Power Injection
Why the far end of a long strip goes dim, and where to feed it
What power injection does
Power injection means feeding the strip's + and - from the power supply at several points along its length instead of only at the first LED, so the voltage reaching the far end stays high enough for the LEDs to run at full brightness.
A strip fed only at the first LED loses voltage along its own copper trace. By the far end the LEDs are running on less than they were designed for, so white turns warm and then orange and the whole tail dims. Each extra feed resets the voltage part way along the run. If you are not yet sure that is what you are looking at, the voltage drop guide separates this loss from the one in the cable feeding the strip, which injection cannot fix.
How far apart
Maximum distance between feeds, in metres. Higher voltage carries the same power at lower current, and denser strips pull the spacing back in.
| Voltage | Up to 12 W/m | 12 to 18 W/m | Over 18 W/m |
|---|---|---|---|
| 5V | 2.5 m | 2.13 m | 1.75 m |
| 12V | 5 m | 4.25 m | 3.5 m |
| 24V | 10 m | 8.5 m | 7 m |
| 48V | 15 m | 12.75 m | 10.5 m |
These are the distances the PSU calculator uses when it marks injection points for your own strip and length. Both inputs are decisions made before the strip arrives: the rail is the voltage guide, and the density comes with the family you pick in the strip types guide. Nothing on this page can recover what those two cost you. A few chips are sold on both rails and let you make the choice late - the WS2814 is the same strip at 12V or 24V, and one feed covers twice as much on the second. The WS2811 is the other way round: one part number sold as a 5V strip and a 12V one that are not interchangeable, and the reach in this table is the 12V figure. The TM1814 is on both rails as well, and there the late choice costs resolution as much as reach - it addresses in threes at 12V and in sixes at 24V.
Brightness does not buy you spacing
A 5 m run of a 14.4 W/m strip at 12V, at each of the calculator's power modes.
| Mode | Of full | Draw | Current |
|---|---|---|---|
| Effects, ~1 channel of 3 | 42% | 30.2 W | 2.5 A |
| Ambient, dimmed | 60% | 43.2 W | 3.6 A |
| Full white | 100% | 72 W | 6 A |
Drop is proportional to current, so a strip at a third brightness loses about a third as much voltage and would reach some three times further. The spacing above is still quoted at full white on purpose. Brightness is a setting and wiring is not: the day someone picks the white preset, a run fed for a third of that goes dim and the strip comes off the wall. The power modes size the supply — a number you can change later — not the feeds.
Four runs worked out
Draw and current are at full white, positions are measured from the start of the strip, and the per-feed figure is what the busiest feed carries — a feed in the middle of a run owns the strip halfway to its neighbour either side, so it carries more than the two at the ends.
WS2812B 60 LED/m, 5 m
The standard WLED build. At 5V a 5m reel needs feeding in four places, which is why so many first projects go dim past the middle.
Draw
60.6 W
Current
12.1 A
Max spacing
2.13 m
Per feed
4 A
Feed at
- 0 m · Start (required)
- 1.7 m · Injection point 1
- 3.3 m · Injection point 2
- 5 m · End (recommended)
WS2815 60 LED/m, 10 m
Same LEDs and density at 12V instead of 5V. The same four feeds now cover twice the length, each carrying less current.
Draw
94.2 W
Current
7.9 A
Max spacing
5 m
Per feed
5.9 A
Feed at
- 0 m · Start (required)
- 5 m · Injection point 1
COB White 528 LED/m (24V), 12 m
Dot-free 24V COB at a moderate 14 W/m. Twelve metres off two feeds.
Draw
168 W
Current
7 A
Max spacing
8.5 m
Per feed
5.3 A
Feed at
- 0 m · Start (required)
- 6 m · Injection point 1
WS2811 FCOB 24V 720 LED/m, 15 m
Addressable FCOB pulls 21 W/m, over the 18 W/m line, so the spacing tightens even at 24V.
Draw
315 W
Current
13.1 A
Max spacing
7 m
Per feed
4.4 A
Feed at
- 0 m · Start (required)
- 5 m · Injection point 1
- 10 m · Injection point 2
- 15 m · End (recommended)
Wiring it up
Five steps, in order. Only the first one changes with your strip.
- 1
Mark the injection points
Divide the run into equal segments no longer than the spacing your voltage and strip density allow, and mark the boundary between each pair. A 5m WS2812B run at 5V splits into segments of about 1.7m, so the marks land at the start, at 1.7m, at 3.3m and at the far end.
- 2
Cut a feed pair for every point
Each point needs its own + and - wire running back to the power supply, long enough to reach without tension. Size it for the current that feed carries and the distance it covers - the wire gauge calculator takes both.
- 3
Solder onto the + and - pads only
Every LED strip repeats its + and - pads along the copper, usually at each cut line. Solder the feed to a pair of those pads. Leave the data pad alone: data travels down the strip from one controller, and a second source on that line corrupts the signal.
- 4
Run every feed back to the supply in parallel
All feeds start at the power supply terminals and fan out from there. Never carry power from one strip section to the next through the strip itself - that copper trace is the bottleneck injection exists to bypass.
- 5
Fuse the supply and check the far end
Fit a fuse rated at 125-150% of the expected current on the positive output for anything over 60W. Then set the strip to full white and compare the last LED with the first: matching colour temperature means the voltage is holding, a reddish or dim tail means a point is missing or a feed is too thin.
That fuse sizing is the supply's output, where one fuse covers everything downstream. Once a run has several feeds, each branch is thinner than the trunk and needs its own — the fuses guide works out what each feed carries and why a fuse sized from the whole strip's draw protects none of them.
Sizing a feed
Each feed carries its share of the current rather than the total, but it has to deliver that share over its own distance while losing less than 5% of the supply voltage on the way. Size every feed from the busiest one: they take a single gauge and a single fuse between them, and that is the one they all have to survive.
Voltage drop (V) = Current (A) × Resistance (Ω/m) × Distance (m) × 2The doubling is the return path: current travels out on the positive and back on the negative, so the run counts twice. The wire gauge calculator takes the per-feed current and that distance and returns the thinnest gauge that stays inside the budget.
Five ways it goes wrong
Daisy-chaining power through the strip
Running + and - from the end of one section into the start of the next puts the current back through the copper trace you were trying to bypass. The far end stays dim and the trace near the feed heats up.
Injecting from a second supply without a common ground
Two supplies with separate grounds give the data signal no shared reference, so the strip flickers or shows random colours. Tie every negative together, always.
Tying two positive outputs together
No two supplies hold exactly the same voltage, so the higher one pushes current into the lower one. Give each supply its own segment and connect only the negatives.
Feeding injections with thin hookup wire
A 22 AWG jumper cannot carry several amps over a few metres without dropping most of the voltage it was meant to deliver. The point is wired and the strip is still dim.
Injecting to fix a strip that is dim everywhere
Even brightness that is simply too low is an undersized supply, not voltage drop. Injection only fixes a gradient along the run.
Several of these announce themselves as flicker or random colours before anything fails outright. Working back from that symptom to its cause is the flickering guide. And when the second supply is the plan rather than a patch - a run split into zones, each with its own supply and fuse - the multiple power supplies guide plans the whole project. One failure worth naming separately: the joint carrying a feed is usually the first thing to run hot, and what a joint actually carries sets each strip's current against what a clip connector is rated for.
Questions
What is power injection?
Power injection means feeding the strip's + and - from the power supply at several points along its length instead of only at the first LED, so the voltage reaching the far end stays high enough for the LEDs to run at full brightness.
When do I need to inject power into an LED strip?
Inject when the run is longer than a single feed can supply: roughly 2.5m at 5V, 5m at 12V, 10m at 24V and 15m at 48V. Dense strips shorten those distances - above 12 W/m cut them by 15%, above 18 W/m by 30%. The visible sign is a gradient along the run: the first LEDs are correct and the far end is dim or shifted reddish at full white.
Where do the injection wires connect?
To a pair of + and - pads on the strip, at the point you marked, with the other end at the power supply terminals. Most strips repeat these pads at every cut line. All feeds run in parallel from the supply, never from one strip section to the next.
Do I need to inject the data line as well?
No. Data flows down the strip from a single controller, LED to LED, and connecting a second source to that line corrupts the signal. Only + and - are injected. If the data itself degrades over a long run, that is a separate fix: a level shifter, a shorter data lead, or a strip like the WS2815 with a backup data line.
Can I use a second power supply for the injection points?
Yes, provided both supplies output the same voltage and all negatives are tied together, including the controller's. Never connect the positive outputs of two supplies to each other - give each one its own strip segment and fuse each positive output separately.
How thick do injection wires need to be?
Size each feed for the current it actually carries and the distance it runs, not for the total. A feed powers the strip halfway to its neighbour either side, so the ones in the middle of a run carry a full segment's worth and the ones at the two ends carry half of that - size them all from the busiest. Keep round-trip voltage drop under 5% - on a 5V strip that is only 0.25V, which is why 5V runs need noticeably thicker wire than 24V ones at the same wattage.
Can I inject less often if I run the strip dimmed?
The arithmetic says yes and the advice here still says no. Voltage drop is proportional to current, so a strip at a third brightness loses about a third as much and would reach roughly three times as far. But brightness is a setting and wiring is not: the spacing above holds at full white, because the day someone picks the white preset a run fed for a third of that goes dim and the strip has to come off the wall to fix it. The power modes in the calculator size the supply, which is a number you can change later, not the feeds.
Does power injection increase how much power I need?
No. The strip draws the same wattage whichever way it is fed. Injection changes how that power gets there, not how much of it there is - size the supply from the full run length plus a 20% margin either way.
Enter your own strip and length to get the points marked for you.
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