Looking at purchasing a few of these:
We have two existing that have stopped working; there is already a hole through the external wall for the cable to come through, but apparently these are known for shipping with a really short cable.
In looking for outdoor-suitable flex to use to replace the short cable with before fitting, I realised I don't really know how to calculate the amps that will pass through from the spec of the lights, to ensure the cable can handle the load. These are being wired straight to mains. The wall is a brick and breeze block, and likely has cavity wall insulation.
So, as an example, would a flex able to carry 20amp be enough?
Best Answer
Current
3 x 30W = 90W
90W / 240V = 0.375 A
So 20A cable is overkill.
A typical lighting circuit uses 5A cable - typically 1 mm² (older installations) or 1.5 mm² (current standards). You could use thinner cable rated for 1A but you might then need a fused connection unit with a 1A fuse to protect the 1A cable from setting fire to your house in the event of a fault in the light fitting.
Outdoor use
What is important for outside use is insulation durability. It has to stand up to UV from sunshine and extremes of hot and cold plus mechanical strength. You need cable rated for outdoor use.
Totally random example
Another random example
Shedding water
Cable needs to be routed so that it exits the wall and any enclosure in a downward direction so that rainwater does not run into the wall nor into any enclosure. Enclosures need to be waterproof (e.g. IP66) and fitted with cable-glands to prevent water ingress.
Walls
Running cable through masonry walls should be protected in conduit. At the inside there should be something to indicate the presence of any electrical cables channelled into the plaster - e.g. a back-box with blanking plate.
Ideally you'd use T&E cable through conduit through the wall and flexible cable to a lamp. Which obviously requires an external waterproof junction box.