Indoor cable outdoors, even under an overhang Running cable through a hosepipe or unrated conduit as a shortcut Any buried supply without knowing where existing services run

Which cable should I use outdoors or underground?

Steel wire armoured cable, or a direct-burial type, at the required depth with RCD or GFCI protection — never ordinary indoor cable.

Outdoor cable faces three hazards indoor cable does not: ultraviolet light, water, and a spade. Ordinary PVC-sheathed indoor cable goes hard and cracks in sunlight, water tracks along the inside of a sheath by capillary action and appears at a socket metres away, and every buried service is eventually dug into by somebody. The controls are correspondingly physical — armoured or purpose-rated cable, a burial depth appropriate to the location, warning tape above the run, and sand bedding so stones do not press into the sheath. Electrically, outdoor circuits require residual current or ground fault protection, and outdoor accessories need an appropriate ingress protection rating with glands entering from below.

The options, ranked

Ranked best-first for the stated job. The third column is why the others exist.
Option	Good at	Bad at
Steel wire armoured (SWA)	Mechanical protection built in, suitable for direct burial, and the armour can serve as the protective conductor where the design allows.	Stiff, needs proper glands, and terminating it correctly is a skill.
Direct-burial cable (UF in North America)	Solid moulded jacket rated for burial without conduit.	No mechanical protection against a spade; depth and marker tape do that job.
Cable in conduit	Allows rewiring later, and protects against damage.	Conduit fills with water unless it is properly drained and sealed; the cable must be rated for wet locations.
Ordinary indoor twin and earth	Nothing outdoors.	Its sheath is not UV-stable or waterproof, and it degrades within a couple of seasons.

Technique that changes the outcome

Use armoured or direct-burial cable, bedded in sand, with warning tape above it
Bury deeper under driveways and anywhere digging is likely
Take a photograph of the trench route before backfilling
Fit RCD or GFCI protection and IP-rated outdoor accessories with bottom entry

What goes wrong

Indoor cable outdoors, even under an overhang
Running cable through a hosepipe or unrated conduit as a shortcut
Any buried supply without knowing where existing services run

Compiled practical guidance rather than a published standard. Product formulations vary by brand and market, so read the label — and where the work is structural, electrical or gas, the regulations that apply where you are govern rather than anything here.

For	Use
Extension leads	One with a conductor rated for the full load, and always fully unwound — a coiled cable reel can be derated to a fraction of its rating.
A socket circuit	Typically 2.5 mm² for a UK ring final on a 32 A device, or 12 AWG on a US 20 A branch circuit — subject to the code and the installation method.
12 V DC and automotive	Size it for voltage drop, not current — at 12 V a small drop is a large percentage, so the cable is far thicker than the amps suggest.
A lighting circuit	Typically 1.0 or 1.5 mm² twin and earth in the UK, or 14 AWG on a 15 A circuit in the US — but the code and the installation method decide.
An electric shower	Commonly 6 or 10 mm² depending on the shower rating, the run length and the installation method — and it is notifiable work in most jurisdictions.
Speakers	16 AWG for short runs and 14 AWG beyond about 15 metres or into low-impedance speakers — and make sure it is copper, not copper-clad aluminium.

Common questions

Which cable should I use outdoors or underground?: Steel wire armoured cable, or a direct-burial type, at the required depth with RCD or GFCI protection — never ordinary indoor cable.
What should I avoid?: Indoor cable outdoors, even under an overhang Running cable through a hosepipe or unrated conduit as a shortcut Any buried supply without knowing where existing services run

Which extension lead should I use for a heavy load? One with a conductor rated for the full load, and always fully unwound — a coiled cable reel can be derated to a fraction of its rating.
Which cable size is used for socket circuits? Typically 2.5 mm² for a UK ring final on a 32 A device, or 12 AWG on a US 20 A branch circuit — subject to the code and the installation method.
Which wire size should I use for a 12 V circuit? Size it for voltage drop, not current — at 12 V a small drop is a large percentage, so the cable is far thicker than the amps suggest.
Which cable size is used for a lighting circuit? Typically 1.0 or 1.5 mm² twin and earth in the UK, or 14 AWG on a 15 A circuit in the US — but the code and the installation method decide.
Which cable size does an electric shower need? Commonly 6 or 10 mm² depending on the shower rating, the run length and the installation method — and it is notifiable work in most jurisdictions.
Which speaker cable should I use? 16 AWG for short runs and 14 AWG beyond about 15 metres or into low-impedance speakers — and make sure it is copper, not copper-clad aluminium.

Sources

Typical range compiled from manufacturer specifications — At One Place

How these figures are compiled and checked