694631 or info@ventsystems.de
Last updated: 08.11.2026
Questions? 040 50694631 · info@ventsystems.de
