Electrical Aluminium Busbar: Selection and Sizing Guide

What an aluminium busbar does and why aluminium is used

A busbar replaces bundles of cable wherever large currents must be collected from several sources or split between several loads. Because it is rigid, it also acts as a structural member: it must resist the magnetic forces of a short circuit, hold its shape between insulators and keep bolted joints tight for decades.

Aluminium became a mainstream busbar material for three practical reasons. First, its conductivity per kilogram is about twice that of copper, so a bar sized for the same resistance weighs about half as much. Second, the larger cross-section gives a bigger surface area, which helps the bar shed heat and often allows aluminium to run at the same current with a similar or lower temperature rise. Third, aluminium is far cheaper per ampere and its price is less volatile, which matters on projects where busbar is a large share of the material bill.

The trade-offs are well understood: aluminium forms a hard surface oxide that must be managed at joints, it expands about a third more than copper with temperature, and pure aluminium is soft enough to creep under bolt pressure. Each of these is solved by grade selection and joint design, covered below. Busbar manufacturers for switchgear, bus duct, transformers, rectifiers, smelter potlines and energy storage all use aluminium routinely.

Aluminium busbar grades compared

The table compares the alloys most often specified for busbar. Conductivity is the figure that sets the cross-section; strength decides span, short-circuit withstand and joint stability. Values are typical or minimum figures from the governing standards; your mill test certificate reports the measured conductivity and strength of the lot.

Within 6101, the temper decides the balance. T6 gives the highest strength with the lowest conductivity of the group (55 % IACS minimum). Over-aged tempers such as T61 and T64 raise conductivity towards 59 – 60 % IACS and make the bar easier to bend, at the cost of strength; T63 and T65 sit in between. For most switchgear bars T6 or T61 is the default, while T64 suits bars that are formed into tight offsets.

ASTM B317/B317M sets the minimum tensile, yield and conductivity values for each 6101 temper. Because published secondary sources disagree on some temper values, design to the edition of the standard named in your contract and to the certificate.

Electrical and physical properties that matter for design

Two consequences follow. For the same DC resistance an aluminium bar needs about 1.64 times the copper cross-section (100 ÷ 61) in 1350, or about 1.82 times in 6101-T6. Multiply that by the density ratio (2.70 ÷ 8.89) and the aluminium bar weighs roughly 50 – 55 % of the copper bar. The higher expansion means long runs need expansion joints or flexible links, and bolted joints need spring washers so clamping force is maintained as the bar heats and cools.

Standard busbar sizes, weight and DC resistance

The table shows calculated values for common flat sizes at 20 °C using the nominal cross-section. Real bars with radiused edges have slightly less area and therefore slightly higher resistance; AC resistance in wide, thick bars is higher again because of skin and proximity effects.

Calculated from resistivities of 0.0283 Ω·mm²/m (61 % IACS) and 0.0313 Ω·mm²/m (55 % IACS) and a density of 2.70 g/cm³.

How to size an aluminium busbar for current

Ampacity is not a property of the bar alone. It is the current at which the bar reaches its permitted temperature in a given installation, so it depends on ambient temperature, the allowed temperature rise, whether the bar is painted or bare, how bars are spaced and oriented, whether they sit in a ventilated or closed enclosure, and AC effects. Published tables are therefore starting points, not guarantees.

1. Fix the limitsTake the maximum ambient and the permitted temperature rise from the product standard. In assemblies to IEC 61439-1, the rise is limited by the materials and insulation the bar touches, and terminals for external cables are limited to 70 K rise.
2. Pick a starting sizeUse a recognised rating table, such as DIN 43670 for aluminium busbars or the Aluminum Association and Copper Development Association conversion charts, for your arrangement and rise.
3. Convert from copper if neededWhen replacing a copper bar, start with about 1.6× the copper area in 1350 or 1.8× in 6101-T6, then check the geometry: increasing width usually cools better than increasing thickness.
4. Correct for AC and groupingFor AC, several thin laminations spaced apart beat one thick bar, because skin effect leaves the centre of thick bars underused. Apply derating for bars stacked close together or in closed ducts.
5. Check short-circuit withstandVerify the bar and supports withstand the peak electromagnetic force and the thermal energy of the fault; this is where 6101 strength becomes important.
6. Confirm by test or calculationType testing or verified calculation to the assembly standard is the final proof of rating.

Making reliable joints and choosing a surface finish

Almost every aluminium busbar failure starts at a joint. Aluminium oxide forms within seconds of exposure and is a good insulator, so contact surfaces must be prepared, protected and kept under steady pressure.

Where aluminium bars must connect to copper, avoid bare metal-to-metal contact in damp conditions, because the two metals form a galvanic couple and the aluminium corrodes. Use tin-plated aluminium, a bimetallic copper-clad aluminium transition piece, or a bimetal connector. See our pages on tin-plated busbar and copper-clad aluminium busbar.

Cutting, drilling, bending and welding busbar

Busbar is normally delivered cut to length, and much of it is punched, drilled and bent to a drawing before plating or insulation. A few material-specific rules prevent rejected parts.

• Bending: 1350 and over-aged 6101 tempers bend most easily; 6101-T6, 6061-T6 and 6082-T6 need larger radii and can crack on the outside of tight edgewise bends. Bend across the extrusion direction where possible and keep holes away from the bend zone.
• Punching and drilling: deburr all holes; burrs reduce contact area and damage insulation. Slotted holes accommodate expansion on long runs.
• Welding: TIG and MIG welding are used for permanent joints and fabricated assemblies. Heat lowers the strength of heat-treated tempers locally, and filler alloy affects joint conductivity, so welded joints are designed with this in mind.
• Friction-stir and explosion bonding: used for aluminium-to-copper transitions and for joining heavy conductors with low added resistance.
• Edge finish: radiused edges reduce local electric field strength and help coatings cover evenly.
• Flexible links: laminated foil or braided connectors take up thermal movement and vibration between rigid bars and equipment terminals.

Applications of aluminium busbar

Aluminium vs copper busbar

Copper keeps the advantage where space is the binding constraint, such as very compact switchgear or high-density power electronics. Aluminium wins where weight, cost and supply stability matter more than a few millimetres of panel depth. For a deeper comparison including copper-clad aluminium, read copper vs aluminium vs CCA. Metline supplies the aluminium and copper-clad aluminium options.

Busbar standards to name on your order

How to order electrical busbar from Metline

1. SpecifyAlloy and temper (e.g. 6101-T61), section and edge radius, length, quantity, minimum conductivity and the governing standard.
2. Add processingDrilling, punching, bending, plating or insulation, with a drawing for each part number.
3. QuoteWe confirm availability, tolerances, certification and terms (EXW, FOB, CIF or DAP).
4. InspectDimensions, surface and conductivity are checked; the EN 10204 3.1 certificate reports chemistry, mechanical properties and conductivity.
5. ShipBars are separated, protected from abrasion and packed in export crates or on pallets for sea or air freight.

Related busbar grades, forms and guides

Aluminium (aluminum) busbar: frequently asked questions