How does outdoor festoon lighting actually work?
Two hundred bulbs strung over a marquee gable on a Friday afternoon, left up through a weekend of showers, and not one of them trips the circuit or lets water in. Nobody is out there fiddling with it between courses. The reason it just works is not the bulb. It is the joint you never look at, the holder the bulb sits in.
Outdoor festoon survives the weather because of three things working together, not one clever bulb. The lampholder is moulded and fixed permanently to the cable rather than a separable screw-fit joint, closing off the exact seam water would otherwise track into. Its IP rating, typically IP44 for a sheltered run or IP65 for a run left out regardless of forecast, states precisely what weather it is built to shrug off. And the circuit feeding it runs on a 30mA RCD, split into several shorter runs rather than one giant daisy chain, so a single damp connector only drops its own section rather than the whole display. LED has also changed the sums: a run draws roughly 10W per 10 metres, a fraction of old tungsten festoon, which is why a full wedding lighting plan now sits easily inside one small circuit.
Festoon looks like the simplest kit on an event site: a string of bulbs on a cable, hooked between two marquee poles or a pergola and a tree. What is easy to miss is that it is also electrical equipment left outdoors, unattended, for as long as the hire runs, in whatever the British weather does over a weekend. The parts of it that matter are the ones built to cope with that, and they are not the parts anyone photographs.
The holder, not the bulb, is the engineering
A cheap garden festoon set typically uses a screw-fit holder: a two-part plastic sleeve the bulb itself unscrews out of, the same basic fitting as a household lamp. That is a perfectly good joint indoors. Outdoors it is exactly the kind of seam that lets water track in over hours of rain, because a screw thread is never a watertight seal, only a mechanical one. UK wiring regulations covering festoon lighting at exhibitions and outdoor shows deal with this directly: they require a lampholder that is moulded and fixed permanently to the cable, not a separable screw-fit type, so there is no loose joint at the bulb end for water to exploit. The trade-off is repairability. You cannot unscrew a fixed holder to swap one dead bulb the way you could at home; a faulty section of hire festoon gets replaced as a sealed unit instead.
What the IP number on the box actually promises
Once the joint is sealed, the IP rating states exactly what that seal is good for. It is two digits: the first, 0 to 6, rates protection against solid objects and dust; the second, 0 to 9, rates protection against water. IP44 means protected against water splashed from any direction, which suits a run sheltered under a marquee gable or a short stretch of settled weather, but is not a promise against hours of driving rain or anything left standing in a puddle. IP65 means dust-tight with no ingress at all, and protected against low-pressure water jets from any angle, the rating built for a festoon run left outdoors for a full weekend regardless of what the forecast does. Marketing copy that simply says "outdoor rated" is not a specification; the two digits are. It is worth asking a supplier for them directly, especially for anything with no cover and no fallback plan if the weather turns.
The circuit behind the bulbs
None of that sealing matters if the circuit feeding the lights is not protected properly, so outdoor festoon runs on the same safety net as any other temporary power at an event: a 30mA RCD, which cuts the supply fast enough to be safe the moment a connector gets damp and starts leaking current to earth. That is not a special festoon rule, it is the same protection a generator feed or a cable run to a marquee gets. What is specific to lighting is how suppliers lay the plan out: rather than one enormous daisy chain looping the whole site, a large job gets split into several shorter circuits, each on its own RCD. A single wet joint anywhere on one giant run can trip the entire chain and kill every light on it at once; split circuits mean a problem in the car-park run never touches the lights over the dance floor.
Why the maths got easier when LED arrived
Connectable festoon sets are rated by total wattage, not bulb count, commonly a cap somewhere in the low thousands of watts, or a limit on how many sets can be linked end to end before the far end of the chain starts to see a meaningful voltage drop. Old tungsten festoon made that limit a real constraint on a big job, forcing a careful count of exactly how many metres could go on one feed. LED has mostly retired that problem: the trade figure is roughly 10W per 10 metres of LED run, so a typical wedding-sized lighting plan, canopy over the dining tables, festoon along the entrance, uplighters round the lining, sits comfortably inside one circuit's limit and well inside a small generator or a spare 13A socket.
When a lower voltage is the safer choice
For the riskiest spots on site, close to standing water, along a wet marquee gutter line, or anywhere a guest could plausibly handle the cable, some suppliers switch that section to an extra-low-voltage system running at 12V or 24V off a transformer, rather than feeding it straight from 240V mains. It costs a transformer and a shorter practical run length, but a fault on a 12V or 24V circuit cannot deliver a dangerous shock the way a mains fault can, which is the same safety logic behind construction sites running power tools at 110V rather than mains voltage, just applied to a string of lights instead of a drill.
What this means for your event
Practically: ask any lighting supplier for the actual IP rating on what they are bringing, not just the word "outdoor", and ask whether a big plan is split across multiple RCD-protected circuits rather than one chain. The lighting hire guide has current guide prices and what else to check before booking, and because lighting and generator feeds are usually drawn from the same site power plan, the generator calculator is worth running alongside it.
Outdoor festoon lighting, questions answered
What does the IP rating on festoon lighting actually mean?
It is two digits, each promising something different. The first, 0 to 6, rates protection against solid objects and dust; the second, 0 to 9, rates protection against water. IP44 means protected against splashing water from any direction, fine for a sheltered gable or a dry forecast but not built for sustained heavy rain or anything standing in a puddle. IP65 means dust-tight and protected against low-pressure water jets from any angle, the rating built for a run left outdoors for a whole weekend regardless of what the sky does. "Outdoor rated" on its own is a marketing phrase; the two digits are the actual specification, so ask a supplier for them before booking anything with no cover plan.
Why do festoon bulbs have fixed holders instead of an ordinary screw-in socket?
A cheap garden festoon set often uses a screw-fit holder, a two-part sleeve the bulb unscrews from, which is exactly the kind of seam water tracks down at a joint left outside for days. UK wiring regulations covering festoon lighting at exhibitions and outdoor shows require a lampholder that is moulded and fixed permanently to the cable rather than separable, so there is no loose joint for water to find its way into. The trade-off is repairability: a fixed holder cannot be unscrewed to swap one dead bulb, so a faulty section of festoon gets replaced as a sealed unit rather than repaired bulb by bulb.
Does outdoor festoon lighting need an RCD, and why split it into several circuits?
Yes, the same way any temporary power at an event does: a 30mA RCD is standard protection for a temporary lighting circuit, cutting the supply fast enough to be safe if a connector gets wet and starts leaking current to earth. It is not a festoon-specific rule, it is the same protection a generator feed or a cable run to a marquee gets. Suppliers split a large lighting plan into several shorter circuits, each on its own RCD, rather than one long daisy chain, because a single damp joint anywhere on one giant run can trip the whole circuit and take out every light on it at once. Several smaller circuits mean one wet connector only drops its own section.
How much festoon can you safely chain together in one run?
Manufacturers rate a connectable set by total wattage, not bulb count, commonly a cap in the low thousands of watts or a limit on how many sets can be linked before the far end of the chain sees a meaningful voltage drop. LED has changed the practical sums here: the trade figure is roughly 10W per 10 metres of LED festoon run, against the much heavier draw of old-style tungsten bulbs, so a typical wedding-sized lighting plan sits comfortably inside one circuit's limit and inside a small generator or a spare 13A feed, where the equivalent tungsten run used to need careful load counting.
Why do some outdoor lighting rigs run on 12V or 24V instead of mains?
For the riskiest spots, anywhere festoon runs near standing water, a wet marquee gutter line, or somewhere a guest could realistically handle a cable, suppliers sometimes switch that section to an extra-low-voltage system running at 12V or 24V off a transformer, instead of 240V mains. It costs a transformer and a shorter practical run, but a fault on a 12V or 24V circuit cannot deliver a dangerous shock the way a mains fault can, which is the same safety logic behind construction sites running power tools at 110V, just applied to a string of lights instead of a drill.
Now work out your site power
Lighting, catering and the bar all draw from the same feed. Size the generator properly, then build the full kit list for your event.