LED strips: 12 V, 24 V and choosing a power supply

Strip voltage, run length and power supply rating are tied together. We walk through the calculation step by step and explain the typical mistakes.

If the strip under your kitchen cabinets is bright white at one end and dim and yellowish at the other, the strip itself is rarely to blame. The cause is usually the choice of voltage and power supply. An LED strip looks like a simple product, but it only works properly when three things match: the strip voltage, the run length and the power supply rating.

12 V and 24 V: where the difference lies

Most strips used at home run on a constant 12 V or 24 V. At the same power, a 24 V strip draws half the current. That matters in practice, because the copper tracks on a strip are thin, and the higher the current, the greater the voltage drop along them.

What does voltage drop actually mean? Power is fed in at one end, and the current spreads along the strip to every LED. The resistance of the copper tracks reduces the voltage the further you go, so the last sections receive less. They glow more dimly and sometimes shift in colour. On a 12 V strip this shows up sooner, which is why 24 V is the easier choice for long runs.

12 V strips have their own strengths. The cut points are usually closer together, so it is easier to trim the strip to a short, precise length. For a small display case, a shelf or a mirror frame that is handy. Many vehicle and battery systems also run on 12 V.

The key rule: the strip voltage and the output voltage of the power supply must be the same. A 12 V strip on a 24 V supply will fail quickly; a 24 V strip on a 12 V supply will either stay dark or glow very faintly.

Sizing the power supply

The packaging or datasheet gives the power per metre (W/m). The calculation is simple:

• multiply the required length by the power per metre;
• add headroom so the supply does not run at its limit all the time; 20–30% is the usual recommendation;
• pick the nearest standard rating above the result.

Example: 5 metres of strip at 14.4 W per metre. That is 72 W in total. With 20% headroom you get about 86 W, so a 100 W supply is the sensible choice. If the strip is 12 V, the full-load current will be about 6 A; for a 24 V strip of the same power it is 3 A. Halving the current reduces losses and heating in the wiring and in the strip itself.

The supply must be a constant voltage type. Constant current drivers intended for individual high-power LEDs are not suitable for strips.

Long runs and wiring layout

Strip manufacturers state a maximum length that can be fed from a single point; for 12 V strips it is often around 5 metres. For longer runs, rather than simply chaining strips end to end:

• feed power in at both ends of the strip;
• split the run into sections and supply each one separately from the power supply with adequately thick cable;
• switch from 12 V to a 24 V strip.

The cable between the supply and the strip matters as well. A long, thin cable loses voltage just like the strip does. If the supply has to be far away, use a larger cross-section or move the supply closer, but remember that it needs ventilation: squeezing it into a closed, hot box shortens its life.

Heat, profiles and moisture

An LED strip gets warm and has to get rid of that heat somewhere. A powerful strip stuck by its adhesive backing straight onto wood or plastic cannot shed heat well; over time the LEDs darken and the adhesive lets go. An aluminium profile spreads the heat, protects the strip from knocks, and together with a diffuser gives an even line of light instead of visible dots.

For moisture, check the IP rating. An IP20 strip is for dry places only. Near a kitchen worktop or in a bathroom, choose a version rated against splashes. The power supply also has to sit outside the wet zone or carry a suitable protection rating.

Checklist before you buy

• strip voltage: 12 V or 24 V;
• power per metre and total length;
• has the supply rating been calculated with headroom;
• is the supply a constant voltage type;
• the maximum length allowed from a single feed point;
• installation spot: is a profile needed, is there moisture;
• if a dimmer will be used, are the supply and the dimmer compatible.

Strips that run directly on 220 V mains are also sold. They need no power supply, but because dangerous voltage is present on exposed contacts, installation is best left to an electrician.

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