I have a new pond with a pump but the cable on the pump is far too short. I have some similar cable which will do the job to extend the length. My only concern is the junction outside. I need to make it dry and safe. What recommendations do people have? Can you buy a junction box that is suitable for outside use? If this helps I live in the UK.
I cannot speak to the UK. In the US the answer is yes, and the principles should be pretty much the same. I'll give you the parts I would use; in the US I would be able to find them at any home improvement store.
You will want to buy a small single-gang metal junction box that is exterior-rated. You will need a green screw for the inside of this box; the box itself will need to be grounded and above grade. (It can't be buried, in other words. Most people stick them on poles if it HAS to be in the middle of the yard, or just attach them to the house.)
The second thing you will want is a fitting that screws into the sockets on the exterior-rated junction box. This fitting will have three parts. The first is the part that screws into it from the inside. The second is a rubber part with a hole in the shape of Romex, which is the standard jacketed cable that we use in the US. The third part is a nut that screws over that and compresses the rubber part between the two metal parts in order to form a watertight seal.
After that, you'd do the normal thing -- wire nut the hot and neutral connectors together, and wire the grounds together while also wrapping one leg of the ground around the green grounding screw.
Electricity is a funny thing and different applications apply for various situations. THis is UK/EU/US/Asia/Africa.. its all the same.
If you are running a low voltage cable 0-48Volts DC/AC
then you can use a pvc outdoor joiner (the screw caps have a rubber seal on them to prevent moisture from entering)
If you are running high voltage (110AC/240AC)
single phase then using a resin joiner is recommended!
You might be tempted to do something like this (this example is of low voltage coax,tv,internet, etc) and is not recommended for outside situations at all! over time water will get in.
That was undergorund application,
you can also use these above ground,
Notice the rubber seals- that is very important!