LED Strip Calculation: Which Power Supply, How Many Feed Points, Which Cable Size?

An LED strip draws its watts per metre times its length, its current is that power divided by the voltage, and you size the power supply with about 20% reserve. Five metres of 14.4 W/m strip at 24 V need 72 W and 3 A, so a 100 W power supply fits. Two lengths then decide the quality: how far the strip may run from one feed point, and how long the cable from the power supply to the strip is. You calculate both in a minute with the free LED strip calculator, then plan the installation in PlanElec all the way to the single-line diagram.

How do you calculate the power and current of an LED strip?

Two formulas are enough:

• power: P = W/m × length
• current: I = P / U

Power per metre is in the datasheet and applies at full brightness. For multi-channel strips (CCT, RGB, RGBW) it is the value with all channels at full load. Plan with that value, even if you rarely run all colours at once. Use the length you actually install: a strip can only be cut at its cut marks, typically every 50 or 100 mm. With a 50 mm cut unit, 2.37 m becomes 2.35 m.

The power-supply column shows the next common commercial rating. Your power supply's datasheet sets the actual range.

How large must the power supply be?

Calculate power supply ≥ P × 1.2. The 20% reserve keeps the power supply cool and long-lived and covers strip and supply tolerances. If the supply sits in a tight suspended ceiling, inside furniture or at high ambient temperature, the datasheet derating applies on top: many units deliver less than their rated power there.

The same formula answers the reverse question of how much strip an existing supply can carry. A 60 W supply delivers 50 W with reserve: just over 5 m of a 9.6 W/m strip or almost 3.5 m of a 14.4 W/m strip. If the largest supply is not enough, split the installation into sections, each with its own power supply.

12 V, 24 V or 48 V?

At the same power, doubling the voltage halves the current, and the voltage drop in the cable falls in the same ratio. For 60 W over a 5 m feed cable of 1.5 mm² copper:

24 V is the standard for homes: widely available, longer runs per feed point than 12 V and, in bathrooms, within the SELV limits (see below). 12 V still makes sense for short furniture and niche strips or existing 12 V components. 48 V plays to its strengths on long lines and profile systems. 5 V is for digital pixel strips and very short pieces.

When does an LED strip need several feed points?

Voltage also drops along the strip itself. Fed too far from one end, the far end becomes visibly dimmer, and RGB and RGBW strips shift in colour. Manufacturers therefore state a maximum length per feed point. It is in the datasheet and decreases as the power per metre rises.

The calculation: feed points = length ÷ maximum length per feed point, rounded up. A 12 m strip with a 5 m manufacturer length needs three feed points; each then carries a third of the current. Every feed point gets its own cable from the power supply, run in parallel. For a light cove around a room this means a central power supply and feed points at the corners, rather than one long band fed from one side.

Which cross-section for the cable from power supply to strip?

Extra-low voltage means high currents at a low voltage, so every metre of cable counts. The formula is the familiar single-phase one without a power factor:

ΔU = 2 × ρ × L × I / S, with ρ = 0.0175 Ω·mm²/m for copper and L the one-way length.

For a 24 V strip drawing 3 A:

We design for at most 3% voltage drop on the feed cable so the strip lights evenly. The AREI/RGIE sets no fixed percentage for extra-low-voltage feed cables; we adopt the 3% customary for lighting circuits as the design limit. Check the current-carrying capacity of thin cables separately against the manufacturer's data; the calculator therefore only proposes cross-sections from 0.75 mm². The basics of voltage drop at 230 V are in Voltage drop calculation.

What changes with RGB, RGBW and tunable white?

Multi-channel strips split their power across channels: two for CCT (tunable white), three for RGB, four for RGBW, five for RGB+CCT. An RGBW strip of 19.2 W/m over 4 m draws 76.8 W and 3.2 A, i.e. 0.8 A per channel. That leads to three design points:

• Controller: it must supply the current per channel. 0.8 A is easy; 15 m of the same strip at 3 A per channel is beyond some controllers.
• Number of cores: the common positive plus one conductor per channel. Mono needs 2 cores, CCT 3, RGB 4, RGBW 5, RGB+CCT 6.
• Voltage drop: the common positive conductor carries the current of all channels, each channel conductor only its share. The effective current is therefore lower than for a single-channel strip of the same power. In the example with 5 m and 0.75 mm², that gives 1.9% instead of 3.1%. The calculator accounts for this automatically.

What does the AREI/RGIE require for LED strips?

The AREI (RGIE in French) is the Belgian wiring regulation, Book 1 of the general regulation on electrical installations.

Voltage and type of current belong on the single-line diagram. Subsection 3.1.2.1 a requires them for every domestic installation. A strip behind a power supply is therefore documented as "24 V ⎓", not only with the 230 V circuit in front of it. Table 2.23 has its own symbol for this, the AC↔DC rectifier; a safety transformer with AC output gets the transformer symbol.

SELV needs an approved source and proper separation. Subsection 4.2.5.3 a defines the approved sources: an LED power supply with a SELV output qualifies, "low voltage" on its own does not. Under 4.2.5.3 c you run SELV conductors 10 mm away from other circuits, with additional insulation, or in a cable that is fully insulated for the highest voltage present. This matters mostly in boxes shared by push button, controller and 230 V wiring.

In bathrooms, zone and voltage count. Table 7.2 under subsection 7.1.4.2 a allows at most 12 V AC or 30 V ripple-free DC in zones 0 and 1, and up to 25 V AC or 60 V DC in zone 2. A 24 V DC strip in a shower niche is therefore permitted if it is suitable for that zone. The SELV source, i.e. the power supply, sits outside zones 0 and 1 according to subsection 7.1.5.2. Which cables go into which zone is covered in Which cables can I use in the bathroom?.

Calculate online

The LED strip calculator shows every value from this article at a glance: power, total and per-channel current, required feed points, power-supply size, recommended feed-cable cross-section and the cross-section comparison with voltage drop, end voltage and maximum length. It covers 5, 12, 24 and 48 V, single-colour, CCT, RGB, RGBW and RGB+CCT strips, starts from typical strip types and takes your own datasheet values.

Every result can be shared as a link or printed, which makes it easy to agree with clients and suppliers. AI assistants in the browser use the calculator directly via WebMCP and return the same shareable link.

The roles are clear: the calculator delivers the design from your datasheet values. The manufacturer's datasheet confirms length per feed point, derating, heat dissipation in the profile and the cable's current-carrying capacity.

Frequently asked questions

Can I feed the strip in the middle?

Yes. A centre feed halves the length in each direction and counts as two feed points over half the strip length each. For long lines it is often neater than two cables to both ends.

Does every LED strip need an aluminium profile?

From medium power upwards, yes: the profile dissipates heat and, together with the diffuser, shapes the light. The manufacturer sets how much power per metre is allowed without a profile.

How many metres fit on a 100 W power supply?

With 20% reserve, just over 83 W is available: about 5.8 m at 14.4 W/m, 8.7 m at 9.6 W/m or 4.3 m at 19.2 W/m (RGBW).

Can I connect two power supplies to one strip?

Plan separate sections, each with its own power supply, instead. Only the manufacturer decides whether supplies may run in parallel or on a continuous strip.

Does the power supply go in the distribution board or next to the strip?

Both work. On the DIN rail in the board it stays accessible and cool, but needs long extra-low-voltage cables, so the cross-section table above decides. Placed near the strip, the feed cable stays short, but the supply must be accessible and suitable for its location.

Which cable goes between controller and RGBW strip?

A five-core cable (common positive plus four channels), sized for the channel current and the length. RGB+CCT needs six cores.

Plan LED lighting in PlanElec

In PlanElec you place luminaires, power supply, switches and push buttons directly on the floor plan, assign them to circuits and generate the single-line diagram, situation plan and material list. The self-check monitors, among other things, the number of light points per circuit and the voltage drop. Controls with Shelly or KNX actuators and push buttons are planned in the same project. Sign up for free and start your first lighting plan.

Related articles

• Voltage drop calculation: how long can your cable be?
• Which cables can I use in the bathroom? (AREI zones)
• Smart home and AREI: what you need to know about home automation

Official basis: FPS Economy, AREI/RGIE Book 1 V06, in particular subsection 3.1.2.1, table 2.23, subsection 4.2.5.3 and subsections 7.1.4.2 and 7.1.5.2.