Sizing an LED strip power supply (and avoiding voltage drop)

You've got a 16-foot LED strip reel, a 12V power supply that says "100W" on the label, and a nagging feeling that the far end of the run is dimmer and warmer-colored than the end plugged into the supply. That's not a bad reel. That's voltage drop, and it shows up almost exactly where a lot of single-reel installs live — a hair past the safe distance for feeding 12V from just one end.

Sizing an LED strip power supply looks like one multiplication: watts per foot times feet equals total watts, buy a supply that covers it. In practice there are two more steps most first-time installs skip — headroom above the calculated load, and a length limit past which the electrical reality of a 12V circuit starts working against you — and skipping either one is why "properly sized" supplies still run hot or produce strips that visibly dim toward the far end.

Total load: watts per foot times feet, times runs

Every LED strip is rated in watts per foot (or per meter), and that number already accounts for a full run at max brightness. Total wattage is simply length times watts-per-foot, and if you're feeding multiple parallel strip runs from a single supply, multiply again by the number of runs. A single 16.4 ft strip (a standard 5-meter reel) at 4.8 W/ft draws about 78.7 W on its own — before you've picked a supply at all.

Why you shouldn't run a PSU at 100% of its rating

A power supply rated for 100W can, in principle, deliver 100W continuously. In practice, running any switching power supply at its full rated output for extended periods shortens its lifespan and pushes it toward running hotter, which compounds the wear. The standard convention is to size the supply to your calculated load plus a safety margin — commonly 20%, sometimes 25% for anything expected to run many hours a day like accent lighting left on overnight.

That headroom percentage gets added to total wattage before you go shopping for a supply size, and then you round up to the next commercially available PSU size — 12W, 20W, 30W, 60W, 100W, 150W, 200W, 350W are typical steps. A calculated requirement of 94W doesn't get you a "94W" supply; it rounds up to the next size on the shelf, which is 100W. If your padded requirement lands just above a size break — say 101W — you jump a full tier to 150W, so it's worth checking whether trimming the run slightly or splitting it across two supplies lands you back under the lower threshold.

The length limit: why long runs need power injection

LED strip copper traces are thin, and thin copper has real resistance. Over distance, that resistance bleeds off voltage as current travels down the strip, so the LEDs near the far end of a long run receive less than 12V (or 24V) even though the supply is putting out full voltage at the injection point. The visible symptom is a strip that's bright and correctly colored near the supply and dimmer, sometimes noticeably warmer or yellower, by the far end — because LEDs shift color output as their supply voltage sags.

The distance at which this becomes noticeable depends on the strip's voltage: 12V strips run into trouble past roughly 16 feet fed from a single end, while 24V strips can push about twice that, roughly 32 feet, before the same sag shows up. This is a real, physical reason 24V strips exist for longer runs rather than just being an arbitrary product tier — doubling the voltage roughly halves the current for the same wattage, and voltage drop scales with current, so the higher-voltage strip tolerates a longer single feed.

Past that threshold, the fix isn't a bigger power supply — it's feeding power into both ends of the run (or at intervals along it) instead of relying on one injection point to carry the whole length. Power injection keeps every LED closer to full rated voltage regardless of position, at the cost of running a second feed wire back to the supply or splitting into a second supply entirely.

Worked example

A single 16.4 ft (5m) reel at 4.8 W/ft, 12V, with 20% headroom, one run on the supply:

• Total load: 16.4 ft × 4.8 W/ft × 1 run ≈ 78.7 W
• Required PSU rating with headroom: 78.7 × 1.20 ≈ 94.5 W
• Recommended PSU size (next size up): 100 W
• Current draw: 78.7 W ÷ 12V ≈ 6.56 A

That 16.4 ft length is past the 16 ft single-feed threshold for 12V — by a small margin, but past it — so this exact reel, at this exact voltage, is a textbook case for injecting power at both ends rather than trusting one connector at one end to carry the full run evenly. Switching the same strip to a 24V product at the same total wattage would cut the current draw to about 3.28 A and push the safe single-feed distance out to roughly 32 ft, comfortably covering the same reel from one end alone — worth knowing before you commit to a voltage on a longer planned run.

Size it once, correctly

Between headroom percentage, the jump between commercial PSU sizes, and the voltage-dependent length limit, there are enough moving parts that eyeballing a supply size is how installs end up either underpowered or visibly dim at the far end. Our LED strip power supply calculator takes your strip length, watts per foot, voltage, headroom percentage, and number of parallel runs, and returns total wattage, the recommended commercial PSU size, current draw, and a direct warning when your run length crosses the voltage-drop threshold for that voltage — so you know before you mount anything whether you need a second injection point.