Most under-cabinet installations fail at the box nobody wanted to look at. An under-cabinet lighting power supply is a small electronic device that produces heat, needs airflow, and will eventually need replacing, and the standard practice of burying it inside a closed cabinet behind the dishes works against all three.
What the driver is doing up there
Low-voltage fixtures need line voltage converted to twelve or twenty-four volts, either as direct current or as a constant current output, depending on the product. A switching supply does that efficiently, but efficiency is never total and the losses appear as heat in a small sealed enclosure. Manufacturers state a maximum ambient temperature and usually require free air around the unit. A cabinet interior above a range or a dishwasher can exceed that ambient on its own before the driver contributes anything, which is how a unit rated for years of service fails in one season.
Access is a code and a maintenance issue
Power supplies for lighting are generally required to be accessible, and permanently concealing one behind finished cabinetry or above a fixed soffit creates a problem the day it fails. Good installations put the supply somewhere reachable: inside a cabinet with a removable panel, in an adjacent utility space, or above a suspended ceiling where access is designed in. The practical test is simple. If replacing the unit would require removing cabinetry or cutting drywall, it is in the wrong place, regardless of how tidy it looks on installation day.
Sizing the supply with headroom
Total the connected fixture wattage and select a supply rated above it rather than exactly at it. Running a switching supply continuously at its maximum rating puts the internal components at their highest temperature, which shortens the life of the electrolytic capacitors that usually determine when it fails. Leaving meaningful headroom costs very little and buys years. Account for any additional runs someone may want to add later, because a kitchen that gets lighting under the wall cabinets frequently gets lighting in a pantry or over a sink shortly afterward.
Class 2 wiring and what it permits
Most under-cabinet systems use a class 2 power supply, and class 2 output wiring has relaxed installation requirements compared with line voltage circuits because the power is inherently limited. That is what allows small unjacketed cables to run through cabinet backs and along rails. It does not mean the wiring can be treated carelessly: class 2 conductors still must be protected from physical damage, kept separate from line voltage conductors except as the code permits, and connected with the fittings the manufacturer supplies. The line voltage side feeding the supply follows all the normal rules.
Dimming without disappointment
The driver, not the fixture, determines what dimming works. Some supplies accept a line-voltage dimmer ahead of them, some require a low-voltage control signal on separate conductors, and some do not dim at all. Mixing a wall dimmer with a supply that does not support it produces flicker, buzzing, or a supply that fails early. Read the specification before choosing a control, and if a designer has specified a particular wall control, confirm the supply is on that control’s tested list before ordering either.
What to check before you act
- Confirm the supply will be accessible without removing cabinetry.
- Check the stated maximum ambient temperature against the location you have chosen.
- Size the supply above the connected load rather than exactly at it.
- Verify the dimming method the supply supports before selecting a wall control.
Common questions
Can I put the driver inside the cabinet above the lights?
Only if that space stays within the stated ambient limit and remains accessible. A closed cabinet above a heat source rarely qualifies.
Why did my under-cabinet lights dim unevenly along the run?
Usually voltage drop along a long low-voltage run, which is a wire size and feed point problem rather than a fault in the fixtures.
The practical takeaway
Decide where the power supply lives before you decide anything else. Give it air, give it access, size it with headroom, and match the dimming method to the control. The fixtures rarely fail; the box you cannot reach is what ends the installation.
References and further reading
- ENERGY STAR: certified lighting products and their performance criteria
- OSHA 1910.304: wiring design requirements including accessibility of equipment
- Department of Energy: choosing efficient lighting products for the home
Numerical scenarios are illustrative unless identified otherwise. Follow the exact product instructions; component ratings and local installation requirements can differ.
