Commercial festoon lighting systems for contractors, venues and distributors. E27/B22, 24V/230V and OEM/ODM options for UK, EU and Australia projects.
Sizing a 24V Festoon System: Driver Wattage and Bulb Count
A 24V festoon layout is a small power-distribution project. The driver converts the mains supply to 24 V, the string carries the current, and the bulbs use it. Sizing starts with simple arithmetic, then moves to the details that decide whether the layout is comfortable: driver headroom, startup current, cable losses and zoning. This guide works through the sums for 100 W, 150 W and 200 W drivers with 1 W bulbs, and explains what the numbers do not tell you. For the product families this applies to, see our 24V festoon lights range.
The basic sum: bulbs, watts and headroom
For a constant-voltage 24V system, each bulb draws its rated wattage, so the load on a driver is the number of bulbs multiplied by the wattage per bulb. Add a margin, then choose the driver size.
A common planning rule is to load a driver to no more than about 80% of its rated output. The margin keeps the driver away from its limit and leaves room for small tolerances between bulbs and for the conditions on site. It is a rule of thumb, not a standard requirement; the driver datasheet always takes priority.
With 1 W bulbs, the arithmetic is:
| Driver rated output | Usable at 80% load | 1 W bulbs supported |
|---|---|---|
| 100 W | 80 W | 80 bulbs |
| 150 W | 120 W | 120 bulbs |
| 200 W | 160 W | 160 bulbs |
The formula is: rated watts x 0.8 / watts per bulb. For 100 W: 100 x 0.8 = 80 W, divided by 1 W = 80 bulbs. The same working gives 120 bulbs for 150 W and 160 bulbs for 200 W. This is an example calculation at 1 W per bulb and 80% load, not a product specification; if the bulbs you choose draw a different wattage, divide by that figure instead. A 2 W bulb, for instance, halves the counts to 40, 60 and 80.
What the table assumes
Every number in the table depends on assumptions, so check them against your own layout:
- all bulbs are the same model and draw their rated wattage at 24 V;
- the driver is loaded continuously, not in short bursts;
- the driver output is rated at the ambient temperature of the installation, not at a laboratory temperature;
- no other load shares the driver: no control gear, no relays, no long extension accessories;
- one driver serves the whole layout, with no second supply point.
If any of these is untrue, redo the sum. The table is a starting point for the project file, not a substitute for the driver datasheet and the installer's calculation.
Startup current: the figure that is not in the simple sum
LED drivers and lamp electronics can draw a brief inrush current when switched on, higher than the steady running current. The running load can sit comfortably below 80% and the switch-on behaviour can still trip a protection device or stress a driver that is already close to its limit.
There is no single safe percentage that fits every driver, because the behaviour depends on the driver design, the number of bulbs and the switching arrangement. The practical response is to keep more margin on large installations, read the datasheet for inrush and any maximum lamp count, and consider switching the layout in groups rather than all at once. If a site has repeated nuisance trips at switch-on, the driver and switching design need review before the bulb count is increased.
Cable losses and voltage drop
At 24 V, the current for a given wattage is much higher than at 230 V. Higher current makes cable resistance and every connection more important: a long run, a thin conductor or a chain of joints loses voltage along the way, and bulbs at the far end can dim or behave differently from those near the driver.
This is a design question, not a table question. Keep 24 V runs sensible for the load, place the driver close to the first bulb, use the cable specified for the system, and keep joints to a minimum. If a layout has long runs or several branches, ask for the calculation to be checked rather than assuming the driver's wattage alone settles it.
Split the layout into zones
When a design needs more bulbs than one driver supports, the usual answer is to split the layout into zones, each with its own driver. Zoning keeps individual runs shorter, reduces the current in each run, makes fault-finding easier and allows separate switching.
Choose zone boundaries by the physical layout as well as the electrical count. Separate garden areas, a terrace and a walkway naturally become separate zones. Then decide where each driver will live: accessible for maintenance, protected from water and heat, and sited according to the driver's own IP rating and installation instructions. A driver buried in a sealed box with no airflow is a service problem waiting to happen.
24V or 230V? The trade-offs
Neither voltage is better in every project. The choice depends on run length, the number of supply points, who will maintain the installation and the documents the client needs.
| What you are comparing | 24V system | 230V system |
|---|---|---|
| Power supply | Needs a 24 V driver, usually one per zone | No separate driver for the string |
| Voltage on the run | Low voltage; whether it is SELV depends on the driver and its documentation | Mains voltage |
| Practical run length | Shorter runs are easier to manage; current is higher for the same wattage | Longer runs are practical; current is lower |
| Components | Driver, string, bulbs, connectors | String, bulbs, plug and connectors |
| Typical projects | Gardens, terraces, hospitality areas and temporary layouts where a driver can be housed | Building fa莽ades, long runs, commercial and municipal schemes |
| Servicing | The driver is an additional item to plan and replace | Fewer components to schedule |
| Documentation | Driver document plus string test report | String test report covers the assembly |
A low voltage on the run can simplify some site discussions, but it does not remove the need for correct IP protection, cable sizing, protection devices and a compliant installation. A 230V system avoids a driver, but it places the full mains voltage on the string and makes the string's own report and installation quality more important.
Documents and IP for a 24V system
A 24V project needs two document tracks: the driver and the string. A driver certificate covers the driver models named in it and does not certify the string, the bulbs or the finished layout. The string report covers the string, and it does not cover the driver.
IP ratings work the same way. A rating on a driver applies to that driver. In our own documentation, the 24V 150 W and 200 W driver models listed in SAA certificate SAA-201654-EA are marked IP67, and the certificate is held by Bright Star Lighting Co., Ltd. That certificate does not extend to the light strings, the bulbs or the complete system, and it names specific models, so check that the exact driver wattage you plan to use is listed rather than assuming the family is covered.
RCM is not a certificate. It is a marking under the Australian EESS arrangements, and it depends on the responsible supplier's registration for the exact product. For Europe, ask for the driver's own CE-LVD and EMC documentation, and the string's report; both belong in the project file. For more on how component ratings differ from system ratings, see our guide to IP65 vs IP67 festoon lights.
A sizing checklist
- List the bulbs per zone and confirm the wattage of the exact bulb model.
- Multiply bulbs by watts, add the 80% planning headroom, then round down to a driver size.
- Check the driver datasheet for maximum load at the expected ambient, inrush current and any maximum lamp count.
- Decide the zones and driver locations before ordering cable.
- Confirm connector and cable compatibility with the string.
- Check the driver and string documents against the destination market.
- Record the calculation, driver model and documents in the project file.
Frequently asked questions
Can a 200 W driver run 200 one-watt bulbs? Not on the 80% planning rule: 200 W x 0.8 = 160 W, so the working figure is 160 one-watt bulbs. Running at the full rated output leaves no margin for startup current, tolerances or ambient conditions.
Is a larger driver always better? No. Each driver model has its own load curve, inrush behaviour and ambient limits. Size to the calculated load plus sensible headroom, then check the datasheet; an oversized driver is not automatically the easier installation.
Does a 24V system mean the whole system is rated IP67? No. IP ratings belong to individual products. In our documentation, specific 24V driver models are marked IP67, while every string model is covered by its own report. The driver's rating does not transfer to the string or to the assembled layout.
Sizing a 24V system is mostly arithmetic plus a set of checks that are easy to document and easy to audit later. Get the bulb count, the headroom and the zoning right, and the driver choice follows; skip those steps and a bigger driver will not fix the layout.
Date: 26 October 2026
Author: Jacky Gong, Sales Manager, WENDADECO
