One to two inches and the first usable gradient
Here the beam clears the lip with room to open, so the band widens into a readable gradient: strong at the top, fading through the middle of the wall, gone by eye level.
This is the tightest band an ordinary painted wall survives, and only where the finish is good. Drywall taped to a standard level shows its seams.
Two to four inches, the range most residential coves land in
Give the light this much room and the beam leaves the trough with space to spread. The band softens, the ceiling reads as a broad lit plane, and the junction where wall meets ceiling stops being a hard line.
Lip height makes or breaks this band, and the relationship is plain geometry: lip height above the emitter, divided by the run from the emitter out to the lip, sets the tangent of the lowest exit angle. That run is not the setback, which is measured the other way, back to the wall. Grow the lip by half and the emitter-to-lip run grows by half to hold the angle. A taller lip added late, to hide a strip someone can suddenly see, is the usual reason a cove looks tighter than the drawing promised.
Strips and drivers produce heat whenever they run, and an aluminum profile helps dissipate heat while holding the tape flat along the trough. It is never mandatory in a cove. It also reduces but does not guarantee that visible dots disappear, because the result rests on LED density, on the depth of the channel or profile, and on the diffuser type as much as on the extrusion. Depth is the dimension to compare across the cove-ready channels and profiles.
Four to six inches and the ceiling takes over
Push out this far and the gradient on the wall largely disappears. The ceiling becomes the lit surface, and the cove reads as a slot with a glow behind it rather than a light on a wall.
Output has to rise with it, because the same lumens now spread across ceiling area instead of the first two feet of wall. A high output series such as the UniPlus high output listing is worth weighing, with its watts per foot and lumens per foot taken from that page. Sizing follows: watts per foot times installed footage, plus at least 20 percent current headroom, before choosing from the power supplies sized to the run. Verify both figures.
A sightline changes too. At a generous setback the opening faces more of the room, so someone tall across the space can see the emitter over the lip. Check that view before the lid goes on.
Pick the strip voltage first and let it govern everything downstream, so a 24V cove strip needs a 24V supply and, where one is used, a 24V controller, while a 12V strip needs the 12V equivalent of each. If the driver is hardwired, the input side stops being a low voltage question. Hardwired supplies, the junction box they land in, cable inside the ceiling build-up and any switch or dimmer plate go to a licensed electrician, whose work answers to local electrical code first, to the NEC where that applies, and to whatever the equipment instructions require, with the circuit off at the panel and verified dead first. Nothing here authorizes unqualified line voltage work.
