Driver and power-supply sizing for daisy-chained LED runs

The single most common installation problem we see in refrigeration LED retrofits isn't the bars themselves — it's the power supply behind them. Drivers sized too tight, chains too long, or supply ends connected the wrong way around. Each one looks fine on day one and starts failing six to twelve months later. This article walks through the planning rules that prevent that.

Why 24V DC is the commercial reference platform

Almost every serious commercial refrigeration LED on the market — including the full CAPST range — runs on 24V DC. Three reasons:

• Safety: 24V is below the SELV (Safety Extra-Low Voltage) threshold. Installation doesn't require electrician-grade qualifications, simplifying refit logistics.
• Loss tolerance: Voltage drop along a daisy-chained run is more forgiving at 24V than at 12V (lower current for the same power), and more efficient than 48V given the typical strip wattages.
• Driver availability: 24V DC commercial power supplies are commodity components — Mean Well, Magtech, and similar manufacturers ship in every wattage tier from 50W to 350W.

The 36W-per-chain rule

Every linear-LED platform has a maximum chain wattage — the total connected load that can run from a single supply end before the voltage drop and current density start damaging the connector or the strip itself. For the CAPST SLK, SNK, and SRL families, that figure is 36W per chain on 24V DC.

The arithmetic is straightforward — but the failure mode if you exceed it is not. At loads above 36W on a single supply end, the connector closest to the driver carries the cumulative current of the entire chain. That connector heats up. In a refrigerated cabinet, the temperature differential accelerates plastic fatigue, eventually leading to intermittent contact and visible flicker.

Driver wattage headroom

The other half of driver sizing is choosing a supply with enough headroom over the connected load. The industry rule is 10–15% above the calculated load:

• A driver running at 95% of its rated load runs hot, ages quickly, and fails early.
• A driver running at 70–85% of rated load lives at its design lifetime — typically 50,000+ hours.
• A driver vastly oversized (running at 30%) is overspend for no benefit; standby losses in the supply itself become disproportionate.

Worked example: 12-cabinet supermarket aisle

Take a typical Bangkok supermarket multi-deck aisle: twelve open-front cabinets, each fitted with one vertical SLK-VL/R bar at 1500mm length, configured at 9.2W per bar. The connected load is:

12 × 9.2W = 110.4W

That's well over the 36W single-chain limit, so the run will be split into chains. Practical division:

• Three chains of four bars = 36.8W per chain. Borderline — closer to a both-end supply than a single-end.
• Four chains of three bars = 27.6W per chain. Comfortably within the single-end limit; safer choice.

For the four-chain layout, each chain takes one supply. With four chains pulling 27.6W each, the total load on a single feeding driver — if you choose to centralise supply — is 110.4W. Add 15% headroom: target driver rating ≥ 127W. A standard Mean Well LRS-150-24 at 156W rated provides 29% headroom — well-sized.

Alternatively, two 100W drivers split across the aisle gives you redundancy: a single driver failure takes down half the aisle rather than all of it. For a continuously-trading supermarket, that's worth the marginal cost.

The four installation mistakes that fail in year one

1. Chain longer than 36W on a single supply end. Connector heat-fatigue, eventual flicker, intermittent failure.
2. Driver running above 90% of rated load. Premature driver failure, often inside the warranty window but always with operational disruption.
3. Mixing wattages on the same chain. Voltage-drop calculations assume uniform load; mixing 9W and 13W bars in one chain produces uneven brightness over time.
4. Daisy-chaining across cabinets with different temperature zones. Connectors at the boundary endure cyclical condensation; sealing degrades faster than the bar lifetime.

What to share with your supplier when you ask for a quote

• Cabinet count and per-cabinet bar quantity.
• Bar length and wattage you're planning to specify.
• Whether you want centralised driver supply or per-cabinet supply.
• Whether the aisle has redundancy requirements (continuous trading sites typically do).

For most projects we'll come back with a wiring diagram showing chain divisions, recommended driver size (with model number), and supply-end placement. The technical pack on each product page also includes a sizing worksheet — request it via the contact form when you're ready to plan a project.