24V window actuator wiring has to be sized for voltage drop, not just current, so long feeds run in heavier cable and step down near the windows. At Royal Harbour Academy in Ramsgate we re-cabled eight 24V window actuator feeds in 4 mm² fire-resistant cable and reinstated the rooftop control cabling during a roof replacement in September 2026.
The school was having its roof replaced. Everything on it had to come off or be disconnected: the air handling units, the kitchen and toilet extract fans, and the cabling to the motorised windows. Our job was to disconnect the controls, replace what was damaged, and put the rest back once the new roof was down. We were on site for three weeks.
New tray on the re-covered roof, with the control cabling laid flat and tied at even spacings.
How long can 24V window actuator wiring run before voltage drop becomes a problem?
It depends on the current the group draws, the cable size and the lowest voltage the actuator will run on, which is on the manufacturer's data sheet. At 24V you don't have much to lose.
BS 7671:2018, Appendix 4, Table 4D2B gives 29 mV per amp per metre for 1.5 mm² two-core cable and 11 mV/A/m for 4 mm². On a 40 metre run carrying 4 A, 1.5 mm² loses 4.64 V, which is 19% of a 24V supply. 4 mm² loses 1.76 V, about 7%. Undersize the feed and the actuators run slow, stall or don't close the window fully.
That's how this school is wired. Eight 4 mm² two-core and earth feeds run from a 24V actuator control panel downstairs, fed from its own transformers, up to eight junction boxes on the roof. Each box splits out on 1.5 mm² or 2.5 mm² to a group of about five window actuators.
How do window actuators integrate with a BMS?
Usually through a window control panel. Most 24V DC actuators are two-wire: one polarity opens the window, the other closes it. Where a BMS controls the windows, its output switches a relay or contactor in the panel, and the panel's own supply drives the motors. The BMS output shouldn't carry motor current for a whole group.
Why does automatic window control matter in a school?
In a naturally ventilated classroom the windows are the ventilation. The DfE's Building Bulletin 101 (2018) says the daily average CO₂ in naturally ventilated teaching spaces should stay below 1,500 ppm, and shouldn't go above 2,000 ppm for more than 20 consecutive minutes in a day. If a group of windows won't open, that's hard to meet. More on this in our guide to BMS for schools and education buildings.
At Royal Harbour Academy the windows are also smoke vents. The actuator control panel has a fire alarm input, which is why the feeds are fire-resistant cable.
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What does a roof replacement do to window actuator wiring and BMS cabling?
Anything wired to the roof has to be isolated and disconnected before the old covering comes off. Here the AHUs were split into sections and craned off, then craned back and rebuilt on the new roof. New cable tray went in along the old routes.
The window feeds were the main problem. They ran across the roof in the open and the sheath on the exposed lengths had weathered and cracked. That cable couldn't be moved and reconnected safely.
The old feed cable after years on the roof. The print on the sheath shows it was BS 7629-1 fire-resistant cable.
Which standards apply to 24V window actuator wiring and rooftop controls?
BS 7671:2018. The wiring regulations cover the whole installation, and Appendix 4 is what you size a long 24V feed against.
BS 7629-1. The product standard for fire-resistant cable. The old cable was made to it, and so is the new: Prysmian FP200 Gold, two-core 4 mm² and earth.
Electricity at Work Regulations 1989, regulation 13. Equipment that has been made dead must be kept from becoming live while you work on it. We isolated the actuator control panel and proved every circuit dead at each roof junction box before disconnecting anything. (HSE guidance HSR25.)
Building Bulletin 101 (2018). Sets the CO₂ limits the windows are there to meet.
Is it difficult to retrofit wiring for automated windows in an existing building?
Not usually. We didn't replace every feed end to end, because most of the cable was fine. The damage was only on the roof. We traced each of the eight feeds back to sound cable, which was inside, in a store room downstairs.
There we fitted a 300 mm termination box on the wall with DIN rail terminals. The eight existing cables come into the bottom. Eight new runs of FP200 Gold come out of the top, go up the riser and across the new tray to window junction boxes one to eight. On the roof we used a seven-metre stub of the old cable as a draw wire to pull the new runs through. Each feed now has one joint, indoors and accessible, instead of old cable lying on a flat roof.
The new termination box in the store room before the cables went in. Existing feeds enter at the bottom, new feeds leave at the top.
One of the eight roof junction boxes. The new feed comes in from the tray and the actuator cables go out to a group of windows.
What else has to be reinstated when plant comes off a roof?
AHU control cabling
You can't crane an AHU off in sections with the cabling still threaded through it. Before the lift, every cable in the units' internal containment was disconnected, pulled out of the control panel and drawn back into its own section. When the sections were back on the new roof and rebuilt, we threaded it all back through, section by section, and terminated it in the panel.
One of the AHUs back in position on the new roof.
The new tray from the fans and AHUs to the roof BMS panel was fitted by others on this job; we install containment too. We re-dressed the serviceable existing cabling onto it with the new runs, glanded everything into the bottom of the panel and terminated it. Every run is flat, parallel and tied at even spacings, so the next engineer can trace a cable in minutes. More on this kind of work in BMS electrical installation in Kent.
Extract fans
The kitchen and toilet extract fans had never been on the BMS. We pulled two new control cable runs, 130 metres in total, from the fans back to the roof BMS panel and terminated them for run and fault monitoring.
An extract fan rebuilt on the new roof and wired back to the roof BMS panel.
Pressure switches and network
The old differential pressure switches were replaced with new ones, and we pulled cable for an extra DP point. Two Cat6 cables went in, one to the roof BMS panel and one to the AHU panel (structured cabling). Then the system was tested and commissioned.
What does good 24V window actuator wiring look like?
- Feeds sized for voltage drop over the actual run length.
- A window control panel powering the actuators, with the BMS (if there is one) only giving the command.
- Joints in proper enclosures with terminals, indoors where possible.
- Sound cable reused, damaged cable cut back and replaced.
- Every group tested open and closed before the scaffold comes down.
When should you bring the controls contractor in?
Before the plant is disconnected. While everything is still working you can prove what each cable does. Once it's all unplugged and stacked at the edge of the roof, every unlabelled cable has to be traced. It's also the cheapest time to add monitoring or replace worn devices, because the access and scaffold are already there.
Frequently asked questions
How long can actuator wiring run before voltage drop becomes a problem?
It depends on load, cable size and the actuator's minimum voltage. BS 7671:2018 Appendix 4, Table 4D2B gives 29 mV/A/m for 1.5 mm² two-core copper and 11 mV/A/m for 4 mm². A 40 m run at 4 A drops 4.64 V on 1.5 mm² but 1.76 V on 4 mm².
Can multiple window actuators share one power supply?
Yes, provided the supply and the cable are sized for the combined current of the group. At Royal Harbour Academy each 4 mm² feed serves a junction box supplying a group of around five actuators.
What power supply is typically used for window actuators?
24V DC is common on commercial buildings, with 230V AC actuators also available. Always work from the manufacturer's data sheet for current draw and minimum operating voltage.
Can automated window wiring integrate with building management systems?
Yes. Typically a BMS output gives the open and close command to relays or contactors in a window control panel, and the panel's own supply powers the actuators. The BMS output should not carry the motor current itself.
Is it difficult to retrofit wiring for automated windows in existing buildings?
Usually not, if sound cable is kept and only damaged sections are replaced. At Royal Harbour Academy weathered roof cable was cut back to a good section and joined to new cable in a termination box indoors.
What happens to the BMS when a roof is replaced?
Control cabling to rooftop plant is isolated, disconnected and drawn back so the plant can be lifted and the roof re-covered, then re-threaded, re-terminated and tested. That can include AHUs, extract fans, BMS panels, pressure switches and window actuator feeds.
Can extract fans be added to an existing BMS?
Yes, if the fan or its starter provides a run and a fault signal, there is a spare input at the nearest BMS panel, and the points are configured on the BMS. At Royal Harbour Academy two new control cable runs were pulled from the kitchen and toilet extract fans back to the roof BMS panel for this.
When should a BMS contractor be involved in a roof replacement?
Before any plant is disconnected. Surveying while the system is live lets every cable be identified, and lets weak points be dealt with while access and scaffold are in place.
Talk to us before the roof comes off
At Royal Harbour Academy that meant eight new 24V feeds, AHU cabling reinstated, extract fans wired back for monitoring, new pressure switches and new network cabling, all dressed onto the new tray.
Alpha Controls is a BCIA member working across Kent, London and the South East. If you've got a roof replacement coming up on a building with plant or motorised windows, get in touch before anything is disconnected, or request a quote.
Alpha Controls Team
Specialist BMS installation, commissioning, and maintenance across London and the South East. SafeContractor Approved, BCIA Member.
