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What Makes a Copper Busbar Joint Stay Tight

September 26, 2026

A bolted busbar joint conducts through the small fraction of its faces that actually touch. Everything about the joint is an attempt to make that fraction larger and keep it that way. The machine shop controls three of the five things that matter.

What decides contact area

  1. Flatness. Two faces that are not flat touch at high spots. This is the machinist’s job.
  2. Surface finish. A smeared or rough face touches less than a cleanly sheared one. Also the machinist’s job.
  3. Plating. A soft tin surface flows into the mating face under pressure and increases real contact. Silver gives the lowest contact resistance and its tarnish still conducts. See tin vs silver plating. Specified by the designer, applied by the plater.
  4. Clamping force. Bolt, washer stack and torque. Designer and assembler.
  5. Time. Copper creeps under sustained load, the bolt loses tension, thermal cycling works the joint. Spring washers are how joints stay tight.

What the drawing has to say

Flatness on the joint faces. Called out on the contact pads, not on the whole bar. On a bent bar, the joint face is machined after bending because forming introduces stress that releases afterwards.

Finish on the joint faces. Ra on the pads, as machined elsewhere. See copper surface finish.

Plating on the joint faces. Finish, thickness at the pad, masked elsewhere, dimensions before or after plating. See plating thickness specification.

Hole tolerance. Clearance holes loose, the two locating holes controlled. The joint does not care about the other holes.

Edge condition. Broken edges so the washer seats flat and nobody cuts a hand.

What the drawing does not decide

Torque. The torque value comes from the fastener size and grade and from the standard your assembly is certified to. A busbar drawing that specifies torque is specifying something the assembler owns. What the drawing does need is the washer stack, because it sets the contact area under the bolt head and it is part of the joint design.

Where the shop matters

Flatness and finish on the pad are the two things a machinist can get wrong that make a joint run hot later. On copper both are harder than on steel: the soft face smears under a rubbing tool and marks under a hard clamp. Ask the shop how it holds the bar and what it does about built-up edge. How to choose a supplier for machined copper parts has the questions.

Cross section, bends and hole patterns are in the copper busbar design guide. The service page is copper busbar machining.

Frequently asked questions

What torque should I use on a copper busbar bolt?

The value comes from the bolt size, grade and the standard your assembly is built to, not from the busbar. Copper is soft, so the joint uses a washer stack to spread the load and often a spring element to hold clamping force as the copper creeps. Specify the stack, then the torque follows from the fastener.

Why does a busbar joint run hot?

Usually because the real contact area is small. Poor flatness, a rough or smeared face, tarnish on bare copper or a loose bolt all shrink the metal to metal area, and current crowds through what is left.

Do I need Belleville washers?

They are common on copper joints because copper creeps under sustained load and the bolt loses tension over time. A spring washer maintains clamping force. Whether it is required depends on the standard and the duty.

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