The most common LED installation mistake isn't a wiring error or a cut in the wrong place. It's a power supply that's too small for the load it's running. The result: flickering, overheating, premature failure, and a callback that costs more than the original job.
Here's how to get the math right the first time.
The Basic Formula
LED power supply sizing has two steps:
Step 1: Calculate total load wattage
Total watts = watts per foot × total run length in feet
Step 2: Apply the 20% safety buffer
Minimum power supply wattage = total watts × 1.2
The 20% buffer is not optional. Running a power supply at 100% of its rated capacity continuously shortens its lifespan, causes overheating, and often results in the flickering that gets blamed on the strip when it's actually a driver problem.
Worked Example
You're installing a single-color LED strip under kitchen cabinets. The strip spec sheet lists 3.0W per foot. The total cabinet run is 24 feet.
- Total load: 3.0W × 24 ft = 72 watts
- With buffer: 72W × 1.2 = 86.4 watts
- Select the next standard size up: 100W power supply
A 60W driver would be undersized. A 100W driver at 72W actual load runs at 72% capacity — well within the safe operating range.
Multi-Zone and Multi-Strip Calculations
If you're running multiple strips from one power supply, add the wattage of every strip on that circuit, then apply the 20% buffer to the combined total.
Example: Three strips of 2.5W/ft, each 12 feet long.
- Each strip: 2.5W × 12 ft = 30W
- Three strips combined: 30W × 3 = 90W total
- With buffer: 90W × 1.2 = 108W
- Select: 120W power supply
Voltage Must Match
The power supply voltage must exactly match the strip voltage. A 24V strip requires a 24V power supply. A 12V strip requires a 12V supply. Mismatching voltages will either under-drive the strip (too dim, 12V supply on 24V strip) or damage it (24V supply on 12V strip).
Never assume. Always confirm the strip voltage from the spec sheet before ordering the driver.
Dimmable vs Non-Dimmable Power Supplies
If your installation requires dimming — and most residential and hospitality installations do — you need a dimmable driver, not a non-dimmable power supply. Non-dimmable supplies are slightly less expensive and fully functional for always-on installations, but they cannot be paired with a dimmer switch.
Dimmable drivers support control protocols including PWM, 0–10V, TRIAC, and ELV, depending on the driver model. Match the driver's dimming protocol to your dimmer switch.
Where to Mount the Power Supply
Power supplies generate heat and need adequate ventilation. Never mount them inside sealed enclosures, in direct contact with insulation, or in areas that regularly exceed 104°F (40°C). For cabinetry installations, a dedicated space inside the upper cabinet run with a vent gap is the standard approach.
For long runs where the power supply needs to be located away from the strip, 24V is strongly preferred over 12V because lower current means less voltage drop over the wire between driver and strip.
Quick Reference: Common Strip Wattages
| Strip Type | Typical W/ft | Power Supply for 20 ft Run |
|---|---|---|
| Standard density single color | 1.5–2.0 W/ft | 48–60W driver |
| High density single color | 2.5–3.5 W/ft | 75–105W driver |
| COB single color | 3.0–4.5 W/ft | 90–135W driver |
| RGB multicolor | 2.0–3.5 W/ft | 60–105W driver |
Note: Always use the actual W/ft figure from your specific strip's spec sheet. The values above are typical ranges, not guaranteed specs.
What Happens When You Undersize
A power supply running above its rated capacity will:
- Overheat, triggering thermal protection and causing flickering or shutdown
- Fail early — sometimes within weeks on installations with continuous operation
- Produce inconsistent output voltage, causing color shift in RGB strips
- In worst cases, become a fire risk if thermal protection fails
UL Listed power supplies have thermal protection and will typically shut down rather than ignite. Non-UL supplies may not. This is one of the practical reasons UL listed LED strip lights and UL Listed drivers are non-negotiable for commercial and permitted residential work.
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Updated June 2026 | HitLights — Factory-Direct LED Strip Lighting Since 2010
