Calculator / What actually goes wrong
What actually causes fires
What actually goes wrong
Holiday lighting fires have a short list of causes, and none of them is the thing most guides spend their time on.
The four real failures
Overloaded chains, where too many sets end to end overheat the first set's cord. This is the one no breaker catches, and it is why the chain limit gets its own bar on our calculator.
Damaged cord, usually from a previous season — cracked insulation from UV and cold, or a cable pinched under a window or crushed by a ladder foot. Inspect every set before it goes up, running the cord through your hand.
Wet connections, where two cords join in a puddle or an outlet fills with water. Height and a proper enclosure solve this; a supermarket bag does not.
Coiled cable reels, which cannot dissipate heat and are rated for a fraction of their capacity while wound. Unwind fully, every time.
The checks that are worth the minute
Test each set on the ground before it goes up. A set that flickers or has a warm plug on the ground will not improve at gutter height.
Use GFCI or RCD protection for anything outdoors, without exception. It is the single most effective protection available and it costs very little to add with a plug-in adaptor.
Put the display on a timer. Beyond the running cost, lights that are off for eighteen hours a day spend eighteen hours not being a risk — and a timer means you are never relying on remembering.
Keep indoor sets indoors. Indoor-rated cord stiffens and cracks below freezing, and an indoor set in a wet gutter is exactly the failure the outdoor rating exists to prevent.
What is worth less worry
Bulb-for-bulb replacement anxiety, mixing brands on separate runs, and the exact wattage of a modern LED set are all low on the list. The failures above account for most of what goes wrong.
One genuine exception: never repair a set by twisting wires together and taping the join. If a set has failed at the cord, it is finished.
Everything on this site concerns what you plug into an existing outlet. Adding a circuit, changing a breaker or wiring an outdoor outlet is work for a qualified electrician, and in most places it is notifiable work regardless of your competence.
The same goes for doubt. If you cannot tell whether a cord is sound, what a circuit already carries or why a breaker keeps tripping, that uncertainty is itself the answer: ask a qualified electrician rather than switching on to find out.
Common questions
What causes Christmas light fires?
Overloaded chains that overheat a set's cord, damaged insulation from previous seasons, wet connections, and coiled extension reels. Overloading the breaker is rarely the cause, because that trips.
Should I leave Christmas lights on overnight?
Better not to. A timer reduces both the running cost and the hours of exposure, and removes the need to remember.
Can I use indoor lights outside?
No. Indoor cord is not formulated for UV or freezing temperatures, and it cracks. Use sets rated for outdoor use, and check the rating rather than assuming.
Do I need a GFCI or RCD for outdoor lights?
Yes, and it is required in most jurisdictions for outdoor outlets. Where the installation is older, a plug-in adaptor provides the same protection cheaply.
Work out your own load
The load calculator checks both limits against your display.