How many electric rooflights can one switch control?
In domestic work, typically three or four from a single control unit, and occasionally more. The switch itself is almost never the limit. It carries a signal, not the current, so a wall plate driving one actuator and a wall plate driving five are the same component. What sets the ceiling is the control unit behind it: its total current output in amps, the size and length of the low voltage cable runs, and the surge every motor draws in the half second it starts.
Get those three right and a whole run of rooflights across a kitchen extension opens together on one press. Get them wrong and you have units that open at different speeds, a controller that trips on the second press, or a sash at the far end that never quite closes hard onto its gasket. The mechanisms involved are described on the electric and opening rooflight page.
What the switch is actually doing
On almost every domestic system the wall plate is a signal device. Pressing it closes a low current contact, or sends a radio message, and the control unit interprets that and switches the real current to the actuators. Nothing meaningful passes through the switch itself.
This is why the wiring question is not “how many can this switch take” but “how many can this control unit drive”. It also explains why adding a second switch in a different part of the room is trivial: two plates can send the same command to one controller without any change to the power side at all. A wired plate, a radio handset and an app gateway can all address the same group.
Current output, and the budget it gives you
Control units carry a stated 24V output, usually somewhere between two and ten amps in domestic products. Divide that by the running current of one actuator and you have the theoretical count. A slim chain drive on a modest roof window draws in the region of half an amp while travelling. A heavy duty spindle drive on a large flat roof unit draws considerably more.
| Controller output | Light chain drives | Heavy spindle drives |
|---|---|---|
| 2A | Two to three units | One unit |
| 5A | Five to seven units | Two units |
| 10A | Beyond most domestic needs | Three to four units |
These are working figures rather than a specification. The actual numbers come from the actuator data sheet and the controller data sheet, and the sensible approach is to add up the real running currents and leave headroom rather than filling the rating exactly.
Inrush, and the half second nobody accounts for
A motor at rest draws far more current at the instant it starts than it does while running, often two or three times as much, because there is no back electromotive force opposing the supply until the armature is turning. That surge lasts a fraction of a second.
With one actuator this is invisible. With four starting simultaneously the combined inrush can exceed the controller’s protection and trip it, which produces the frustrating symptom of a group that works fine most of the time and fails on cold mornings when the mechanisms are stiffest. Better controllers stagger their outputs by a few hundred milliseconds for exactly this reason, and where they do not, the answer is a larger supply rather than a smaller group.
Volt drop, and the unit at the far end
Extra-low voltage is unforgiving over distance. At 24V, losing two volts in the cable is losing more than eight per cent of the supply, and the actuator at the end of that run is slower, weaker and less able to compress its gasket at the close.
The variables are cable cross section, run length and current. A short run to a single unit is fine in a small cable. A twenty metre run across a ceiling void feeding three actuators is not, and it wants stepping up in size. On a long glazing run the sensible layout is a controller sited centrally with cables radiating out, rather than a controller in a corner cupboard with one unit twenty five metres away. The symptom of getting this wrong is specific and recognisable: the nearest unit closes smartly and the furthest one arrives late and lands soft.
Wiring several actuators from one output
Actuators on a shared output are wired in parallel, each taking the full 24V and drawing its own current. They are not put in series, which would divide the voltage between them and leave every unit underpowered.
The junction is made where it can be reached, typically in a ceiling void or a cupboard rather than buried above plasterboard. That is worth insisting on, because a connection that cannot be reached is a connection that stays wrong. Where units are also being driven by a synchronisation module, because two actuators share one wide sash, that module counts as its own load and sits between the controller and the pair.
Identical units, and why mixing causes trouble
Two actuators on the same output start at the same moment and stop when each reaches its own limit. If they are the same model with the same stroke on similar sashes, they travel together and arrive together, and the effect from the floor is one movement.
Mix a 200mm stroke with a 400mm one, or a chain drive with a spindle drive, and they visibly diverge: one pane is fully open while the other is still travelling. Nothing is damaged, and some people never notice. Across a run of glazing in a single ceiling it looks wrong, and it is worth specifying matched units on any group that will be seen together, even where one opening is smaller than the rest.
Groups, scenes and separate control
Beyond a single output, controllers offer channels. Each channel is an independently switched group, so a kitchen extension might have three rooflights on channel one, driven together by the switch beside the door, and a fourth over a utility on channel two with its own plate.
This is generally a better arrangement than one enormous group. A room with all its openers on a single command loses the ability to crack one unit for background ventilation without opening every one of them, and on a windy day that is exactly what you want. The rule that serves most extensions is: group the units that are always used together, and give anything with a different purpose its own channel.
Sensors across a group
A single rain sensor can close an entire group, and usually should. The sensor reports to the control unit rather than to any individual actuator, so one plate on the most exposed unit protects everything on the channel.
Where units face genuinely different conditions this needs thought. A sensor on a sheltered slope may not wet at the same moment as a unit exposed to driven rain from the estuary, and the group will close a few seconds late. Siting the sensor on the most exposed pane rather than the most convenient one solves it. Temperature and humidity inputs work the same way, reporting to the controller and acting on whichever channels they are assigned to.
What a survey settles before any of this is wired
Four numbers decide the whole arrangement. How many actuators, and of what running current. How far each one is from the intended controller position, which fixes the cable sizes. Whether any of the sashes needs two synchronised drives, which changes the load. And whether the units are all wanted on one command or want splitting into groups.
Those answers come out of a survey, and they are worth having before first fix rather than after, because a controller position and a set of cable routes are trivial in an open ceiling and disruptive in a finished one. Sunspire quotes in two stages, a range by email and a fixed written figure after the survey, and the control layout is settled at the second of those. The certification of the mains side is covered on the electrician page.
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