Copper Busbar Design for Machining: Sizing, Bolted Joints, Bends and What to Put on the Drawing

01Material and Temper

Busbar is C110 (UNS C11000, electrolytic tough pitch) copper almost without exception, bought to ASTM B187, the specification written for bus bar, rod and shapes. It carries 100% IACS conductivity minimum in the annealed condition, and the CDA's busbar handbook notes that hard-drawn bar should be specified at 97% IACS minimum, because cold work costs a little conductivity in exchange for a bar that stays straight and resists denting.[1][2]

Specify the temper. Half-hard (H02) or hard (H04) bar machines more cleanly than annealed, holds flatness at bolted joints and does not creep under bolt load the way soft copper does. The handbook's guidance on creep is the reason: tough pitch copper creeps under quite low stress once it is warm, and a bolted joint is a bar held under stress while warm. Copper also begins to soften above about 150 °C, which is one of the reasons busbar temperature limits exist.[2]

Oxygen-free copper (C101, C102) adds nothing to a bolted busbar; it matters only when the bar will be brazed or welded in a hydrogen-bearing atmosphere. Our C110 vs C101 guide has the numbers. Aluminum busbar (6101-T61 and similar) is common where weight or cost dominate; it is a different joint design problem, with oxide film and creep concerns of its own, and is not covered here.

02Sizing: The CDA Ampacity Table

Current rating is the electrical engineer's decision, but the shop is often asked to confirm a size or quote alternatives, so the reference is worth having on hand. The CDA publishes ampacities for rectangular C110 bar; the figures below are a selection from that table. Conditions: indoors, 40 °C ambient, bar running horizontally on edge, emissivity 0.4, 60 Hz, free of external magnetic influence. Rise is bar temperature above ambient.[1]

Two things the table shows that matter to the design. First, a bar gets more ampacity per pound from width than from thickness: 1/4 × 4 carries more than 1/2 × 2 at every rise, on the same weight of copper, because the wide bar has more surface to shed heat from. Second, which rise you are allowed decides everything. The handbook cites ANSI C37.20 as permitting a 65 °C rise above 40 °C ambient only where the bolted terminations are silver-plated or an acceptable alternative; without that, 30 °C. The same bar is rated at 1,250 A or 1,950 A depending on that one decision.[1][2]

The full table, all 68 sizes from 1/16 × 1/2 to 3/4 × 12 with DC resistance and skin-effect ratios, is on our busbar ampacity chart, with a size picker and a bar finder. Blank weight for any size is in our metal weight calculator.

03Bolted Joints: Overlap, Holes and Contact Pressure

A bolted busbar joint conducts through a large number of tiny metal-to-metal contact spots, which the handbook estimates occupy about 1% of the overlap area. Everything in joint design is about creating enough of those spots and keeping them: pressure, flat clean faces, and holes that do not cut off the current path.[2]

Hole size follows the bolt: the handbook's arrangements use clearance holes from 7 mm for M6 up to 20 mm for M16. Slots instead of holes are common where two bars must meet with tolerance to spare; a slot costs a little more to cut than a hole and reduces cross-section along its length, so use them where the assembly needs them and round holes elsewhere.[2]

04Joint Faces: Flatness, Finish and Burrs

Because the current passes through contact spots, the two faces have to actually touch. A bowed bar, a burr standing at a hole edge, or a washer sitting on a raised edge all hold the faces apart and shrink the contact area. Three callouts handle it:

Flatness at the joint. Put a flatness tolerance on the overlap zone rather than on the whole bar; a long bar can bow slightly between supports without harm, but the joint faces need to lie flat under the bolts. Rolled bar as delivered is usually adequate for narrow joints; wide joints on thick bar are often face-milled to be sure.

Finish. A machined or as-rolled face is what the joint needs: clean metal, not polished. Polishing does not help; it reduces the peaks that form the contact spots. What does matter is that the face is free of oxide and grease at assembly, which is an assembly-procedure note, not a machining one.

Burrs and edges. Copper burrs are soft and large. Specify "deburr all edges" and a specific edge break on the joint faces so a raised hole edge cannot lift the mating bar. On bars at higher voltages, a radius on the long edges reduces field concentration at the corners and is a note the electrical engineer should make deliberately; a 1/16 in radius is a single pass with a corner-rounding cutter.

05Bends and Where the Holes Go

Flat is cheapest, and a design that routes with flat bars and machined angle blocks is often cheaper in total than one with formed bends. Where bends are needed, the handbook gives minimum inside radii for high-conductivity copper in half-hard or hard temper:[2]

Bending is forming, not machining, so on a bent bar the sequence has to be decided. Holes drilled before the bend are cheaper to machine (the part is still flat and clamps easily) but the bend moves them slightly and can distort a hole near the bend line. Holes machined after the bend are exactly where the drawing says, at the cost of fixturing a bent part. The usual answer is to machine everything that is at least a couple of thicknesses away from the bend first, and put any hole that must be precise after bending on the drawing as a post-bend operation. Say which on the drawing, or the two vendors will each assume the other did it.

06Plating, Briefly

The handbook's position surprises people: plating copper-to-copper joint faces is not recommended unless corrosion in the environment requires it, because the soft plating prevents the copper cold welds that form the best contact and can flow under load. Plating earns its place where the bar must be protected against a corrosive atmosphere (it must then be continuous, including at joints, and is typically 2 to 5 µm thick), where the joint mates with a different metal, or where a standard's higher temperature rise is conditional on plated terminations. Tin is the everyday finish, with the caution that pure tin can grow whiskers; nickel is hard and durable but needs higher joint pressure; silver is the best conductor and is used where sulfur compounds are absent.[2]

Plating is done by an outside processor after machining, so the drawing needs the specification, thickness and any masking. Masking a joint face is a real cost; masking a whole bar except two faces is a bigger one. Our tin vs silver vs nickel guide covers the choice in detail, and the finishes and coatings reference covers how to write a plating note.

07What Machining Copper Bar Involves

Copper rates about 20 for machinability on the scale where free-cutting brass is 100. It does not cut so much as smear: the chip welds to the tool, the exit side of every hole grows a burr, and a dull drill pushes a crater through the far face instead of cutting a clean hole. None of this makes copper hard to machine well; it makes it hard to machine well by accident. Sharp, polished, high-rake tooling, generous coolant, and a deburring pass that is planned rather than hoped for are the difference.[3]

What keeps the cost down on a machined busbar:

For tolerances on the hole pattern, position from a common datum end rather than chaining hole to hole down a long bar; the stack-up calculator shows why in a minute. For the general rules on hole depth, edge breaks and tolerances, see the DFM guide for CNC milling.

08Drawing Checklist

The rest of the quote package is covered in How to Write an RFQ.