Sizing a Power Supply for LED Strip: 80% Rule
How to size a 12 V or 24 V driver for LED strip: load calculation, the 80% derating rule, voltage drop on the low-voltage side, inrush current and dimming.
HFY Applications Engineering · Published October 13, 2026 · 3 min read
Size a constant-voltage driver at 80% of its rated output for the connected strip load, then check inrush and voltage drop. A 100 W driver should carry 80 W of strip. In a sealed cabinet, derate further to 70% because driver output falls about 10% above 40 °C ambient and capacitor life halves for every 10 °C rise.
Key points
- Add up W/m × length, then divide by 0.8 — that is the minimum driver rating.
- Inrush on long COB reels trips marginal drivers; two smaller drivers often beat one large one.
- Cable between driver and strip is part of the voltage-drop budget, not separate from it.
- PWM frequency decides whether dimming will flicker on camera, not the driver wattage.
The load calculation
Total load (W) = power per metre (W/m) × total length (m)
Driver rating (W) = total load ÷ 0.8Three worked examples:
| Installation | Load | Minimum driver | Practical choice |
|---|---|---|---|
| 8 m of 12 W/m at 24 V | 96 W | 120 W | 150 W, single zone |
| 24 m of 10 W/m at 24 V | 240 W | 300 W | 2 × 150 W, two zones |
| 3 m of 14.4 W/m at 12 V | 43 W | 54 W | 60 W |
The 80% rule is not conservatism for its own sake. Drivers are rated at 25 °C ambient, and a strip installation usually puts them in a cabinet at 40–50 °C, where output derates and electrolytic capacitors age faster.
Inrush, the failure nobody plans for
A 50 m COB reel at 12 W/m has a cold resistance roughly one-tenth of its operating resistance. At switch-on it draws a surge several times the steady current for a few milliseconds.
Consequences:
- A driver sized at exactly the load will current-limit or shut down at switch-on, then restart — visible as a flicker on power-up.
- A 24 V 100 W driver feeding 90 W of long reels will trip intermittently in cold weather.
- Two 60 W drivers feeding separate halves cost about the same as one 120 W unit and avoid the problem entirely.
Zone splitting also means a driver failure takes out one zone, not the whole elevation.
Cable is part of the circuit
The voltage drop calculation does not stop at the strip’s copper. A 5 m run of 1.0 mm² cable carrying 5 A loses 0.85 V; at 12 V that is 7% before the strip’s own loss is counted.
Practical rules:
- Keep the driver within 2 m of the first strip feed where possible.
- Use 1.5 mm² for runs up to 5 A, 2.5 mm² above that.
- Do not use bell wire, even for 3 m of strip — the drop appears as dimming at the far end, which is then blamed on the strip.
For long runs, the same physics described in voltage drop applies to the cable as to the FPC.
Dimming and flicker
| Method | Where it acts | Flicker risk | Best for |
|---|---|---|---|
| Phase-cut (trailing edge) | Mains input | Moderate at low levels | Domestic retrofit |
| 0–10 V | Driver output | Low | Commercial, simple control |
| DALI-2 | Driver output | Low, logarithmic curve | Projects with a control system |
| PWM on the low-voltage side | Strip supply | High if below 1.5 kHz | Effects and colour control |
Camera-visible flicker is a frequency problem, not a driver-quality problem: a 200 Hz PWM output flickers at 1/1000 s shutter even when it looks steady to the eye. Specify ≥ 1.5 kHz for anything that will be filmed, and 3 kHz for broadcast.
Heat, again
The driver is a heat source and the strip is a heat source, and putting them in the same sealed cabinet is the standard mistake. A 150 W driver dissipates 12–15 W; that is enough to raise a small cabinet 15 °C above ambient, which then derates the driver and accelerates the strip’s ageing.
Mount the driver where air moves, or in a separate enclosure with a vent. The relationship between temperature and strip life is covered in heat management, and the power and driver range page lists the stocked 12 V and 24 V units.
This article describes general manufacturing practice for machined and fabricated components. It is not a substitute for a drawing-specific engineering review — tolerances, material selection and process capability depend on the individual part.