Copper vs aluminium busbars: conductivity, weight and cost

Compare like with like

A copper bar and an aluminium bar of the same size do not carry the same current. Aluminium has about 61 per cent of the conductivity of copper by volume, so a straight substitution of the same width and thickness gives a higher resistance, more heat and a larger voltage drop. The comparison only means something when both bars are sized for the same resistance.

That single change of basis moves every other number. Once the aluminium bar is enlarged to match resistance, its mass, surface area, stiffness, cost and space claim all change with it. A supplier who quotes a price per kilogram is not answering the question a designer needs to ask, which is the cost per amp over the life of the run.

The properties that drive the choice

The table below uses typical values for electrical grade copper and 1350 grade aluminium. They are illustrative, because alloy, temper and temperature change the numbers, but the ratios are close enough to show the trade.

Weight, space and stiffness

The area ratio of 1.64 is the central number. A 50 x 10 mm copper bar has a section of 500 mm². An aluminium bar of equal resistance needs about 820 mm², which could be 80 x 10 mm or 100 x 8 mm. The aluminium bar is larger in one or both dimensions, and that changes the enclosure layout, the support spacing and the clearance to other parts.

The mass ratio is about 0.49, so the larger aluminium bar weighs roughly half as much as the copper bar it replaces. On a long run, or in a vehicle or a roof-mounted enclosure, that saving can decide the project. It also changes the support load and the effort needed to install the bar.

Stiffness does not follow the mass. Aluminium has a lower modulus, so a bar of equal resistance may deflect more between supports even though it is lighter. If the bar has to stay flat, resist a fault force or hold a joint face square, check the section modulus and the support spacing rather than assuming the larger bar is stiffer.

Handling is part of the same trade. A lighter bar is easier to lift and position, which can shorten an installation and reduce the risk of a dropped part. The larger envelope can offset that gain if the route is tight or the bar has to pass through a small opening.

The termination is where the comparison is won or lost

Aluminium forms a hard oxide layer within minutes of being cleaned. That layer is an insulator, so an aluminium joint has to break it during assembly or be plated at the interface. Copper also oxidises, but its oxide is less troublesome at the pressures used in a bolted joint.

A direct aluminium to copper joint also creates a galvanic pair. In the presence of moisture, the aluminium side can corrode preferentially. The usual answers are a bimetallic transition plate, a plated interface or a joint design that keeps the dissimilar metals out of the wet path. Each answer adds a part, a process and an inspection point.

Aluminium also creeps under a bolted load more than copper. A joint that is tight at installation can lose clamp force over thermal cycles. Belleville washers, a defined torque and a re-torque step are common responses, but they add labour. The copper bar may cost more at the counter and less at the joint.

Cost per amp, not cost per kilogram

Metal prices move, so use a dated illustration rather than a fixed rule. If copper is £9 per kilogram and aluminium is £2.50 per kilogram, the raw material in an equal-resistance aluminium bar costs about 14 per cent of the copper. That looks decisive, and it is why aluminium is attractive for large sections.

The fabricated cost is a different number. The aluminium bar needs more extrusion or rolling capacity, a larger enclosure, different tooling for bends, a joint treatment and often a plating step. Its scrap value is lower and its recycling route may be less valuable. Freight is lower because the bar is lighter, which helps on imported sections.

The honest comparison is the installed cost per amp over the programme: metal, fabrication, plating, joints, supports, enclosure space, installation labour and the cost of the space the bar takes. A 14 per cent material ratio can become a 60 or 70 per cent total ratio once those lines are added. That is still a saving in many projects, but it is a different claim.

Currency and metal index also move the answer. A copper price at a high point flatters aluminium, while a narrower spread can remove the case for a larger section with a more complex joint. Fix the price date, the exchange rate and the quantity basis before the comparison is circulated. A cost model without those notes is a snapshot that cannot be defended six months later.

Heat, expansion and fault behaviour

At equal resistance, both bars generate the same heat per metre at the same current. They do not dissipate it in the same way. The aluminium bar has a larger surface, which helps, but its lower emissivity and its larger section change the balance. The only safe approach is to run the heat balance for the actual surface and mounting.

Thermal expansion is higher for aluminium, at about 23 ppm/K against 17 ppm/K for copper. Over a long run, that means more movement at the ends and more stress on a rigid joint. Slotted holes, flexible links or a defined expansion joint may be needed. The supports also have to allow the bar to move without wearing through the insulation.

Under a short circuit, the lower melting point of aluminium is a disadvantage, and the larger section changes the adiabatic heating. The fault check has to use the actual section, the actual duration and the actual support spacing. A bar that passes the continuous current check can still fail the fault check, whichever metal is chosen.

A practical selection sequence

Start with the electrical duty, then compare sections at equal resistance. Do not start with a metal price. The sequence below keeps the comparison honest and makes the trade visible to the person who signs the order.

1. 01

   Fix the duty

   Continuous current, ambient, allowable temperature, fault level and duration.

2. 02

   Size both metals

   Choose a copper section and the aluminium section that gives the same resistance.

3. 03

   Check the heat balance

   Use the actual surface, mounting and enclosure for each option.

4. 04

   Design the joint

   Decide the transition, plating, clamp load and re-torque step for each metal.

5. 05

   Check space and support

   Compare the envelope, stiffness, expansion movement and support spacing.

6. 06

   Price the installed amp

   Add metal, fabrication, finish, joints, supports, labour and freight.