LED strip: sizing the supply, and why the far end goes dim
Two numbers off the reel decide almost everything about how a run of strip behaves. Get them wrong and you either buy a supply that runs hot or a run whose last metre is visibly duller than its first.
Scope: everything here is about the low-voltage side — the 12 V or 24 V between your supply and your strip. We do not cover mains wiring, opening power supplies, or anything on the AC side. If your install needs work on the mains side, that is an electrician's job in most jurisdictions, and in many it is a legal requirement.
Step one: what your strip actually draws
Find the watts per metre. It is on the reel, the box, or the listing, usually as "W/m" or "watts per metre". Multiply by your length:
Total load (W) = watts per metre × metres
A 14.4 W/m strip over 5 m is 72 W. Now add headroom. A supply run continuously at its rated output gets hot, and heat is what kills supplies:
Supply size = total load × 1.25, rounded up to the next size sold
72 W × 1.25 = 90 W, so buy a 100 W supply. This is not padding for its own sake. A 100 W supply running at 72 W sits well inside its comfortable range, runs cooler, and lasts considerably longer than a 75 W supply running at 96% forever.
If W/m is not stated, do not guess from the LED count. Two strips with the same LEDs per metre can differ in draw by two or three times depending on the LEDs used. A strip with no stated power figure is a strip you cannot size a supply for — and that alone is a reason to be cautious about it.
Step two: check the controller, not just the supply
This is the step that gets skipped, and it is the one we see fail.
The controller sits between the supply and the strip, and every controller has a maximum current per output channel. Your 100 W supply will happily deliver more than the controller can pass. When people say "I bought a big enough supply and it still dimmed and got hot", the bottleneck is almost always the controller output rather than the supply.
To check, convert your load to current:
Current (A) = watts ÷ supply voltage
72 W on a 12 V system is 6 A. On a 24 V system the same 72 W is 3 A. Compare that against the controller's per-channel rating. If your strip is a colour type, the total splits across the channels — but do not assume it splits evenly, because full white drives every channel at once, which is the worst case and the one to size for.
The white test. Whatever effect you plan to run, set the strip to full white at full brightness for ten minutes and feel the controller and the supply. Colour effects rarely drive every channel hard at the same time, so a marginal install can look perfectly healthy for weeks and then overheat the first time somebody picks white. If it is comfortable on white, it is comfortable on everything.
Why the far end is dimmer
The copper inside a strip is thin, and it has resistance. Every LED along the way draws current through the copper that precedes it, and each stretch of copper drops a little voltage. By the far end, the LEDs are receiving less than the LEDs at the head, so they run dimmer.
Three things follow from that, and all three are useful:
- It is worst at the start of the run, not the end. The first metre of copper carries the current for the whole strip, so most of the total drop happens there. This is why adding a thicker feed cable to the head of the strip helps and adding one at the tail does not.
- It shows on white before colour. White uses all channels at once, so it draws the most current and drops the most voltage. A run that looks even on blue can be visibly graded on white.
- Colour shifts as well as dims. As voltage sags, blue and green LEDs fade before red because they need a higher forward voltage. So the far end does not just get darker, it goes warmer and eventually pinkish. If you see a run drift toward red along its length, that is voltage drop, not a colour setting. The same effect reads as a colour-temperature drift on a dual-white run, where the warm and cool emitters do not fade at the same rate as the supply sags.
Fixing it: inject power, do not add supplies
The fix is to stop making all the current travel the whole length of the strip. You run a separate pair of wires from the same supply directly to a point further along — usually the far end — and connect them to the strip's power rails there.
- Feed both ends from the same supply. One supply, two cable runs. The effective distance current has to travel is now half the strip, and the drop falls dramatically.
- Keep polarity identical. Positive to positive, negative to negative, at both ends. Reversed polarity at the second feed is a short across the supply.
- Use a cable that is genuinely thicker than the strip's copper. The whole point is to provide a lower-resistance path. Thin bell wire achieves very little.
- Do not carry the data line to the second feed. On addressable strip, power can be injected anywhere but the data signal must still enter at the head and travel in one direction. Injecting data at the far end does not help and will confuse the chain.
Never connect two separate supplies to the same strip unless you know exactly what you are doing. Two supplies with slightly different output voltages — and they always differ slightly — will push current into each other through the strip. If you must use two supplies for a very long run, split the strip into electrically separate sections and join only the data line, never the power rails.
How to know whether you need it
Set the strip to full white, then measure the voltage across the strip's pads at the far end with a multimeter. If it reads below roughly 90% of nominal — under about 10.8 V on a 12 V system, or 21.6 V on 24 V — the tail is being starved and injection is worth doing. Below 85%, you will see it with your eyes, and so will everyone else.
Extending a run, or starting a second one
At some point a run is long enough that the answer is not more strip. Deciding between the two is straightforward if you ask the right question.
| Situation | Do this |
|---|---|
| You are under the strip's stated maximum run length and under the controller's channel rating | Extend the existing run |
| You are past the maximum run length but the effect must be continuous | Extend, and inject power at the far end |
| You are past the controller's current rating | Second controller. The supply is not the limit here |
| The new section is in a different room or on a different surface | Second run, its own controller. Trying to make one run serve two spaces means cable routed where you will regret it |
| You want the two sections to do different things | Second controller, obviously — one channel does one thing at a time |
Every addressable strip also has a maximum number of pixels its controller can address. Past that count the extra LEDs either stay dark or repeat the pattern from the beginning. That limit is in the controller's specification, and it is not adjustable in the app.
The cost of running strip at full brightness
Worth a paragraph because people ask. Decorative strip is a small load. A 5 m run of 14.4 W/m strip at full white draws 72 W — comparable to a single old-fashioned filament bulb. Run four hours a day, that is under 0.3 kWh, and most decorative effects use well under half of the full white figure because they never drive all channels at once. Whatever your electricity costs, this is not the thing to worry about. The supply's idle draw when the lights are off is a more interesting number over a year, and it is why we would rather you switched a run off at the socket than left a supply idling permanently.
Common questions
Can I use a supply rated higher than I need?
Yes, and it is generally a good idea. A supply delivers only the current the load draws; the rating is a ceiling, not a setting. The one thing to match exactly is voltage — a 24 V supply on a 12 V strip destroys it immediately.
My strip works but the supply buzzes
Audible buzzing usually comes from the supply being driven near its limit or from a dimming frequency the supply's components respond to mechanically. Check the load figure first. If the load is comfortable, it is worth trying a different supply before assuming the controller.
Is it safe to leave strip on all night?
On a correctly sized low-voltage supply, yes — that is what these are designed for. The check worth doing once is the white test above. A supply that is comfortable at full white for ten minutes is comfortable overnight.
Ask us if you want a run checked before you buy the supply.