Reflected ceiling plan for a backlit feature wall in a JP Nagar living room: the electrical rough-in architects forget

A lacquered or textured-glass feature wall backlit at 2700K reads as a sculptural object in a JP Nagar living room, not a flat plane. The light comes from behind—from a cavity that sits between the glass and the structural wall. That cavity is not a detail the MEP engineer typically owns. It is not a finish specification the interior designer typically coordinates. It lives in the gap between the RCP and the shop drawing, and when it is forgotten, the entire wall goes dark on handover.

Why the RCP needs a dedicated electrical layer for backlit glass

The reflected ceiling plan shows downlights, diffusers, air vents, and the grid. It does not, by convention, show the wiring that sits inside a cavity wall. But a backlit feature wall is not a cavity wall in the conventional sense. It is a shallow plenum—typically 120 to 150 mm deep—that must house LED strip, power feed, and junction logic without interfering with the glass installation itself.

The architect's RCP shows the finished ceiling height and the position of the glass panel. What it does not show is where the LED feed enters the cavity, where it terminates, whether it runs horizontally or vertically, and crucially, where the dimmer and power-distribution logic lives. This absence is not an oversight; it is a gap in the conventional drawing hierarchy. The RCP assumes all electrical is roughed in before the ceiling is closed. But a backlit wall cavity is open from the side—accessible only during the glass installation phase. The electrical rough-in must happen in parallel with the atelier's measurements and shop drawing.

Cavity depth and LED strip specification

Measuring the plenum

A backlit feature wall in a Bangalore residential project typically sits 120 to 150 mm proud of the structural wall. This depth is determined by three constraints: the LED strip width (typically 10 mm), the reflector or diffuser panel behind it (if used), and the clearance needed for the glass itself to sit flush without trapping dust or condensation at the joint.

The hard-water TDS in Bangalore's supply (200–300 ppm from the Cauvery) means mineral deposits will accumulate on any horizontal ledge inside a cavity. A 120 mm cavity leaves room for a 10 mm LED strip, a 10 mm air gap, and a 100 mm reflector panel. That reflector panel—typically white-painted MDF or anodised aluminium—bounces light forward and prevents the LED from being visible through the glass edge. If the cavity is less than 120 mm, the reflector becomes too thin to be rigid, and the LED sits too close to the glass, creating a hot spot or visible line.

LED strip selection and wiring path

Specify the LED strip by lumen output and colour temperature, not by brand. A 12V or 24V constant-voltage strip is standard for residential backlit walls; 24V is preferred for runs longer than 5 metres because it reduces voltage drop. In a JP Nagar living room with a 2.5-metre feature wall, a 24V 14W/metre strip at 2700K will deliver 1200–1400 lumens across the panel with minimal flicker.

The wiring path must be drawn on the shop drawing, not assumed. If the cavity is open at the top, the feed can enter from above and run down the back face of the reflector. If the cavity is closed at the top (because the ceiling structure does not permit a cutout), the feed must enter from the side—from within the wall itself—which means the MEP rough-in must include a conduit stub at the point where the glass panel begins.

Junction-box placement and dimmer logic

Where the power feed terminates

The LED strip does not simply plug into the wall. It requires a 24V DC power supply, a dimmer (if dimming is specified), and a junction point where the low-voltage feed from the power supply meets the strip itself. This junction cannot sit inside the cavity; it will be inaccessible after the glass is fitted. It must sit either within the wall cavity at a marked and accessible point, or outside the cavity entirely—in a small recessed or surface-mounted junction box that sits behind the glass panel at the top or side edge.

In most Bangalore residential projects, the junction box sits at the top of the cavity, just above the glass panel, concealed by the ceiling or a fascia. The dimmer (if wall-mounted) sits at the living-room switch plate, typically 1.2 metres above floor level, next to the main lighting dimmer. If the dimmer is a remote wireless unit, it can sit on a side table, but it must be specified and coordinated with the client before the rough-in is closed.

Dimmer type and control logic

A backlit feature wall is almost always dimmed. Specify whether it is dimmed by a hardwired trailing-edge dimmer (which requires a neutral wire and a dedicated 10A circuit), or by a wireless DALI or Zigbee receiver built into the LED driver itself. A hardwired dimmer is cheaper and more reliable; a wireless dimmer is easier to retrofit and allows the client to control the wall from anywhere in the room using a remote or a phone app.

The electrical rough-in must anticipate this choice. If hardwired, the MEP engineer must run a 2.5 mm twin-and-earth cable from the main board to the junction box, with a 10A MCB and an RCD. If wireless, the same cable is needed, but the dimmer sits at the power supply itself, not at the wall switch. Either way, the cable run must be shown on the RCP as a dashed line or a note, because it determines whether the cavity can be fully closed or must remain accessible at a specific point.

Coordination between the RCP and the shop drawing

The atelier receives the RCP from the architect and the structural drawing from the engineer. From these, we produce a shop drawing that shows the glass panel dimensions, thickness, edge finish, and the cavity depth. But the shop drawing cannot be finalised until the MEP rough-in is confirmed. Specifically:

• The cavity depth must be locked in, because it determines the reflector panel size and the LED strip length.
• The LED feed entry point must be marked, because it determines whether the cavity is open at the top or closed, and whether a conduit stub is needed in the wall.
• The junction-box location must be confirmed, because it determines whether the glass panel can be fitted flush to the ceiling or must have a 50–100 mm clearance at the top for access.
• The dimmer type and control wiring must be specified, because it determines whether the power feed is 240V AC or 24V DC, and whether it enters the cavity from above, from the side, or from behind.

In practice, this coordination happens in a series of RFIs (requests for information) between the architect, the MEP engineer, and the atelier. The RCP is issued first; the MEP engineer responds with a cavity-wiring diagram; the atelier produces the shop drawing; the architect reviews and approves. This cycle typically takes 2–3 weeks. If it is skipped or compressed, the rough-in is done blind, and the glass installation becomes a problem-solving exercise on site.

Site dimensions and joint tolerance at handover

Once the rough-in is complete and the drywall is closed, the cavity is sealed. The atelier then measures the finished cavity on site, to the millimetre. The glass panel is cut to sit within a 3 mm joint tolerance on all sides—this allows for plumb variation in the wall and for the sealant joint, which is typically a 3 mm bead of clear silicone. The LED strip is fitted to the reflector panel in the atelier workshop, tested at full brightness and dimmed to 10%, and then delivered to site as a complete assembly.

On site, the assembly is lifted into the cavity and secured to the top and sides with mechanical fixings (typically M6 stainless-steel screws into pre-drilled holes in the reflector panel). The glass panel is then fitted over the assembly, with the joint line sealed with silicone. The power feed is connected to the junction box, tested, and the dimmer is commissioned. The entire process takes 4–6 hours on site, assuming the rough-in is correct and the cavity is accessible.

If the cavity is inaccessible, or if the junction box is in the wrong place, or if the LED feed does not reach the junction, the wall cannot be lit on handover. The client receives a dark feature wall and a schedule of remedial works. This is not uncommon in Bangalore residential projects, particularly in the tech-corridor towers around Whitefield and Sarjapur Road, where the architectural and engineering drawings are often produced in parallel and handed to the contractor with gaps.

A worked example: backlit lacquered gold in Sadashivanagar

A recent Sadashivanagar project specified a backlit abstract geometric gold-lacquered glass feature wall above the sofa. The wall was 2.8 metres wide and 1.2 metres tall. The RCP showed a 150 mm cavity, open at the top. The MEP engineer roughed in a 24V DC feed from the main board (located in the kitchen, 8 metres away) and terminated it in a junction box at the top-right corner of the cavity, concealed by the ceiling soffit.

The atelier measured the cavity on site and found it to be 148 mm deep (2 mm less than specified, due to drywall taper). A 24V 14W/metre LED strip was fitted to a 100 mm white-painted MDF reflector panel. The strip was connected to the junction box via a 1.5 mm twin-core cable, with a 10A wireless DALI receiver in the power supply itself, allowing the client to dim the wall from a remote control or a mobile app.

The lacquered gold panel was fitted over the assembly, with a 3 mm silicone joint on all sides. The wall was commissioned and tested at full brightness (1400 lumens) and at 10% brightness (140 lumens). The monsoon humidity in Bangalore (June–September) meant the cavity was sealed with a 3 mm bead of clear silicone to prevent moisture ingress, which also prevents dust accumulation on the reflector panel.

The project was completed on schedule and on budget. The client received a fully functional backlit feature wall on handover, with no remedial works. This outcome is the result of coordination between the RCP and the shop drawing, not luck.

Questions we get asked

Can a backlit feature wall be retrofitted into an existing ceiling without a cavity?

Not practically. A backlit wall requires a minimum 120 mm cavity to house the LED strip and reflector panel without creating a hot spot or visible line. If the existing ceiling structure does not permit a cavity, the wall cannot be backlit. A surface-mounted LED strip (running on the face of the wall, below the ceiling) is an alternative, but it will cast a shadow line and will not read as a unified luminous panel.

What happens if the LED strip fails after handover?

The strip is accessible only if the cavity was designed with an access point. If the cavity is fully sealed, the glass panel must be removed to replace the strip, which is costly and disruptive. Specify the LED strip with a 10-year warranty and ensure the power supply is located in an accessible junction box. The dimmer and control logic should be specified as a separate module, so it can be replaced without touching the cavity.

Does the dimmer need to be hardwired, or can it be wireless?

Both are viable. A hardwired trailing-edge dimmer is cheaper and more reliable; a wireless DALI or Zigbee receiver allows remote control and is easier to retrofit. The choice depends on the client's preference and the MEP engineer's capacity to run the control wiring. Specify the dimmer type on the RCP and coordinate it with the MEP engineer before the rough-in begins.

How does humidity affect a backlit feature wall in Bangalore?

Bangalore's monsoon humidity (June–September) and hard-water TDS (200–300 ppm) mean condensation and mineral deposits will accumulate inside the cavity if it is not sealed. Specify a 3 mm silicone joint on all sides of the glass panel, and ensure the cavity is ventilated with a small weep hole at the bottom to allow any trapped moisture to drain. Do not use foam sealant; it absorbs moisture and will degrade the reflector panel.

What is the typical cost impact of a backlit cavity on the overall project budget?

The cavity framing, LED strip, power supply, and dimmer typically add 8,000–15,000 rupees per square metre to the cost of the feature wall, depending on the LED specification and the dimmer type. This is a material cost, not a labour cost. The MEP rough-in (the conduit and cable run) is typically the responsibility of the MEP contractor and should be costed separately. Do not assume the MEP contractor will anticipate the cavity wiring; specify it explicitly on the RCP and in the MEP scope of work.

Commissioning the wall on site

Before the client takes handover, the backlit feature wall must be commissioned in a darkened room. Test the wall at full brightness, at 50%, and at 10%. Check for flicker, hot spots, or visible LED lines. Verify that the dimmer responds smoothly and that there is no audible hum from the power supply. Document the commissioning with photographs and a sign-off sheet, and provide the client with a maintenance guide that covers cleaning the glass (use only distilled water and a microfibre cloth), checking the silicone joint annually, and replacing the LED strip if it fails (which is rare, but possible after 10+ years).

Commission a backlit feature wall with the same rigour you would apply to a shower enclosure or a kitchen splashback. The wall is not just a surface; it is a luminous object that will be the focal point of the living room. Get the electrical rough-in right, and the wall will perform flawlessly for decades. Get it wrong, and the client will spend years looking at a dark panel and wondering why.

If you are designing a backlit feature wall for a Bangalore residential project—in JP Nagar, Sadashivanagar, Indiranagar, or elsewhere—talk to the atelier about the RCP and electrical coordination before the MEP rough-in begins. We can review the drawings, flag the gaps, and produce a shop drawing that anticipates the cavity wiring. Commission a fitting with us, and we will ensure the wall is lit and functional on handover.