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Why Are Copper Busbars Tinned or Silver-Plated?

By DHYN Engineering Team  ·  August 12, 2026

Quick answer: Copper busbars are plated because bare copper oxidizes at the joint, and copper oxide conducts poorly, so contact resistance climbs and the joint runs hotter every year. Tin plating is the standard finish on busway joints: cheap, soft, and reliable up to about 105 degrees C at the contact surface. Silver plating costs more and is worth it above roughly 4000A, in hot or corrosive atmospheres, and on critical connections like generator and transformer terminals. The plating is not cosmetic. It is what keeps joint resistance low for thirty years.

Buyers sometimes ask us why the copper bars inside a bus duct look silver-colored, and whether they can save money by skipping the plating. The short version: you can skip it, and you will pay for it later at the joints. Here is what is actually going on at the contact surface, and how to choose between tin and silver without guessing.

What happens at an unplated copper joint

Two flat copper bars bolted together touch only at microscopic high spots. The real contact area is often less than 1 percent of the overlap, and all the current squeezes through those tiny spots. Bare copper reacts with oxygen and forms copper oxide right at the spots, and copper oxide is a semiconductor at best. Resistance rises, the spots heat up, heat accelerates oxidation, and the cycle feeds itself. We covered the visible symptoms in our piece on busway joint overheating. The root cause is usually this film.

The physics is not subtle. A joint that picks up just 60 micro-ohms of extra resistance, carrying 1000A, dissipates 60 watts inside a volume the size of a matchbox. That is a soldering iron tip sitting inside your bus duct, every hour of every day the load runs.

Tin plating: the industry default

Almost every busway joint you will ever buy is tin plated, typically 3 to 10 micrometers of electroplated tin under ASTM B545 or an equivalent spec. Tin works for three reasons. It resists oxidation far better than bare copper at normal operating temperatures. It is soft, so under bolt pressure it flows into the surface roughness and enlarges the real contact area. And it is cheap enough that plating a whole production run adds very little to the price of a duct.

The limits matter though. Above roughly 105 degrees C at the contact, tin oxidizes and diffuses into the copper fast enough that the protection degrades over years. Tin is also soft to a fault: serious vibration can cause fretting wear at the interface. For a normal indoor sandwich bus duct running within its IEC 61439-6 temperature rise limits, neither limit is a problem. For a duct feeding a furnace transformer or sitting on a vibrating ship deck, they start to be.

Silver plating: when it earns its price

Silver plating, usually specified under ASTM B700 at 5 micrometers or more, behaves differently in the ways that matter at the extreme end. Silver tarnishes in sulfur-bearing air and turns dark, which looks alarming, but silver sulfide is still a reasonable conductor, unlike copper oxide. Silver also holds up at contact temperatures well beyond what tin tolerates, and its contact resistance at a given pressure is the lowest of any common plating.

In practice we recommend silver on four situations: ratings above 4000A where every micro-ohm of joint resistance turns into real heat, generator and transformer terminal connections where a hot joint takes out the most expensive asset on site, hot environments where the contact will live above 100 degrees C, and humid coastal or industrial atmospheres that eat ordinary finishes. The cost premium on a typical project runs a few percent of the busway price. On a 6300A transformer feed, that is cheap insurance.

The options compared

Contact finish	Typical thickness	Continuous contact temp limit	Relative cost	Watch out for
Bare copper	None	Degrades from day one	Lowest	Oxide film, rising joint resistance
Tin plating (ASTM B545)	3 to 10 micrometers	About 105 degrees C	Low	Fretting under vibration, high-temp oxidation
Silver plating (ASTM B700)	5 micrometers and up	Around 200 degrees C	Moderate	Cosmetic tarnish in sulfur air (harmless)
Nickel plating	5 to 10 micrometers	Above 200 degrees C	High	Hard surface, needs high contact pressure; rare in LV busway

Nickel exists mostly for switchgear and high-temperature specialties. If a supplier proposes nickel on a standard low voltage busway, ask why; the answer is usually interesting.

What about aluminum busbars?

Aluminum is a different story, and a stricter one. Fresh aluminum grows an insulating oxide film within seconds of exposure to air, so an untreated aluminum joint is unreliable from the start. Proper aluminum busway joints are tin plated (over a zincate or copper strike layer so the plating actually sticks), or they are cleaned, abraded and coated with oxide-inhibiting compound at assembly. Mixing aluminum bars with copper hardware also raises galvanic corrosion questions in damp service. If you are weighing conductors, our aluminum vs copper busway comparison covers the rest of the trade-offs, but on plating specifically: never accept bare aluminum contact faces on a joint that has to last.

What to write into your purchase specification

Five lines cover it. State the plating type and the standard (tin to ASTM B545, silver to ASTM B700). State the minimum thickness, 5 micrometers is a sensible floor for joints. Ask whether the whole bar is plated or only the joint areas; whole-bar plating protects the duct inside humid enclosures and is what we do as standard. Require the manufacturer's bolt torque values in the installation manual, because plating performance assumes correct contact pressure. And ask for a salt spray or adhesion test reference if the project is coastal or tropical.

Mistakes we see on site

The most common one is a well-meaning electrician wire-brushing a plated joint face to make it "clean," removing in ten seconds the coating the joint needed for thirty years. Plated faces get wiped, not abraded. Second: mixing finishes, a bare copper field-made connection bolted to a plated factory joint, so the bare side becomes the failure point. Third: over-torquing tin-plated joints until the soft tin extrudes out of the interface and contact pressure drops. The torque value exists for a reason. And finally, no joint compound on outdoor connections. Plating plus a thin film of electrical joint grease on an exposed joint is belt and suspenders, and outdoors you want both.

If your specification is quiet on plating, most factories will ship tin and most projects will be fine. When the amps climb past 4000, the ambient climbs past 40 degrees, or the site sits near the sea, that is when a two-line plating clause in the purchase order pays for itself many times over.