Quick answer
The Australian 7-pin flat and 7-pin large round carry the same seven circuits: indicators, tail lights, stop lights, reversing lights, electric brakes and earth. They’re just different shapes. A 12-pin flat carries those same seven circuits plus five more, typically a permanent 12V feed (for a caravan battery or fridge), an ignition-switched feed, a second earth, rear fog lights and a spare. Choose the connector that matches your trailer, or use an adaptor that carries every circuit you need.
Is this a DIY job?
- Difficulty: Easy to identify; Medium to change connectors; Pro for charging circuits on modern vehicles
- Time: 5 minutes to identify; 30 to 90 minutes to rewire a connector
- Tools: trailer circuit tester, owner’s manual, screwdriver
- Skills: recognising the connector and checking each circuit
Which one do you have?
- 7-pin flat: a flat, rectangular plug. The most common connector on Australian light trailers, boat trailers and many caravans.
- 7-pin large round: a large round plug, common on older trailers, some boat trailers and heavier equipment.
- 12-pin flat: a larger flat plug, common on caravans and campers that need power from the tow vehicle as well as lights and brakes.
- 7-pin small round (European): a smaller round plug used on imported European vehicles and trailers. It’s wired to a different standard and isn’t interchangeable with the Australian 7-pin round without the correct adaptor.
Step by step: matching a vehicle and trailer
- Identify both connectors. Look at the socket on the vehicle and the plug on the trailer.
- List what the trailer needs. Lights only? Electric brakes? Reversing lights? A caravan battery or fridge charged while towing?
- If the connectors differ, use an adaptor that carries every circuit you need, especially electric brakes (pin 5). Secure it so it can’t drop onto the road.
- Don’t feed power through a 7-pin connector. Pin 2 is for reversing lights. Battery charging or fridge power needs a 12-pin connector or a separate Anderson plug with heavy cable. (see How do I wire an Anderson plug?)
- Test every function with a trailer tester or a helper before you tow.
- Consider a 12-pin upgrade if you’re towing a caravan with an onboard battery. The extra pins are only useful if the vehicle side is wired for them, with suitably sized cable and fusing.
Specs table
Pin functions (Australian connectors)
| Pin | Function | Colour | 7-pin flat | 7-pin large round | 12-pin flat |
|---|---|---|---|---|---|
| 1 | Left indicator | Yellow | Yes | Yes | Yes |
| 2 | Reversing lights | Black | Yes | Yes | Yes |
| 3 | Earth | White | Yes | Yes | Yes |
| 4 | Right indicator | Green | Yes | Yes | Yes |
| 5 | Electric brakes | Blue | Yes | Yes | Yes |
| 6 | Stop lights | Red | Yes | Yes | Yes |
| 7 | Tail, clearance and side marker lights | Brown | Yes | Yes | Yes |
| 8 | Permanent 12V (battery charging, accessories) | Orange | – | – | Yes |
| 9 | Ignition-switched 12V or auxiliary | Pink | – | – | Yes |
| 10 | Second earth | White | – | – | Yes |
| 11 | Rear fog light | Grey | – | – | Yes |
| 12 | Spare or auxiliary | Violet | – | – | Yes |
Pins 8, 9 and 12 are described slightly differently by different sources, and installers sometimes use them differently. Always test what your vehicle and trailer actually carry before relying on them.
Sockets with or without a reed switch
Trailer sockets are sold in two versions: plain, or with a built-in reed switch. The switch is a small magnetic sensor inside the socket. When the lid opens or a plug goes in, the switch changes state, so the vehicle knows a trailer is connected. Its most common job is to switch off the reversing sensors so they don’t beep at the trailer the whole time you’re reversing.
Reed switch sockets come as normally open or normally closed. Which one you need depends on how the vehicle’s circuit is wired, so check the vehicle or harness instructions before buying. The wrong type will do the opposite of what you want: the sensors stay on while towing and switch off when you’re not.
Connector rules for light trailers: under the national guide for small trailers (VSB 1, Revision 6), 7-pin and 12-pin plugs on trailers up to 3,500 kg ATM must comply with AS 4177.5-2004. It recommends using pin 5 only for service brakes, and a 12-pin connector if you need auxiliary circuits.
What you’ll need
- 7 pin flat to 7 pin round adaptor or 12 pin to 7 pin trailer adaptor
- 12 pin flat trailer socket / 12 pin flat trailer plug
- 12 pin flat socket reed switch if your vehicle has reversing sensors (check normally open or normally closed)
- trailer circuit tester
- trailer plug cover
When to call an auto electrician
- You want a 12-pin socket with working battery-charging and auxiliary circuits on a modern vehicle. Those circuits need heavy cable, fusing at the battery, and often a vehicle-specific harness or DC-DC charger.
- Your vehicle is European and has a small round socket.
- The trailer has electric brakes and you need a brake controller fitted. (see How do I install an electric brake controller?)
- The lights or brakes don’t work properly through the adaptor.
Need an auto electrician? Find local auto electricians and mobile services in your city.
Related questions
Q: Can I plug a 7-pin trailer into a 12-pin socket? A: Yes, with a 12-pin to 7-pin adaptor. Pins 1 to 7 carry the same functions, so lights and brakes work. The extra 12-pin circuits simply aren’t used.
Q: Will a 12-pin plug charge my caravan battery? A: Only if the vehicle’s pin 8 is wired with heavy enough cable from the battery and fused properly. Long, thin cable loses a lot of voltage, so many people use a DC-DC charger in the caravan. (see Why isn’t my caravan battery or fridge charging while towing?)
Q: Is the European 7-pin round the same as the Australian one? A: No. They look similar but are wired differently. Using one with the other without a proper adaptor can make the trailer brake when you switch on your lights.
Q: How do I wire a 7-pin plug? A: See our step-by-step guide with a pin chart. (see How do I wire a 7-pin flat or 7-pin round trailer plug?)
Sources
- ExplorOz, “Trailer wiring diagrams” – 7-pin flat, 7-pin large round and 12-pin flat pin functions and colours. https://www.exploroz.com/vehicle/electrics/trailerwiringdiagrams.aspx (checked October 2026)
- Wikipedia, “Trailer connectors in Australia” – 12-pin Type 3 pin allocations, pin 2 caution, ISO 1724 small round differences. https://en.wikipedia.org/wiki/Trailer_connectors_in_Australia (checked October 2026)
- Department of Infrastructure, VSB 1 Revision 6 (August 2026), section 7.2 – plugs must comply with AS 4177.5-2004 for trailers up to 3,500 kg ATM; 7-pin and 12-pin pin functions and colours; pin 5 for service brakes only. https://www.infrastructure.gov.au/sites/default/files/documents/vsb1-revision-6-trailers-with-an-aggregate-trailer-mass-of-4.5-tonnes-or-less-august-2026.pdf (checked October 2026)
- Narva, 12-pin flat trailer socket with reed switch for normally open circuits (82074BL) – reed switch deactivates reversing sensors while a trailer is connected. https://www.narva.com.au/products/82074BL (checked October 2026)
- Narva, 12-pin flat trailer socket with reed switch for normally closed circuits (82073BL) – the normally closed version. https://narva.com.au/products/narva-12-pin-flat-quickfit-trailer-socket-with-reed-switch-for-use-with-normally-closed-circuits-1-pack (checked October 2026)