Busbar vs cable: current, space, cost and installation

Compare on the same basis

The useful question is not whether a busbar is better than cable. It is whether a busbar or a cable is the better way to move a stated current, over a stated distance, inside a stated enclosure, with a stated fault level and a stated maintenance regime. Change any of those and the answer can change with it.

That sounds obvious, yet most comparisons are made from two different tables. The cable figure comes from a manufacturer catalogue for free air at 30 °C with a single circuit. The busbar figure comes from a laboratory test at 25 °C with the bar in open air. Neither number describes the sealed cabinet at 50 °C that the design actually has to survive.

Normalise first. Fix the ambient, the allowable conductor temperature, the installation method and the fault duty. Then compare the conductor size, the voltage drop, the space used and the installed cost for each option.

Current density and space

A busbar puts copper where it is needed. A cable spends part of its overall diameter on insulation, bedding, armour and the gaps between strands. For the same conductor area, the bar is usually the more compact package, and that advantage grows when several circuits run in parallel.

The thermal side is more important than the geometric side. A cable is limited by the temperature its insulation can survive, and it is derated for grouping, enclosure, ambient and installation method. A busbar is limited by the bar temperature, the temperature of nearby insulation and the touch temperature that the standard allows. Because a bar can often run hotter than a cable, it can carry more current for the same metal area, provided the heat has somewhere to go.

For illustration, a copper bar and a copper cable of the same conductor area might be compared at 1.5 A/mm² and 1.0 A/mm² respectively in a ventilated panel. Those figures are illustrative only. The real values depend on the installation and must come from the applicable standard or a test.

Installation and routing

Cable is flexible. It can follow a route around obstacles, pass through a gland, enter a device from any direction and absorb a small misalignment between two fixed points. That flexibility is valuable in a one-off panel or a retrofit where the layout is not fully known.

A busbar is not flexible. It is bent, drilled and supported to a fixed geometry. That is a disadvantage during installation and an advantage afterwards: the path is repeatable, the spacing is fixed, the joints are accessible and the assembly can be built and tested on the bench before it goes into the enclosure.

Labour is the hidden variable. A cable run needs cutting, stripping, crimping, lugging, glanding, supporting and testing. A busbar run needs cutting, bending, drilling, deburring, plating, supporting, bolting and torque marking. Which is cheaper depends on how many joints each option needs and how much of the work can be done before assembly.

• Cable suits long routes, changes of direction and equipment that may move.
• Busbar suits short, high-current links and repeated builds of the same layout.
• Cable terminations need a qualified crimp and a pull test.
• Busbar joints need flat contact faces, the correct torque and a contact resistance check.
• Both need support against fault forces, but the busbar support is a design feature rather than an accessory.

The cost structure

Cable cost is dominated by copper, insulation and the terminations. A long run multiplies all three. A short run is dominated by the labour at each end, which is why a two-metre cable link can cost more than its copper suggests.

Busbar cost is dominated by copper, forming, drilling, plating and supports. The bar itself may be dearer per kilogram than cable, but it uses less insulation and fewer terminations. At high current and short length, the busbar often wins on total installed cost. At low current and long length, cable usually wins because the bar needs more supports and more joints.

Do not compare piece price. Compare the installed cost of the complete link: conductor, insulation, supports, terminations, labour, testing and the space the link occupies. Add the cost of a rework if the routing changes, because a cable can be rerouted and a formed bar cannot.

Fault current and thermal behaviour

A short circuit loads both options in two ways. The current heats the conductor, and the magnetic field between conductors produces a force that tries to push them apart. Cable makers publish a short-circuit withstand for a stated duration. Busbar designers have to check the bar temperature rise and the force on the supports.

The busbar has an advantage in thermal mass, because a solid bar has more metal in the same length than a stranded conductor of similar rating. It has a disadvantage in force, because a rigid bar transfers the whole load to its supports. The support spacing, the support material and the strength of the fixing become part of the fault design. A cable can flex, but it can also whip into a live part or damage its own insulation.

Neither option is automatically safer. The fault check has to be done for the installation, and the result has to be reflected in the support drawing or the cable cleat schedule.

Maintenance and modification

A bolted busbar joint can be inspected, re-torqued and, if necessary, dismantled. That is a real advantage in equipment that is serviced on site or extended over time. The flip side is that a joint is only as good as its last assembly: a loose bolt, a contaminated face or a missing washer can turn a sound bar into a hot spot.

A cable termination is harder to inspect without disturbing it. A crimp is either right or wrong, and the evidence is the tool record and the pull test. Once installed, the connection is usually left alone until a fault appears.

For a product with a long service life and a planned extension path, the busbar is often easier to live with. For a temporary installation or a route that may change, cable is more forgiving.

A short decision framework

Use cable when the run is long, the route is uncertain, the current is modest or the equipment may move. Use a busbar when the run is short, the current is high, the layout will be repeated and the joints need to be accessible.

If the answer is not clear, compare three numbers for each option: the conductor size after derating, the total installed cost and the space envelope. If one option wins on all three, the decision is easy. If they split, the tie-breaker is usually maintenance access or the fault support detail.

1. 01

   Fix the duty

   State the current profile, ambient, allowable temperature, fault level and duration.

2. 02

   Size both options

   Use the same installation assumptions and the same temperature limit for a fair comparison.

3. 03

   Lay out the route

   Check bend radius for cable and support spacing for the bar. Count the joints each option needs.

4. 04

   Price the installed link

   Include conductor, insulation, supports, terminations, labour and test.

5. 05

   Check the fault case

   Confirm the cable cleats or the busbar supports can take the force and the thermal duty.