The short formula
Total the strip's power draw, then oversize the supply. In practice: watts per foot times total feet gives load watts; divide load watts by 0.8 to add 20 percent headroom, and that is the minimum supply wattage. Then confirm the supply's voltage matches the strip and that its amp rating covers your current.Step 1: Find the strip's power per foot
Every quality strip lists a power figure, usually as watts per foot or watts per meter. If you only have watts per meter, divide by 3.28 to get watts per foot.- Typical indoor strips range from about 1.5 to 5 watts per foot.
- High-output and high-density strips can reach 6 watts per foot or more.
- Do not guess. Two strips of the same length can differ two or three times in draw.
Step 2: Total the wattage
Multiply watts per foot by the exact length you will power from one supply.Example: a strip that draws 4 watts per foot over 15 feet uses 4 x 15 = 60 watts.
If you are running several separate strips from one driver, add each strip's wattage together.
Step 3: Add headroom
Never run a power supply at 100 percent of its rating. Continuous full-load operation shortens the supply's life and adds heat.- Divide your load by 0.8 to build in 20 percent headroom (the common minimum).
- For fixtures that run for hours or sit in warm spaces, 30 percent headroom (divide by 0.7) is safer.
Step 4: Match the voltage exactly
The supply's output voltage must match the strip.- A 12V strip needs a 12V supply; a 24V strip needs a 24V supply.
- Mismatching voltage either leaves the strip dim or destroys the LEDs.
- Voltage is not something you oversize. Only the wattage and amperage carry headroom.
Converting watts to amps
Some supplies are rated in amps rather than watts. Convert with amps = watts divided by volts.- The 60-watt example on a 12V system draws 60 / 12 = 5 amps.
- With headroom, you want a 12V supply rated for at least 6.25 amps.
- The same 60-watt load on a 24V system draws only 2.5 amps, which is one reason 24V is preferred for long runs: less current means thinner wire and less voltage drop.
A worked example
Say you are lighting 20 feet of 24V strip rated at 3 watts per foot:Constant-voltage drivers and Class 2 limits
Standard LED strips use a constant-voltage driver (a fixed 12V or 24V output). When you shop, note a few points:- Class 2 supplies limit output to 100 watts or under for safety, so larger installs may need multiple Class 2 drivers or a larger non-Class-2 supply with proper protection.
- If the strip is dimmable, the driver and dimmer must be compatible; some need a PWM dimmer on the low-voltage side, others a triac dimmer ahead of the driver.
- For damp or wet locations, use a driver with the correct enclosure rating.
Safety note
Low-voltage strip wiring is relatively safe, but the driver still connects to 120V household power. If you are hardwiring a driver, adding a junction box, or installing in a ceiling or wall cavity, turn off power at the breaker and verify it is off before touching anything. Hardwiring, new circuits, or in-wall drivers should be handled by a licensed electrician, and low-voltage runs must still follow local code. This is general guidance, not a replacement for professional advice.Get the voltage exact, size the wattage with headroom, and confirm the amperage, and your LED strip will run bright, cool, and reliably.
