Copper Busbar: Ampacity, Grades and When Copper Is Still the Right Call

A copper busbar is a solid copper conductor, normally a flat rectangular bar, used to move current between a transformer, a main breaker, a distribution block or a battery pack where cable would be too bulky, run too hot, or be impossible to support properly. Specify copper when joint quality, available space or fault level matter more than weight and invoice value; specify aluminium when the run is long, the pack is weight-critical, and the joints can be engineered for it.

What a copper busbar actually does in a panel

A busbar is not simply a thick wire with square corners. It is a structural conductor: it carries current, it locates the devices bolted to it, and it holds the geometry of the board together. That triple role is why the bar is specified on the drawing rather than picked from a list of current ratings.

Three consequences follow, and they explain most of the arguments that happen between a panel builder and a supplier. First, the bar has to fit a space that is already crowded, so width and thickness are constrained independently of current. Second, every hole is a stress raiser and a future loosening point. Third, the bar is usually the last item to be modified on site, which means an error in the hole pattern is expensive.

In low-voltage assemblies copper is the default because it is forgiving: it conducts well, it tolerates repeated tightening, and a joint made with clean surfaces and correct torque stays stable for years without special preparation. Those properties matter more than the conductivity figure that appears in every comparison table.

The four specification fields buyers get wrong

Most copper busbar orders that go wrong do not fail on current rating. They fail on one of four details that were never written down.

• Temper and hardness. Soft annealed bar bends easily and is the right choice for formed links; hard bar holds its shape and resists deformation under short-circuit forces. A drawing that says only "copper 40 × 5" leaves the mill free to supply either.
• Surface finish. Bare copper is fine in a dry, clean switchroom. Tinned bar is the safer default where humidity, sulphur or ammonia is present, because the tin layer keeps the joint surface oxide-free and keeps contact resistance predictable.
• Edge condition and burr. Sheared bar carries a burr that lifts a lug off the surface and creates a point contact. Deburred or radiused edges are a line item, not an assumption.
• Hole pattern and hardware. Hole diameter, pitch, slot versus round, and the bolt grade all belong on the drawing. Slotting gives the installer adjustment; round holes give a defined contact area. Both are legitimate; only one is right for a given board.

If a quotation arrives without these four items confirmed in writing, the price is not comparable with a quotation that does confirm them.

Sizing: ampacity is a system property, not a catalogue number

A busbar does not have an ampacity in the way a cable does. The number depends on how the bar is mounted, how many bars run side by side, what the enclosure temperature is, whether the bar is painted or insulated, and what the installation altitude is. A figure quoted without those conditions is a starting point, not a rating.

The practical first pass is current density. For naturally cooled copper bar inside a ventilated enclosure, designers commonly work between 1.5 and 2.5 A/mm², taking the higher figure only for short runs with good air movement and the lower figure for enclosed, grouped or high-ambient conditions.

Those figures come from the stated current density only. They are a way to choose a starting cross-section, not a substitute for the manufacturer's tested table or for a short-circuit temperature-rise check. Two derating factors then decide the real answer: grouping (bars mounted close together cannot shed heat as freely as a single isolated bar) and skin effect at higher currents, which pushes current to the outside of the bar and makes a wide, thin section less effective than its area suggests.

Finally, check the bar thermally against the fault current it must survive for the clearing time, and mechanically against the force a short circuit puts on it. A bar that carries normal load comfortably can still bend between supports under fault conditions.

Copper versus aluminium, without the folklore

The comparison that matters is not "which metal conducts better" — copper does, by a wide margin — but "what does the assembly cost and weigh once the joints are designed properly".

Read the table as a design instruction rather than a verdict. Aluminium is not a cheaper copper; it is a different conductor that needs a different joint design. A pack that swaps copper for aluminium without changing the joint will pass its first test and fail in service, usually at the interface, usually after a year of thermal cycling.

Joints, plating and the failure that shows up in year five

Almost every busbar failure reported in the field is a joint failure, not a conductor failure. The mechanism is consistent: the contact surfaces oxidise, contact resistance rises, the joint runs hotter, the hotter joint oxidises faster. Nothing visible happens for months, then the joint discolours and the resistance reading jumps.

Four habits prevent it. Clean and abrade the contact faces immediately before assembly, never the day before. Apply a joint compound that excludes air, and apply it to both faces. Use hardened washers — a plain washer relaxes as the joint thermally cycles, and the clamp load that held the contact area together quietly disappears. Torque to the specified value with a calibrated wrench and record it, then re-torque once after the first thermal cycle on any joint carrying high current.

Tinning is the other half of the answer. On tinned bar the oxide that forms is a tin oxide, which is far less resistive than copper oxide, and the joint stays stable without field maintenance. In coastal, industrial or agricultural installations this is usually the difference between a busbar that is inspected once a year and one that is repaired once a year.

What to put on the drawing before you ask for a price

• Conductor material, temper and surface finish, stated separately
• Bar cross-section in millimetres, with a tolerance
• Overall length and the bend geometry, with radii rather than angles alone
• Hole or slot pattern as a dimensioned drawing, including pitch and edge distance
• Insulation, if any: sleeving, heat-shrink, epoxy coating, and whether it covers the joint faces
• Marking and phase identification
• Quantity per board and the total order quantity, because quantity drives process choice
• Inspection requirements: dimensional report, material certificate, finish check

A supplier who asks for the fault current and the enclosure type as well as the drawing is doing the right thing. A supplier who quotes from a cross-section alone is quoting a metal price, not a busbar.

Rigid copper busbars in the YOMIN catalogue

YOMIN lists rigid copper busbars in the rigid busbar category, supplied to drawing with a minimum order quantity of 100 pieces. The catalogue entries describe a rigid bar in copper or aluminium for fixed installations, battery packs and switchgear, available insulated or bare.

• ym-0458 — customised rigid copper electrical busbars, rigid, copper or aluminium, for fixed installations and switchgear
• ym-0443 — copper bus, flat, high current, insulated, rigid construction for high-current distribution
• ym-0585 — insulated rigid copper busbar, insulated variant of the same rigid family

Because these are made to drawing, the specification fields in the previous section are the ones that determine the price. Send the drawing, the material and finish, the quantity and the application, and the quotation can be compared line by line with any other supplier's.

Frequently asked questions

What size copper busbar do I need for 400 A?

At a design current density of 2.0 A/mm² that is roughly 200 mm² of copper, which a 40 × 5 mm bar provides. Treat that as a starting point only: the final size depends on enclosure temperature, how many bars run together, and the short-circuit duty, so confirm it against the manufacturer's tested data for your mounting arrangement.

Is tinned copper busbar worth the extra cost?

In a dry, clean switchroom it changes little. In humid, coastal, industrial or agricultural environments it is usually cheaper over the life of the installation, because the tin layer keeps the joint surface free of copper oxide and removes the need for periodic re-torquing and cleaning.

Can copper and aluminium busbars be bolted directly together?

They can be bolted, but not left bare. The copper-aluminium interface is where galvanic corrosion starts, and aluminium also creeps under sustained clamp load. Use a bimetallic transition, plated surfaces and a joint compound, and treat the joint as a designed component rather than a connection detail.