LED Facade Power Supply Sizing: Continuous Load, Derating and Inrush-Current Planning
Power engineering guide · Evidence reviewed August 27, 2026
LED Facade Power Supply Sizing: Continuous Load, Derating and Inrush-Current Planning
Adding fixture watts is only the first line of a safe power design. A supply that looks large enough on its label can still be undersized after temperature or input-voltage derating, while several supplies that run normally can trip a branch breaker when energized together. This guide separates DC load, usable output, AC input and startup behavior.
Answer in 30 seconds
- Calculate fixture watts and DC current by branch, not only for the entire facade.
- Apply the selected PSU manufacturer's actual ambient and input-voltage derating curves [S4].
- Treat an 80% loading target as a project assumption, not a universal standard.
- Check rated AC input current separately from inrush peak, duration and breaker curve [S2][S3].
- Do not approve the design while the exact PSU, enclosure temperature, protection and startup sequence are unknown.
1. Four numbers that must not be confused
| Quantity | Meaning | Design use |
|---|---|---|
| Fixture load (W) | Maximum approved scene load | DC output demand |
| PSU nameplate output | Rated output under stated conditions | Starting point only |
| Available derated output | Output allowed at actual ambient/input | Capacity acceptance |
| AC inrush | Short capacitor-charging pulse at energization | Breaker and startup coordination |
2. Capacity equations
Fixture load = quantity × fixture rated watts
Preliminary PSU rating ≥ fixture load ÷ planned loading fraction
DC current = fixture load ÷ nominal DC voltage
Available output = nameplate output × applicable derating factor
The loading fraction is a documented design assumption. It does not replace the selected model's curves, installation instructions or thermal verification. IEC 61347-1:2024 provides general controlgear safety requirements and includes thermal requirements, but the project still needs the actual manufacturer's conditions [S1].
3. Worked example: 24 wall washers
Use the published LIKELIGHT L-W-10036-DMX value of 36 W at DC24 V only as a public product reference [S5]. For an illustrative layout:
- 24 fixtures × 36 W = 864 W total fixture load.
- Three branches of eight fixtures: 8 × 36 W = 288 W per branch.
- Nominal DC current per branch: 288 W ÷ 24 V = 12 A.
- With a preliminary 80% loading assumption: 288 W ÷ 0.80 = 360 W minimum nominal PSU per branch.
Now test the assumption. If a chosen 360 W unit were allowed only 70% output at the actual enclosure temperature or input voltage, its usable output would be 360 × 0.70 = 252 W, below 288 W. The 70% figure is illustrative, not a LIKELIGHT or universal rating. Obtain the exact curve for the approved PSU model.
4. Why startup trips a breaker
Switching supplies charge input capacitors at turn-on, producing a short pulse that can be far above steady operating current [S2]. Do not multiply a headline peak value and compare it directly with a breaker ampere rating. Coordination needs each model's peak, pulse duration/T50, cold or warm start condition, number of units, breaker make/model and trip curve, supply impedance and whether starts are simultaneous [S3].
5. Engineering workflow
- Freeze the maximum simultaneous lighting scene and fixture quantity per DC branch.
- Confirm fixture worst-case input power; do not substitute typical consumption.
- Select an exact PSU model and collect output, efficiency, ambient/input derating, cooling and orientation limits.
- Check DC branch current, cable ampacity, fuse/protection and voltage drop separately.
- Collect rated AC input current, power factor if relevant, inrush peak and duration.
- Coordinate MCB/fuse and upstream distribution with a qualified electrical designer.
- If needed, evaluate staged energization or a compatible inrush limiter; document failure mode.
- Commission at maximum scene and worst credible thermal condition; record voltage, current, enclosure temperature and trips.
6. Architecture options
| Option | Benefit | Risk/verification |
|---|---|---|
| One large central PSU | Fewer AC starts | Long DC runs, larger fault domain, voltage drop |
| Distributed PSUs | Shorter DC runs and modular service | More simultaneous inrush sources |
| Staged startup | Reduces coincident energization | Sequencer ratings, delay logic and restart behavior |
| Redundant/parallel supplies | Potential continuity | Requires approved current sharing/ORing; not automatic |
7. When not to approve the design
- The BOM says only “24 V power supply” without manufacturer and model.
- A fixed 20% spare rule is used without ambient/input derating evidence.
- Inrush peak/duration or breaker curve is missing.
- All supplies start together, but no branch coordination or startup test exists.
- Parallel or redundant operation is assumed without manufacturer approval.
- DC cable, fuse, voltage drop and fault isolation are absent.
8. Evidence ledger
| ID | Source | Supports | Reviewed |
|---|---|---|---|
| S1 | IEC 61347-1:2024 | General safety requirements for controlgear for electric light sources, including thermal requirements within its stated voltage scope. | 2026-08-27 |
| S2 | ICL-16 Inrush Current Limiter and Circuit Breaker Selection | Explains capacitor-charging inrush in switching power supplies and why breaker selection must consider both steady input current and inrush behavior. | 2026-08-27 |
| S3 | Application Q&A: Power Supply Selection | Shows that rated AC input, inrush current and duration must be taken from the selected supply specification and verified in the application. | 2026-08-27 |
| S4 | SP-200 Series Specification | Official example datasheet includes model-specific ambient-temperature and input-voltage derating curves; it is not a specification for another model. | 2026-08-27 |
| S5 | 36W DC24V RGB DMX Dimmable LED Wall Washer | Publishes L-W-10036-DMX as 36 W, DC24 V, RGB and DMX512; project PSU and protection details remain to be confirmed. | 2026-08-27 |
9. FAQ
Can I just add all fixture watts?
No. That finds load, but not derated capacity, DC current, AC input or startup coordination.
Is 20% spare always enough?
No. It is an assumption until checked against the selected model's curves and operating conditions.
Why does the breaker trip only at startup?
Input capacitors can cause a short inrush pulse even when steady current is acceptable [S2].
Will delay relays always solve inrush?
No. Device ratings, restart sequence and failure behavior must be engineered.
Is an SPD the same as an inrush limiter?
No. They address different transient phenomena and are not interchangeable.
Who authored this guide?
Author: LikeLight Tom, Operations. Technical reviewer: 待确认.
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