Tutorial
Fit an LED strip under kitchen cabinets
A 24V strip in an aluminium channel removes the shadow your body casts on the worktop. Here are the numbers that keep it flicker-free, with no dim, yellow end.
12V or 24V?
For the same wattage, a 24V strip draws half the current of a 12V one. Voltage drop in the wires is proportional to current, so it halves in volts and falls to a quarter as a percentage.
- 24V: up to 5 m fed from one end with no visible difference. The right choice for a kitchen.
- 12V: only for very short runs (display cabinet, niche) or a 12V battery system.
COB strips give a continuous line with no dots reflected in a glossy worktop.
Voltage drop and wire size
Formula for a two-core cable (out and back):
ΔU = 2 × L × I × ρ ÷ S — L cable length (m), I current (A) = power ÷ voltage, ρ = 0.0172 Ω·mm²/m (copper), S cross-section (mm²). In the US, 0.5 mm² ≈ 20 AWG, 0.75 mm² ≈ 18 AWG, 1.5 mm² ≈ 16 AWG.
Example: 3 m of 10 W/m strip = 30 W, driver 3 m away, 0.5 mm² cable. At 24V: I = 1.25 A, ΔU = 0.26 V (1.1%), invisible. At 12V: I = 2.5 A, ΔU = 0.52 V (4.3%), plus the drop inside the strip itself.
Cable voltage drop for 30 W of strip (target ≤ 3%)
| Cable length | Cross-section | 12V strip (2.5 A) | 24V strip (1.25 A) |
|---|---|---|---|
| 2 m | 0.50 mm² | 0.34 V (2.9 %) | 0.17 V (0.7 %) |
| 2 m | 0.75 mm² | 0.23 V (1.9 %) | 0.11 V (0.5 %) |
| 2 m | 1.50 mm² | 0.11 V (1.0 %) | 0.06 V (0.2 %) |
| 3 m | 0.50 mm² | 0.52 V (4.3 %) | 0.26 V (1.1 %) |
| 3 m | 0.75 mm² | 0.34 V (2.9 %) | 0.17 V (0.7 %) |
| 3 m | 1.50 mm² | 0.17 V (1.4 %) | 0.09 V (0.4 %) |
| 5 m | 0.50 mm² | 0.86 V (7.2 %) | 0.43 V (1.8 %) |
| 5 m | 0.75 mm² | 0.57 V (4.8 %) | 0.29 V (1.2 %) |
| 5 m | 1.50 mm² | 0.29 V (2.4 %) | 0.14 V (0.6 %) |
Sizing the power supply
Driver rating ≥ strip power × 1.25. 30 W × 1.25 = 37.5 W: a 40 W unit is enough, a 60 W unit runs cooler and leaves headroom. Check three markings: constant voltage (not constant current), SELV output, and Class II (double insulation). In the US, choose a UL-listed Class 2 power supply: its output is power-limited, which is what makes low-voltage DIY wiring acceptable under the NEC. To dim from a wall dimmer you need a TRIAC-dimmable driver; otherwise dim on the 24V side with a PWM dimmer.
Switching
- 24V inline switch under the cabinet.
- Hand-wave infrared sensor: works with wet hands.
- High-frequency PWM dimmer (several kHz) to avoid flicker.
- Zigbee or Wi-Fi controller on the 24V side.
Electrical safety (UK and US)
- Everything in this tutorial is extra-low voltage (24V SELV), downstream of a driver plugged into an existing socket.
- UK: hard-wiring the driver, adding a socket or a 230V switch is work on the fixed installation under BS 7671. In England, adding a new circuit is notifiable under Part P of the Building Regulations: use a registered electrician (NICEIC, NAPIT or similar).
- US: line-voltage work follows the National Electrical Code and usually needs a permit; hire a licensed electrician.
- Keep the driver and joints away from sink splashes and from hob, oven and hood heat. Above the sink, use an IP65 strip.