When it comes to designing wire harnesses and multiconductor cables, one of the most overlooked yet critical features is the drain wire. Whether used for grounding, shielding, or providing structural strength, the drain wire plays a key role in ensuring the performance, reliability, and longevity of the cable assembly.
But when specifying a drain wire, designers often face the question: Should it be solid or stranded? The answer depends on the application, flexibility needs, and overall design goals.
In this blog post, we’ll explore the role of drain wires in multiconductor cables, why they matter, and the pros and cons of solid versus stranded options.
What Is a Multiconductor Cable?
A multiconductor cable is exactly what it sounds like: a cable that contains two or more insulated conductors bundled together within a single jacket. These cables are used in countless applications across industries, from industrial automation and robotics to medical equipment, aerospace systems, and consumer electronics.
By combining multiple conductors under one outer jacket, multiconductor cables help reduce installation complexity, save space, and improve cable management in harness assemblies. Many of these cables also include shielding provisions to protect signal integrity in electrically noisy environments; this is where drain wires become essential.
Multiconductor cable with stranded wires throughout.
Understanding the Shielding Layer
The shielding layer of a cable is designed to protect the internal conductors from electromagnetic interference (EMI) and radio frequency interference (RFI). In sensitive systems, such as medical diagnostic devices, avionics, or high-speed data applications, shielding is vital to maintaining signal clarity and preventing crosstalk.
Common Shielding Options:
- Tinned Copper Braided Shield: A woven mesh of tinned copper wires, this shield provides excellent coverage (typically 70–95%), mechanical durability, all while still flexible. It’s ideal for applications where shielding effectiveness is most critical.
- Metalized Foil Shield: A lightweight aluminum or copper foil laminated to a plastic layer (often polyester). Foil shields offer nearly 100% coverage, making them great for high-frequency EMI protection but less durable than braided shields.
- Combination Shields:
Some cables use both foil and braided shields, delivering the high coverage of foil with the mechanical robustness and low-resistance grounding path of a braid. Many cable designs employ both a tinned copper braided shield and a mylar foil shield.
Shielding on this USB cable is exposed and visible.
How To Properly Ground the Shield of a Cable
A shield is only as effective as its connection to ground. Without proper grounding, a shield simply acts like an antenna, potentially amplifying interference instead of suppressing it.
So how is grounding typically achieved?
- In foil-shielded cables, a drain wire is included to provide a low-resistance path to ground. The drain wire runs in direct contact with the foil shield and is typically connected to ground via a soldered joint or crimped terminal.
- In braid-shielded cables, the shield itself can be connected directly to ground. In some cases, the braid is soldered or crimped to a separate ground wire, which is then terminated to ground.
Solid vs. Stranded Drain Wires: Which Is Better?
When specifying a drain wire, designers must decide between solid or stranded construction. Each has unique advantages depending on the application.
Benefits of Solid Drain Wires:
- Stronger and More Rigid: Solid drain wires can handle greater mechanical stress, making them suitable for applications where the cable may double as a structural element.
- Memory Retention: They retain their shape when bent or formed, which can be useful in custom harness assemblies. Romex and other building wires include a large gauge ground wire, which technically acts as a drain.
- Reduced FOD Risk: Fewer loose strands during trimming reduces the potential for Foreign Object Debris (FOD) in critical applications such as medical devices.
- Higher Current Carrying Capacity: A solid conductor can often carry slightly more current than an equivalent gauge stranded wire.
- Strength Member Capability: Solid drain wires can serve as a secondary strength member to absorb tension in the cable.
Romex wire uses a solid bare copper wire for grounding.
Benefits of Stranded Drain Wires:
- Flexibility: Stranded drains are much more flexible than solid wires, making them better suited for applications with frequent bending or movement.
- Improved Bend Radius: They can achieve tighter bend radii without risking conductor damage or jacket deformation.
- Common Material: Stranded drains use raw wire that is common in other parts of the wire harness. For raw cable manufacturers, using the same standard wire reduces complexity and the number of types of wire needed.
- Ease of Assembly: Stranded drains can be fanned out during termination for cleaner, more efficient connections. They’re easier to cut and strip during termination steps.
Stranded drain wire within a multiconductor cable.
Which is More Common?
In practice, stranded drain wires are more common, primarily because they align with the stranded conductors in most multiconductor cables and provide superior flexibility for dynamic applications.
Additional Uses for Drain Wires
While their primary role is shielding and grounding, drain wires can also be used as strength members to improve a cable’s mechanical durability. In applications where the cable may experience frequent pulling or tension, the drain wire can absorb some of that load, reducing stress on the conductors and extending the cable’s lifespan.
For more on enhancing cable durability, check out our blog post on improving strength and performance in multiconductor cables.
Stranded drain wire is shown between the foil shield and braided shield.
Summary
Choosing between solid and stranded drain wires isn’t about which one is universally better; it’s about selecting the right option for your application. Solid drain wires offer greater rigidity, memory, and current capacity, while stranded drains provide flexibility, improved bend performance, and easier integration with other stranded conductors.
By understanding the purpose of the drain wire, the shielding options available, and how grounding impacts system performance, engineers can design wire harnesses that deliver reliable EMI protection and mechanical resilience. Whether your project calls for a highly flexible multiconductor cable in a robotics assembly or a rigid harness for x-ray equipment, specifying the correct drain wire is essential to meeting performance and reliability goals.
Key Takeaways
- Drain wires provide grounding and simplify shield termination in foil-shielded and combination-shielded cables.
- Solid drain wires are stronger, more rigid, and reduce FOD risks, making them ideal for static or structurally demanding applications.
- Stranded drain wires are more flexible and better suited for dynamic applications requiring tight bend radii.
- The shielding layer must be properly grounded to prevent EMI interference; drain wires are critical in this process.
- Drain wires can also serve as strength members, improving cable durability under tensile loads.
