Understanding Cable Sizing and Selection

When designing or setting up an electrical system in a vehicle, it's essential to select the right cable size and type for each circuit. Ensuring this not only enhances the system's efficiency but also its safety. This article offers a guide on how to specify electrical cables effectively and ensure their suitability for the desired purpose.

Why Vehicle Cables are Flexible
Most people have observed that electrical cables in vehicles are more flexible than those found in household walls. This flexibility is crucial. Continuous vibration and shocks in a vehicle can lead to 'work hardening' of the cable, making it brittle over time. To counter this, vehicle cables consist of multiple small strands of copper wire rather than one solid wire. Such cables are aptly named 'stranded' cables. Their construction prevents work hardening, making them ideal for vehicles.

Note: When handling stranded cables, ensure you don’t cut or misplace any strand. This could reduce its overall efficiency.

How Cables are Specified
Electrical cables come with specific properties, such as:
  • Conductor's Cross-Sectional Area: Usually described in mm², it shows the total area of the copper conductor. It's essential to understand that "1mm" or "2mm" doesn't represent the cable's diameter.
  • Conductor Number and Size: Defined as the number of conductors and their respective diameters, e.g., 28/0.30 refers to 28 strands of 0.30mm each.
  • Nominal Current Rating: This is the cable's safe maximum continuous current capacity, represented in Amperes (A).
  • Overall Diameter: Indicates the cable's total diameter, including its insulation.
  • Resistance: Showcased in Ohms per meter (Ω/m), it plays a role in determining voltage drop.
Some cables also come with additional features such as temperature tolerance and resistance to various chemicals.

Choosing the Right Cable
When selecting a cable, consider the following:
  • Current Carrying Capacity: The cable should efficiently handle the power requirements of all connected components. For instance, a 50W light drawing 4.17A needs a cable rated at least 4.17A. However, it's always safer to opt for a cable with a bit more capacity than the exact requirement.
  • Voltage Drop: Every element in an electrical circuit, including the cable, contributes to some energy loss. In low voltage systems, even a few meters of cable can cause significant voltage drop, affecting device performance. So, ensure your chosen cable's length doesn’t induce more than a 3-4% voltage drop.
  • Materials: Cables can be of various materials, each with its pros and cons. For instance, thin-wall cables are lighter and withstand higher temperatures than standard PVC ones but might be less flexible. Tinned copper cables resist oxidation better than plain copper ones, ensuring longevity in moist conditions.
To Conclude
Ensure your chosen cable:
  • Can handle the circuit's current, with a safety margin.
  • Won't induce more than a 3% voltage drop.
  • Is made of materials suitable for its intended use.
AWG vs. Metric Cable Size
In Europe, stranded cables typically use metric sizes, while North America uses the AWG standard. For instance, a 2.0mm² 28/0.30 cable in Europe corresponds to a 16 AWG 7/24 cable in North America. The table below provides a comparison between the two for commonly used automotive and marine applications.

Choosing the right cable size and type is paramount in ensuring an efficient and safe vehicle electrical system. This guide aids in making informed decisions, ensuring longevity and reliability.