Which wire size should I use for a 12 V circuit?
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.
The arithmetic is what surprises people. A 0.5 V drop on a 230 V circuit is negligible; the same 0.5 V on a 12 V circuit is over 4% and is enough to dim lights, slow a pump and make a winch behave as though its battery is flat. Because power is voltage times current, a 12 V circuit also carries roughly twenty times the current of a 230 V circuit doing the same work, and cable resistance loss rises with the square of that current. So 12 V wiring is thick, and its length matters enormously — a run to the back of a vehicle needs both directions counted. Fusing is the other half: every circuit is fused as close to the battery as practical, because it is the cable that is being protected, not the device.
The options, ranked
| Option | Good at | Bad at |
|---|---|---|
| Stranded automotive cable (GXL/TXL or thin-wall) | Flexible, and its insulation resists heat, oil and abrasion in a way household cable does not. | Sized in different systems on either side of the Atlantic, so check what you are buying. |
| Crimped terminals | A properly crimped joint is gas-tight and survives vibration; it is the automotive and aviation standard. | Requires a decent ratchet crimper — pliers produce joints that fail later. |
| Soldered joints | Excellent electrical contact. | Solder wicks up the strands and creates a stiff point that fatigues and breaks under vibration. |
| Solid household cable | Cheap and available. | Work-hardens and fractures under vibration, and its insulation is not rated for engine bay heat or fuel. |
Technique that changes the outcome
- Calculate total circuit length, counting both the positive and the return path
- Aim for under about 3% voltage drop, and less on anything motorised
- Fuse at the battery end, sized to the cable rather than to the appliance
- Crimp with a ratchet crimper and use adhesive-lined heatshrink where damp can reach
- Protect every point where the loom passes through metal with a grommet
What goes wrong
- Sizing from current alone
- Solid household cable in a vehicle
- Unfused connections directly to a battery, which are a straightforward fire risk
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.
Other cable and wire questions
| 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. |
| 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. |
| Outdoor and buried runs | Steel wire armoured cable, or a direct-burial type, at the required depth with RCD or GFCI protection — never ordinary indoor cable. |
| 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 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.
- What should I avoid?
- Sizing from current alone Solid household cable in a vehicle Unfused connections directly to a battery, which are a straightforward fire risk
Related pages
- 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 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 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.
- 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