LED strip power supply sizing and wiring: what size adapter do you need?

Short answer: Required supply = watts per metre × metres × 1.2 (20% headroom). Amps = watts ÷ volts. For example, 5 m of 14.4 W/m (60 LEDs/m, 5050) strip is 72 W; at 12 V it draws 6 A, so with 20% headroom you need at least 7.2 A — choose an 8–10 A adapter. A 24 V strip draws half the current for the same power and runs more evenly over long lengths.

Step-by-step sizing

1. Find the strip's power per metre (printed as W/m). Common values: 4.8, 7.2, 9.6, 14.4 and 19.2 W/m.
2. Total power: W/m × metres.
3. Add 20% headroom: total × 1.2. A driver running at full load all the time gets hot and wears out sooner.
4. Amps: watts ÷ volts (12 V or 24 V).

Worked examples (12 V strip)

• 5 m × 14.4 W/m = 72 W → 6 A → at least 7.2 A: 8–10 A adapter
• 10 m × 14.4 W/m = 144 W → 12 A → at least 14.4 A: 15 A adapter
• 5 m × 4.8 W/m = 24 W → 2 A → at least 2.4 A: 3 A adapter

The other way round, keeping 20% headroom:

• 12 A (144 W) supply: about 8 m of 14.4 W/m strip or 24 m of 4.8 W/m strip
• 30 A (360 W) supply: about 20 m of 14.4 W/m strip

The 24 V version of the same strip halves the current: 10 m × 14.4 W/m = 144 W draws 6 A at 24 V.

12 V or 24 V?

A 24 V strip draws half the current for the same power, so there is less voltage drop along cable and strip and less difference in brightness and colour between start and end of a long run. 24 V is preferred for cove, shelf and ceiling lighting; 12 V is common for short runs, vehicles and hobby use. Strip and supply must match: a 24 V strip will not light on 12 V, and a 12 V strip on 24 V will burn out.

How to wire an LED strip

• Polarity: strip + to the supply's + (V+), − to − (V−). A reversed strip will not light.
• Never straight to mains: 12/24 V strips must be fed by an LED driver; the driver's input is connected to mains (L, N and earth if present) by an electrician.
• Run length: up to 5 m from one end at 12 V and 5–10 m at 24 V (check the datasheet). For longer runs, feed from both ends or the middle instead of chaining strips from one end.
• Cutting and joining: cut only at the marked points; join with suitable connectors or solder, and cut-and-join at corners rather than bending.
• Heat: higher-power strips (14.4 W/m and up) should be mounted on an aluminium profile.
• Wet areas: use IP65 or higher strip and a suitable driver in bathrooms, near kitchen sinks and outdoors.

Choosing the supply

• Constant voltage (CV): LED strips run on a constant 12 V or 24 V; do not use a constant-current driver.
• Enclosed: indoors, in furniture and ceilings use an enclosed driver with overload and short-circuit protection.
• Dimming: use a dimmable driver or a suitable LED dimmer.
• Access: mount the driver where it can be serviced.

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