Tin plating
The standard contact finish. Gives a stable, low-resistance surface, resists humidity and is the usual answer when aluminium must mate with copper.
Busbar selection guide
Flexible Busbar: Materials, Types and Selection
A flexible busbar is a high-current link made from stacked foils or braided wires that bends to absorb vibration, thermal expansion and misalignment between equipment. The conductor can be copper, aluminium or copper-clad aluminium (CCA), and the choice changes weight, size, cost and how joints must be made. This guide compares the three materials and the main constructions so you can choose the right flexible busbar for your system.
- Conductors
- Al, CCA (Cu compared)
- Types
- Laminated, braided, rigid-flex
- Plating
- Tin, nickel, silver
- Insulation
- Heat shrink, PVC, epoxy
What a flexible busbar does
Rigid busbars move current efficiently but cannot accept movement. Wherever two pieces of equipment move relative to each other, whether through vibration, heating and cooling, battery swelling or simply loose assembly tolerances, a rigid bar transfers that movement into the terminals. Over time the result is loosened bolts, cracked welds and hot joints.
A flexible busbar breaks that load path. It keeps solid, flat contact ends for bolting or welding and a flexible section in between made from many thin layers or fine wires. Because each layer is thin, the section bends with little force and little stress, yet the total conductive area can be as large as a rigid bar.
Flexible busbars appear in electric vehicles, battery energy storage, inverters, UPS systems, transformers, switchgear, rail traction, welding equipment and electrolysis plants. In each case the questions are the same: how much current, how much movement, how much space, and what the conductor material should be.
Copper, aluminium and copper-clad aluminium compared
The conductor material sets the cross-section needed, the weight, and the joint method. The figures below are approximate values for annealed conductor material. CCA values depend on the copper fraction and are calculated here by rule of mixtures for 10 – 15 % copper by volume.
| Property | Copper (reference) | Aluminium 1350 / 1060 | Copper-clad aluminium |
|---|---|---|---|
| Density | about 8.9 g/cm³ | 2.70 g/cm³ | about 3.3 – 3.6 g/cm³ |
| Conductivity | about 100 % IACS | about 61 – 62 % IACS | about 65 – 67 % IACS (calculated, DC) |
| Resistivity, 20 °C | 0.01724 Ω·mm²/m | about 0.028 Ω·mm²/m | about 0.026 Ω·mm²/m (calculated) |
| Area for equal resistance | 1 × | about 1.6 × | about 1.5 × |
| Mass for equal resistance | 1 × | about 0.5 × | about 0.55 – 0.6 × |
| Contact surface | Copper | Aluminium oxide film; needs plating or prep | Copper skin on the surface |
| Galvanic match | Copper terminals | Aluminium terminals and cells | Copper terminals, with an aluminium core |
| Relative material cost | Highest | Lowest | Between the two |
In practice: copper gives the smallest part and the most forgiving joints but is heavy and expensive. Aluminium gives the lightest, cheapest solution and suits systems with aluminium terminals, at the price of a larger section and more careful contact design. CCA puts a copper surface on an aluminium core, so contacts behave like copper while weight stays close to aluminium. At high frequencies, where current crowds into the outer skin, CCA performs closer to copper than its DC figures suggest.
Metline supplies aluminium and copper-clad aluminium conductors. Copper is shown only as the reference material that most flexible busbar designs are converted from.
Laminated, braided and rigid-flex constructions
| Construction | How it is built | Strengths | Limitations |
|---|---|---|---|
| Laminated foil | Stack of thin foils (about 0.05 – 0.30 mm) with ends welded solid | Compact; high current in low height; low resistance; easy to insulate | Flexes mainly in one plane; foils can fatigue under severe continuous vibration |
| Braided | Fine wires braided into a flat or round strap, terminals crimped or welded | Flexes in all directions; best vibration tolerance; good for earthing straps | Bulkier for the same area; open braid can trap moisture |
| Rigid-flex | Short laminated section welded into a solid bar | Defines routing; flexibility only where needed; good for long runs | More complex part; requires accurate geometry |
| Stranded flexible bar | Fine stranded conductor in a rectangular insulated sleeve | Can be bent by hand on site to any route | Lower current density; ends need fitted terminals |
Choose laminated construction for compact, high-current connections that move in a predictable direction, such as battery module links and inverter DC connections. Choose braided construction where movement is multi-directional or continuous, such as earthing across hinges or links on vibrating machinery. Rigid-flex suits transformer and switchgear connections where a defined path must still absorb expansion.
Rigid busbar or flexible busbar?
| Consideration | Rigid busbar | Flexible busbar |
|---|---|---|
| Movement and vibration | Transfers movement to terminals | Absorbs movement in the flexible zone |
| Assembly tolerance | Requires accurate hole positions | Takes up misalignment and height differences |
| Current over long distances | Most economical for long straight runs | Used in short links and expansion joints |
| Space | Needs supports at intervals | Fits into tight, irregular spaces |
| Cost per metre | Lower | Higher, due to lamination, welding and insulation |
| Typical role | Main distribution bars | Links between equipment, battery modules, expansion joints |
The two are complementary rather than competing. A well-designed power system runs rigid aluminium busbar for the main distribution and inserts flexible links at each piece of equipment and at expansion points. Over-using flexible links where a rigid bar would do adds cost and joints without adding reliability.
Surface plating and insulation options
Nickel plating
Harder and more heat resistant than tin. Used for higher operating temperatures and where contact faces see wear.
Silver plating
The lowest contact resistance and good high-temperature behaviour, used for demanding power electronics contacts.
Heat-shrink sleeving
Polyolefin or PE/EVA sleeves shrunk over the flexible zone. Flame-retardant grades are available; wall thickness is set by voltage and abrasion risk.
PVC dip coating
A dipped, flexible PVC layer that follows the shape of formed laminates and seals the foil stack.
Epoxy coating
Powder or fluidised-bed epoxy for rigid sections, giving a hard, thin insulating layer.
Working voltages of 1,000 V AC or 1,500 V DC are common design points for EV, solar and storage equipment, but the insulation system must be confirmed by dielectric testing of the actual design and by the clearance and creepage rules of the equipment standard, such as IEC 61439 for switchgear assemblies.
How to choose a flexible busbar
- Define the electrical dutyContinuous current, overloads, short-circuit current and duration, working voltage and allowed temperature rise.
- Define the mechanical dutyDirection and size of movement, vibration levels, number of thermal cycles and available space.
- Choose the conductorAluminium for weight and cost, especially with aluminium terminals; CCA where copper-type contacts and low weight are both needed. Size the section using the area factor from the material table.
- Choose the constructionLaminated for compact links moving in one plane, braided for multi-directional movement, rigid-flex for defined routes.
- Specify contacts and finishHole pattern, pad size, plating and torque, with special care for aluminium-to-copper joints.
- Specify insulation and testsMaterial, thickness, flame rating, dielectric test, and resistance or temperature-rise test on first articles.
Applications of flexible busbars
- Electric vehicles: module-to-module and pack connections where vibration and cell swelling must be absorbed
- Battery energy storage: rack and module links, often insulated aluminium laminates
- Solar and wind: inverter, combiner and converter connections exposed to thermal cycling
- Switchgear and distribution: expansion links in long bus runs and connections to breakers and transformers
- Industrial equipment: welding machines, furnaces and drives with heavy currents and vibration
- Data centres and UPS: compact high-current links in battery cabinets and power distribution units
- Rail and marine: traction and onboard power systems subject to constant vibration
How copper-clad aluminium conductors work
Copper-clad aluminium is a metallurgically bonded composite, not a plated product. A copper layer is bonded to an aluminium core by roll cladding, cladding during drawing or similar processes, so the two metals cannot separate under bending or heating. For wire, ASTM B566 classifies CCA by nominal copper content of 10 % or 15 % by volume, in annealed and hard-drawn conditions; strip and bar follow the same principle with the copper ratio agreed by drawing.
Because the copper sits on the outside, CCA terminals can be bolted, crimped or soldered using copper practice, and the copper skin carries a large share of high-frequency current. The aluminium core keeps density low. The trade-offs are that cut edges expose aluminium, which should be protected or plated, and that very heavy machining or deep punching can break through a thin copper layer, so hole and edge details are agreed at the design stage.
Typical points to agree for CCA busbar
- Copper fraction
- By volume or by thickness per side, e.g. 10 % or 15 %
- Form
- Strip, flat bar, or wire for braids
- Temper
- Annealed for flexible parts, harder tempers for rigid bars
- Edges and holes
- Protection or plating of exposed aluminium at cut faces
- Testing
- Bond integrity (bend or peel), conductivity, dimensions
Common mistakes when specifying flexible busbars
- Converting a copper design to aluminium at the same cross-section, which overheats the link
- Bolting bare aluminium to bare copper without plating or a transition piece
- Making the flexible zone too short for the real movement, which overloads the welded ends
- Choosing the thinnest foil for a connection that only moves during assembly, adding cost without benefit
- Leaving out spring washers on aluminium joints, so bolt pressure drops as the metal relaxes
- Specifying insulation without dielectric testing of the finished, bent part
References and standards
- IEC 61439-1 – Low-voltage switchgear and controlgear assemblies: general rules
- ASTM B566 – Standard Specification for Copper-Clad Aluminum Wire
- ASTM B236 – Aluminum Bars for Electrical Purposes (Bus Bars)
- ASTM B317/B317M – Aluminum-Alloy Extruded Bar, Rod, Tube and Profiles for Electrical Purposes
- The Aluminum Association – Aluminum Electrical Conductor Handbook
Flexible busbar: frequently asked questions
Is an aluminium flexible busbar as good as copper?
Electrically it can do the same job if the cross-section is about 1.6 times larger for equal resistance. It then weighs about half as much and costs less. Copper remains the more compact option where space is the hard constraint.
What is copper-clad aluminum used for in flexible busbars?
CCA gives a copper contact surface on an aluminium core. It suits designs that need copper-like bolted joints and lower weight than copper, and it behaves well at higher frequencies where current flows mainly in the copper skin.
Laminated or braided: which should I choose?
Laminated busbars are more compact and carry high current in a low height; they flex mainly in one plane. Braided busbars flex in every direction and tolerate continuous vibration better but take more space.
Why are flexible busbars tin plated?
Tin gives a stable, low-resistance contact surface that resists oxidation and humidity, and it protects aluminium-to-copper joints from galvanic corrosion.
Can flexible busbars be insulated?
Yes, with heat-shrink sleeving, PVC dip coating or epoxy, leaving only the contact pads bare. The thickness and material follow working voltage, clearances and flame-retardancy needs.
Does Metline supply copper flexible busbars?
No. Metline supplies aluminium and copper-clad aluminium conductors only: foil, strip, wire, busbar and CCA stock, cut, punched, formed and plated to drawing, shipped worldwide with mill certificates.
Planning a flexible busbar in aluminium or CCA?
Send the current, movement, space and terminal details. We will recommend a conductor and construction and quote material to drawing.
