SmartGlass wiring conduit routing through a load-bearing partition: why the relay placement matters more than the wire gauge in a retrofit dimming spec

A home office in Sarjapur Road, ground floor, 14 feet by 12 feet, with a load-bearing brick partition running north-south down the middle. The architect wants Notte switchable glass on the south-facing wall—clear by day, blackout by evening—but the electrical panel is on the north wall. The shortest path between power and the glass control relay cuts directly through the structural partition. No in-wall conduit. No chasing. The decision, then, is not about whether 2.5 mm² or 4 mm² copper will fit the spec. It is about where the relay itself sits, and how the signal and power loops separate.

This is a retrofit problem specific to Bangalore's post-2010 residential boom, where older properties and new layouts collide. And it teaches a principle that applies to every SmartGlass dimming job: the relay placement governs the conduit strategy, not the other way around.

Why the relay is not just a component—it is the geometry of your install

A SmartGlass dimming system has three electrical paths: mains power in, low-voltage signal in (from the wall switch or app), and the switched output to the glass. In a new-build where you chase walls during construction, you can run all three through a single 20 mm conduit, keeping everything behind plaster. In a retrofit through a load-bearing partition, you cannot. You must choose: does the relay live on the north side or the south side of the wall?

If the relay sits north-side (near the panel), you run only the low-voltage signal wire through the partition—typically 0.75 mm² twisted pair, no more than 4 mm diameter with insulation. The mains-switched output then runs surface-mounted on the north face to reach the dimming glass on the south. If the relay sits south-side (near the glass), you run mains power through the partition and the signal wire separately—more complex, more conduit, more site coordination.

Most retrofit jobs in Bangalore favour north-side relay placement. The reasons are practical: the electrical panel is already north, the installer can test and commission the relay before leaving site, and if the glass ever needs service, the power source is not buried behind the partition.

Conduit sizing and the hard-water problem in Bangalore

Once you have decided relay placement, conduit size follows. For a north-side relay, you need a single 16 mm PVC conduit through the partition—large enough for the signal pair (0.75 mm²) plus a spare draw-wire for future additions, with room for thermal expansion. The mains output from the relay to the glass then runs in a second surface-mounted 20 mm conduit, typically clipped to the south face of the partition or, if the room layout allows, along the ceiling soffit.

Here is where Bangalore's water chemistry becomes relevant. The Cauvery supply has a TDS of 200–300 ppm—harder than most Indian metros. If you are running conduit through an external wall or a space near a bathroom, mineral deposits will accumulate inside the plastic over time, making future wire pulls difficult. Specify stainless-steel conduit clips and, where the conduit passes through any zone with moisture risk (near a window, near a bathroom wall), use a 20 mm conduit instead of 16 mm. The extra 4 mm of internal diameter costs nothing in the spec and saves a site call in year three when humidity has settled.

The joint tolerance and the partition interface

Drilling through brick and keeping the partition intact

A load-bearing brick partition in a Bangalore home is typically 230 mm thick (one brick plus plaster on each face). Drilling a clean 25 mm hole for conduit entry requires a diamond-core bit and a structural engineer's sign-off if the partition is post-tensioned or has any embedded reinforcement. Most residential partitions do not, but you must verify on site. The architect or site engineer should confirm this before the drill arrives.

The hole itself should be sealed with a flexible fire-rated mastic (not cement mortar—it cracks). The conduit should pass through with a 5 mm clearance on all sides. This tolerance matters: if the hole is tight and you force the conduit, you risk crushing the insulation on the internal wires, which will only show up as a short-circuit after the plaster is closed.

The relay box and the surface-mounted path

On the north side, the relay box itself (typically 200 mm × 150 mm × 80 mm) can be recessed into the wall if there is no structural depth constraint, or surface-mounted on a 40 mm deep backplate if the wall is solid. If you recess it, the conduit entry should be from below or from the side, never from above—water pooling inside the box during monsoon is a real risk in Bangalore's June-to-September humidity.

From the relay box to the glass, the surface conduit should run vertically where possible, with a gentle curve near the glass to avoid sharp bends that stress the internal wires. If the conduit must run horizontally across a ceiling soffit, clip it every 800 mm and slope it slightly toward the relay box so condensation drains back, not toward the glass.

Signal and power: keeping the loops separate

A common retrofit mistake is running the low-voltage signal wire (from the wall switch to the relay) inside the same conduit as the mains output (from the relay to the glass). This creates electromagnetic interference, especially in older homes with unshielded wiring and high-frequency dimmers. The signal wire becomes noisy, the dimming becomes jerky, and the architect gets a call three months after handover.

Specify two separate conduits: one for the signal pair (0.75 mm²), one for the mains output (2.5 mm² minimum, 4 mm² if the run is longer than 20 metres or if you are using a retrofit smart film installation that draws higher current). The signal conduit can be 16 mm PVC. The mains conduit should be 20 mm. If both must pass through the partition, drill two holes, not one oversized hole with both conduits squeezed together.

A worked example: HSR Layout, home office retrofit

A 2015-built home in HSR Layout, 13 feet by 11 feet office, north-facing window, south-facing load-bearing partition (brick, 230 mm). The client wants blackout glass for video calls. The electrical panel is in the kitchen, on the north side of the partition, approximately 8 metres away through the wall.

Spec: north-side relay placement. Run a 16 mm PVC conduit through the partition (hole drilled and sealed with fire-rated mastic). Inside the conduit, pull a 0.75 mm² twisted-pair signal cable from the wall switch (mounted on the south face, near the door) back to the relay box (recessed into the north face, 1.2 metres up). From the relay, run a 4 mm² mains output cable in a separate 20 mm surface conduit, clipped to the north face of the partition, then up and across the ceiling soffit to the glass, which sits in a 50 mm aluminium frame on the south wall. Total conduit length: 12 metres (8 metres through and across the partition, 4 metres along the soffit). Total wire: 20 metres of signal pair, 15 metres of 4 mm² mains. Cost of the electrical install: approximately 8–10 labour days, including testing and commissioning.

If the architect had specified south-side relay placement instead, the mains cable would have to run through the partition (4 mm² needs a 20 mm conduit minimum), and a second signal conduit would still be needed from the switch back to the north-side panel for control logic. That doubles the partition drilling and adds complexity. North-side placement was the right choice here.

Commissioning and the as-built record

After the conduit is run and the wires are pulled, before the glass is fitted, commission the relay with a test load (a standard 500 W incandescent lamp, if you can still find one, or a resistive load box). Check that the signal wire carries the switching voltage correctly (typically 12 V DC or 24 V AC, depending on the relay model), and that the mains output switches cleanly without arcing or voltage drop. Document the relay location, conduit routing, wire gauges, and any deviations from the spec in the as-built drawing. This record matters when the glass is eventually replaced or when the homeowner upgrades to a different control system.

In retrofit work especially, the as-built is not optional. It is the contract between the current installer and the next one.

Questions we get asked

Can I run the signal and mains wires through the same conduit if I use shielded cable?

Shielded cable reduces but does not eliminate interference, and it adds cost and bulk. In a retrofit where you are already drilling partitions, the time and cost saving of a second conduit is minimal—typically 1–2 labour days and 1500–2000 rupees in materials. Separate conduits are cleaner, more testable, and easier to troubleshoot later. Specify them.

What if the load-bearing partition is reinforced concrete, not brick?

Concrete partitions are harder to drill but structurally safer (no risk of destabilising the wall with a large hole). You will need a diamond-core drill and a structural engineer's clearance to confirm there is no post-tensioning or critical reinforcement in the drilling zone. Once drilled, the hole is sealed the same way: flexible fire-rated mastic, 5 mm clearance all around. The conduit routing remains unchanged.

Does the relay box need ventilation if it is recessed into the wall?

Modern relay boxes dissipate very little heat (the components are solid-state), but in a recessed installation in Bangalore's monsoon season, condensation can form on the internal circuitry. Specify a small 10 mm diameter vent hole at the top of the recessed box, fitted with a desiccant breather (a small cartridge that absorbs moisture). This costs 300–500 rupees and prevents corrosion on the relay contacts over time.

If I am retrofitting Privato switchable bathroom glass, does the electrical spec change?

Privato draws slightly more current than standard Notte (due to the larger surface area typical of bathroom partitions), so specify 4 mm² mains cable even if the run is under 15 metres. The signal and mains conduits remain separate. The relay box should be mounted outside the bathroom (on the corridor side) to keep moisture away from the control electronics. If the relay must be inside the bathroom, use a sealed stainless-steel enclosure rated IP65.

What happens if I specify the wrong relay placement and realise it mid-install?

It is expensive to correct. If you have already drilled the partition for a north-side relay and then decide to move the relay south, you need to drill a second hole, abandon the first conduit, and re-run all the wiring. The partition will have two holes—one sealed, one active. This is why the relay placement decision must happen during the design phase, not on site. Coordinate with the electrical contractor and the structural engineer before any drilling begins.

Bringing it together: the retrofit mindset

A retrofit SmartGlass install is not a simplified version of a new-build. It is a different problem, with different constraints and different solutions. The wire gauge matters, but it is secondary. The relay placement governs everything: the conduit routing, the partition drilling, the commissioning sequence, the future serviceability. Get the relay location right, and the rest follows. Get it wrong, and you are drilling again in six months.

If you are working on a Bangalore residential project with an existing load-bearing partition and dimming glass in the brief, talk to the atelier before you lock the electrical layout. We can walk through the relay placement logic, confirm the conduit routing, and flag any structural or moisture-risk issues specific to your site and micromarket. Commission a consultation with our team.