Everything you need to know about N and S Coils

When it comes to commercial vehicle electrical systems, few components are as essential as N and S coils. Used across trucks, trailers and heavy goods vehicles, these coils have become the long-established solution for connecting truck and trailer electrics safely and reliably.

The N coil (ISO 1185) has been in use since 1975, while the S coil (ISO 3731) followed just five years later. Despite their age, both standards are still widely specified today and remain a trusted choice for HGV electrical connections throughout the transport and logistics industry. Together, they form the backbone of many truck and trailer lighting systems.

One of the first things people notice when comparing N and S coils is the visual difference between them. The 24N coil is typically fitted with a dark permanent colour ideally black with pin 1 always configured as a female contact. The 24S coil plug is usually a light and permanent colour, preferably green or white, with pin 1 always being a male contact. These clear differences in colour and pin layout make it easy to identify the correct trailer electrical coil during installation or maintenance.

Beyond appearance, performance is driven by what’s inside the cable. All Autac truck and trailer coils are manufactured using bare (virgin) copper conductors, ensuring excellent electrical conductivity and long-term reliability. High-quality copper is essential for maintaining consistent power delivery across commercial vehicle wiring systems, particularly in demanding working conditions.

To suit different applications, our coiled electrical cables are available with a range of sheath materials, including TPU, TPC-ET, and different grades of PUR. Each material offers different benefits depending on how and where the vehicle operates. Soft PUR cable, for example, remains highly flexible in cold and freezing environments, making it ideal for vehicles operating in winter conditions. A higher grade of PUR, on the other hand, provides a more rigid structure, helping the coil retract more effectively, sag less and retain its shape over time.

Cable colour also plays a role in durability. In general, black electrical cable sheaths provide improved UV resistance, making them a popular choice for vehicles that spend long periods outdoors or operate in hotter climates.

All Autac coiled trailer cables are reversed, meaning they maintain their coiled memory and retractability throughout their service life. Standard working lengths range from 3 metres to 4.5 metres, with bespoke coil lengths available for specific vehicle or trailer setups. Certain cable class designs are ADR approved, making it suitable for vehicles carrying hazardous goods, and all products are manufactured in line with ISO 4141 standards. We also offer multiple temperature-rated cables, ranging from -40°C up to +125°C, to suit a wide range of operating environments.

The plug and connector are just as important as the cable itself. For most applications, we recommend fully injection-moulded plugs, which provide excellent durability and waterproof protection in harsh working conditions. Plastic and metal trailer electrical connectors are also available to suit different requirements. Inside the plug, our brass terminals are offered in both standard and circlip designs. Standard terminals are well suited to trucks that don’t change trailers frequently, while circlip terminals are ideal for vehicles with high trailer changeover rates, helping to reduce wear on both the plug and socket.

Every Autac N and S coil is fully tested before leaving the factory. Each coil undergoes electrical testing at up to 1,000 volts, ensuring correct wiring, safe operation and reliable performance. This rigorous testing process is a vital part of delivering high-quality commercial vehicle electrical components you can trust.

Although N and S coils for trucks and trailers have been around for decades, they continue to be one of the most reliable solutions for HGV trailer electrics when manufactured to high standards. With proven ISO compliance, high-quality materials and thorough testing, they remain an essential part of modern commercial vehicle electrics.

If you have any questions about our N & S coils, please give our team a call on 01625 619 277.