How to Size a Power Supply for LED Strip Lighting: Watts, Voltage Drop and Safety Margin

Most failed LED strip installs we get called about aren’t strip failures. They’re power supply failures. An LED strip lighting power supply sized to exactly the load runs hot and dies early, and that’s the mistake we see most: somebody added up the watts, bought a supply with exactly that rating, and wondered why it cooked in eight months.

Here’s the whole method in four steps.

The short answer: four steps to a correctly sized supply

1. Total the load. Watts per metre times run length, for every run on that supply.
2. Divide by 0.8. That’s the 80% loading rule. It leaves 20% of the supply’s capacity unused.
3. Check the Class 2 ceiling. In Canada a Class 2 output is capped at 100 VA. Cross that and your wiring rules change.
4. Check voltage drop against run length. A supply that’s big enough can still leave the far end dim.

Round up to the next standard size after step 2. Then confirm steps 3 and 4 before you order.

That’s the formula. Every one of those four rules comes straight out of how the strip is built, so it’s worth two minutes on the electronics first.

How do LED strip lights work?

An LED strip is a flexible printed circuit board carrying small LED emitters, current-limiting resistors and two copper power rails. LEDs are roughly 3-volt devices, so the strip wires them in short series groups with a resistor, then repeats that group in parallel down the length of the tape. Feed the rails a steady DC voltage and every group lights independently.

That parallel-repeating structure is the whole design. It’s why you can cut the tape, why the load scales with length, and why voltage drop happens.

[IMAGE: Annotated close-up of a cut section of 24V LED tape showing the copper rails, a series group of six emitters, the current-limiting resistor and the cut mark] Alt text: “How LED strip lights work: flexible PCB showing copper rails, series LED group and current-limiting resistor”

Why the wiring inside decides your voltage

A 12V strip puts three LEDs in series with a resistor. Three emitters at about 3V each is 9V, and the resistor takes the remaining 3V or so. A 24V strip does the same trick with six LEDs in series. Waveform Lighting’s schematic breakdown walks through the arithmetic.

Two practical consequences fall out of that:

• The cut mark sits at the end of each series group. Cut mid-group and you’ve orphaned LEDs that can’t reach forward voltage. That section stays dark. Our guide on whether you can cut LED strip lights covers where those marks fall.
• The resistor does the current limiting, not the supply. This is the reason strip needs constant voltage. Give it the rated voltage and the resistors settle the current themselves. Give it more, and they can’t.

That second point explains the failure mode people ask about most. Put 24V into a 12V strip and every group is overdriven well past what its resistor was sized for. The emitters burn out, sometimes within minutes.

SMD or COB: what’s actually on the board

Two construction types dominate, and they behave differently in a channel.

COB stands for chip on board. Because the phosphor gel runs the length of the strip rather than sitting over individual chips, a COB LED strip reads as one unbroken line even with no diffuser over it. Super Bright LEDs has good side-by-side photos of the difference.

Pick on exposure. If the strip is hidden in a channel behind a good diffuser, an SMD strip is fine and cheaper. If it’s visible in a cove, a handrail or a reveal where somebody will see the emitters directly, COB earns its price. Our comparison of tray ceiling lighting options goes through where each one lands.

Either way, the sizing method below is identical. What changes is watts per metre.

Step 1: Add up the load, not the reel

Watts per metre is printed on the reel or the spec sheet. Multiply it by your actual installed length.

A 5 m reel of 4.8 W/m tape is 24 W. Install 3.2 m of it and your load is 15.4 W, not 24 W. Size for what you install.

Because the groups sit in parallel, load rises in a straight line with length. Double the tape, double the draw. No surprises there.

Two things trip people up:

• Selectable-CCT strip draws differently at different settings. If the strip has a 3000K/4000K/5000K switch, size for the highest draw, not the setting you plan to use. Somebody will move that switch.
• One supply feeding several runs carries all of them. Add every run on that circuit together. A 40 W supply with three 15 W runs on it is already over.

Step 2: Add headroom with the 80% rule

Never load an LED strip lighting power supply to 100% of its rating. The industry standard is to load it to 80% and leave the remaining 20% as buffer, which the lighting trade calls derating. It covers heat, efficiency loss and inrush.

The arithmetic is a single division:

Supply size = total load watts / 0.80

Dividing by 0.8 picks a supply 25% larger than the load. A 67 W load wants an 84 W supply, so you’d order the next standard size up, typically 96 W or 100 W.

Run a supply flat out and every component inside it sits hot continuously. That’s what shortens its life, and it’s why a driver dies years before the strip it feeds. Flexfire LEDs and aspectLED both put the same 20% buffer at the centre of their sizing guidance.

One caveat worth knowing. Some premium drivers are built to run efficiently closer to full load and are rated to a 90% factor instead. When the manufacturer publishes a derating figure, that figure wins over the generic 80% rule. Check the datasheet.

Step 3: Check the Class 2 ceiling before you go bigger

This is the step most sizing guides skip, and it’s the one that matters most on a commercial job in Canada.

Rule 16-320 of the Canadian Electrical Code limits a Class 2 power circuit to an output of 100 VA and 60 V dc. Stay inside that and you’re on Class 2 wiring, which is why low-voltage strip work is as simple as it is.

Work out what the ceiling means in metres:

The watt ceiling is the same either way. What changes is the current, and current is what drives everything in step 4.

Push past 100 VA and you’ve left Class 2 behind. The circuit now falls under Class 1 rules for conductor sizing, protection and enclosure, per Rule 16-330 and the surrounding section. That’s a design decision, not a detail. Two 96 W Class 2 supplies are often the cheaper and faster install than one 200 W supply that drags Class 1 requirements onto the job.

Step 4: Check voltage drop against your run length

A correctly sized supply still can’t fix a run that’s too long. The copper rails on a flexible board are thin, so voltage falls off along the tape itself. The far end goes dim and shifts colour.

Two figures to work with:

Single-feed run limits. Keep a 12V run under about 5 m (16 ft). A 24V run of the same strip goes to roughly 10 m (32.8 ft), and on heavier copper some manufacturers publish 10 to 15 m. The reason is straightforward: at 24V the same wattage draws half the current, and voltage drop follows current.

The code figure. CEC Rule 8-102 caps voltage drop at 3% in a branch circuit, 5% from supply to point of use. Worth noting that Canada uses “shall” here, so unlike the NEC’s informational guidance, these limits are enforceable.

Three fixes when a run is too long:

• Feed from both ends. Halves the effective distance from the supply.
• Split into shorter home runs. Two 7 m runs off one supply beat one 14 m run.
• Move up in wire gauge on the feed. 14 or 12 AWG instead of the usual 18 AWG on longer leader runs.

Pick 24V by default on anything commercial. It’s the single decision that buys the most run length for free.

A worked example: 14 metres of retail soffit

Say you’re lighting a 14 m soffit in a retail fit-out with 4.8 W/m tape at 24V.

Step 1, load. 14 m x 4.8 W/m = 67.2 W

Step 2, headroom. 67.2 / 0.8 = 84 W. Order the next standard size up: a 96 W or 100 W supply.

Step 3, Class 2. 84 W sits under the 100 VA ceiling, so this stays a Class 2 circuit. Good. If the run had been 20 m, the load would be 96 W, headroom would push you to 120 W, and you’d be into Class 1 territory. Two supplies would be the better answer.

Step 4, voltage drop. 14 m exceeds the ~10 m single-feed guidance for 24V. So don’t run it end to end. Split it into two 7 m runs fed from a common supply at the centre, or feed the 14 m from both ends. Either works. Both cost less than a callback.

Now run the same job at 12V. Load is identical at 67.2 W, but current doubles from 2.8 A to 5.6 A, and 14 m is nearly triple the 5 m single-feed limit. You’d be injecting power in three places to do what 24V does in two.

Constant voltage or constant current?

An LED strip lighting power supply has to be constant voltage. It puts out a fixed voltage, 12V or 24V DC, and the resistors on the strip settle the current. Constant current drivers hold a fixed milliamp output and feed fixed LED arrays like downlight modules, where there’s no resistor doing that job. Put a constant current driver on a strip and you’ll get flicker, wrong output, or nothing.

The words on the box vary and mean the same thing: power supply, driver, transformer, adapter. What matters is the label reading constant voltage and the DC output matching your strip.

What the label has to say in Canada

An LED strip lighting power supply takes 120V on its input side, which makes it line-voltage equipment. Any line-voltage product installed in Canada needs a certification mark from a body accredited by the Standards Council of Canada. For LED drivers that means cUL, cULus, CSA or cETL.

A US-only UL mark isn’t enough. It’s the most common problem with grey-market supplies bought online, and an inspector will fail it. Drivers listed for Canada are tested to CSA C22.2 No. 141 alongside the US standard. Maple Electric Supply has a clear breakdown of which marks cover what, and Health Canada publishes the federal position on electrical product safety.

Also check the enclosure rating against where it’s going. A driver in a damp parking garage or an exterior soffit needs a wet or damp rating and a sealed enclosure. An indoor plastic brick will not survive a Canadian winter outdoors.

Five mistakes that kill supplies early

1. Sizing to exactly the load. Covered above. It’s the big one.
2. Ignoring ambient temperature. Every driver has a rated ambient range. An unheated garage in Winnipeg and a sealed ceiling cavity above a kitchen are both outside what a generic adapter expects. Read the range on the datasheet and match it to the actual location.
3. Burying the driver with no air around it. These need to shed heat. Stuffed inside insulation or a sealed box, they run hot and fail.
4. Mismatching the dimming method. A 0-10V driver needs a 0-10V control. A TRIAC driver needs a compatible forward-phase dimmer. Mixing them is the usual cause of flicker, which our post on flickering LED lights works through in more detail. If you’re wiring the control side, see how to install a dimmer switch. We stock a 0-10V dimmer switch for commercial driver work.
5. Sizing for the dimmed load. Dimming doesn’t change the sizing math. Size for full output. Somebody will run it at 100%.

When you can skip the driver entirely

Worth asking before you size anything: does this job need low-voltage strip at all?

On commercial work, often it doesn’t.

Line-voltage neon rope. Our flexible neon rope runs on 120V direct, at 9 W per metre, IP65, with switchable 3000K/4000K/5000K, in 50 m rolls. No driver, no Class 2 calculation, no voltage-drop budget across a 24V run. For signage, cove and architectural outlines it removes this entire problem.

Integral-driver linear fixtures. For garages, shops, warehouses and back-of-house, a hardwired linear strip fixture takes AC120-347V straight in, with the driver already inside and matched to it. The 4ft unit is field-selectable at 12W, 20W, 30W, 35W or 40W, 130 Lm/W, 5-CCT from 3500K to 5700K, and links unit to unit. Nothing to size, nothing to derate. It’s usually the faster install and the fewer failure points. Browse the full strip lighting range to compare.

Outdoor 12V landscape work is its own case. Our landscape lighting transformer is 88 W, 120V to 12V AC, 7.3 A, with a photocell and a 4/6/8-hour timer in a weatherproof housing. Note that’s 12V AC for landscape fixtures, not the DC output a flexible strip needs. Different job, different device. Don’t substitute one for the other.

Low-voltage flexible strip still wins where you need a tight bend radius, a shallow channel or fine dimming control. Just size it properly.

Frequently asked questions

What to take away

• Strip is series groups of LEDs repeated in parallel. That single fact explains the cut marks, the linear load, and the voltage drop.
• Total the load, divide by 0.8, round up. That’s the sizing calculation.
• 100 VA is the Class 2 ceiling in Canada under CEC Rule 16-320. Two smaller supplies often beat one large one.
• Voltage drop is a separate check from capacity. 5 m per feed at 12V, about 10 m at 24V.
• Choose 24V by default on commercial jobs, and COB wherever the strip is visible.
• The mark has to be cUL, cULus, CSA or cETL. A US-only UL mark fails inspection here.
• On garages, shops and signage, an integral-driver fixture or line-voltage rope skips the whole exercise.

Sizing a run for a specific project? Send us the length, the watts per metre and where it’s going, and we’ll spec the supply and confirm whether it stays Class 2. Request a quote and you’ll hear back within one business day.