Key takeaways
- Tin does not improve conductivity. It slows oxidation and keeps the strand surface easier to terminate.
- Bare copper is the better choice for a dry, sealed joint that is assembled once and never disturbed.
- Tinned braid earns its cost in humid, sulphur-bearing or salt-laden air, and where the joint will be reworked.
- The finish on the terminal matters as much as the finish on the braid. Mixing them adds a corrosion cell.
The short answer
Bare copper braid is woven or plaited copper with no coating on the strands. Tinned copper braid starts with the same copper and adds a thin tin layer to each wire. Both are flexible conductors with the same nominal cross-section, and both will carry the same current when they are new.
The difference appears at the surface. Copper oxidises in air, and the oxide grows faster in heat, humidity and industrial atmospheres. Tin also oxidises, but its oxide is thin, soft and easier to displace under pressure. That single difference decides most of the practical behaviour of a braid in service.
The choice is therefore not a conductivity question. It is a question about the environment, the joint, the service temperature and how often someone will open the connection.
What the tin layer actually does
The tin coating is applied to the individual strands, either by hot dipping or by electroplating before the braid is constructed. A typical coating is a few micrometres thick. It covers the full circumference of each strand, so the braid keeps its flexibility and the strands do not stick together.
The coating works as a barrier. It keeps oxygen away from the copper and it keeps sulphur compounds away from the copper. It also keeps the surface solderable after storage, which matters for braids that are soldered into a terminal or bonded to a screen.
The coating is not a miracle. It is soft, it wears, and it can be damaged by abrasion or by repeated flexing at a tight radius. Once the copper is exposed, the protection is local and the exposed area behaves like bare copper. Tin also melts at about 232 C, which sets a ceiling for any joint that relies on the coating staying solid.
A second effect is contact behaviour. Tin is softer than copper and flows plastically under a bolted joint. That helps the joint make contact over a larger area at a given torque, but it also means the joint can relax over time if the clamping system has no spring element.
Corrosion: where each finish belongs
In a dry, heated switchgear cabinet, bare copper is usually fine. The oxide that forms is thin, and a properly prepared joint with a contact paste or a wiped surface will hold its resistance. Many bolted busbar and braid joints in indoor equipment run for decades on bare copper.
The picture changes outdoors, at the coast, in food processing, in wastewater plants and anywhere hydrogen sulphide is present. Copper reacts with sulphur to form a dark, poorly conducting film. In a braid the problem is worse than on a flat bar because there are thousands of strand surfaces, and the film forms on all of them. The resistance of the braid rises, the joint runs warmer, and the warmth accelerates the film growth.
Tinned braid is the usual answer in those locations. The tin layer slows the reaction and keeps the strand surface clean for longer. It is not a seal, so a tin coating on a braid that is permanently wet with salt water will still fail eventually. For that duty, a closed connector or a different conductor construction is the better route.
There is also a galvanic point. Tin and copper are close in the galvanic series, so a tin coating on copper creates only a small couple. That is one reason tinning is preferred over a thick metallic coating with a much larger potential difference.
Contact resistance and joint preparation
The resistance of a braid joint is the sum of the braid body, the interface between the braid and the terminal, and the terminal itself. The coating changes only the interface, but that is often the largest and least stable part of the path.
Bare copper forms an oxide that is harder to break than tin oxide. A bolted bare copper joint therefore needs a clean, abraded surface, a suitable contact compound and enough clamp force to break through the film. If the joint is made without that preparation, the initial resistance may look acceptable and then climb after a few thermal cycles.
Tinned surfaces are more forgiving. The oxide is thinner and softer, and a normal bolted joint will usually displace it. This is why tinned braid is common in equipment that is assembled on a line rather than in a controlled workshop.
The terminal finish should match the braid. A tinned braid bolted to a bare copper lug is workable, but the bare copper side still needs preparation and the joint now has two different surface behaviours. A tinned braid on a silver-plated terminal is a common and reasonable pair, provided the joint is dry. A bare copper braid on a tin-plated terminal is the least comfortable combination because the copper side is the one that will oxidise.
The joint, not the braid, sets the resistance
Two braids with the same cross-section can differ by more at the terminal interface than in the braid body. Specify the terminal finish, the preparation and the torque, then measure the joint, not just the wire.
Temperature and current limits
Current capacity is set by the copper cross-section, the number of strands, the mounting arrangement and the ambient temperature. The finish has almost no effect on the current rating at moderate temperature. It matters at the top of the range.
Bare copper oxidises more quickly as temperature rises. Above roughly 100 C in air the oxide film grows at a rate that starts to matter for a joint that is not sealed. Tinned copper holds its surface better in that range, which is one reason tinned braid is often specified for equipment that runs warm.
The limit for tin is the melting point and the creep behaviour of the coating. A soldered joint on tinned braid should not be run close to the melting point of tin. For high-temperature duty, silver-coated copper braid is the usual upgrade, not a thicker tin layer.
For an illustrative comparison, take two 50 mm2 braids carrying the same current in the same enclosure. Their temperature rise will be similar because the copper is the same. The difference is what happens to the joint after a few hundred cycles between ambient and 90 C. The tinned joint is likely to stay closer to its initial resistance, while the bare joint depends heavily on how well it was prepared and how much clamp force remains.
Choosing between the two finishes
The table below turns the discussion into the questions that usually decide a specification. It assumes the same copper cross-section and the same braid construction on both sides, so the only variable is the strand finish.
| Question | Bare copper braid | Tinned copper braid |
|---|---|---|
| Dry indoor joint, assembled once | Good choice | Works, but the coating adds cost |
| Humid, coastal or sulphur-bearing air | Oxide grows on every strand | Preferred finish |
| Joint preparation on the line | Needs abrasion and contact compound | More forgiving of normal preparation |
| Soldering or bonding | Needs flux and a clean surface | Easier to solder after storage |
| Continuous temperature near 100 C | Oxide growth accelerates | Surface holds up better |
| Continuous temperature above 150 C | Use silver coating instead | Tin is not the right answer |
| Repeated rework of the joint | Surface degrades each time | Better tolerance to re-termination |
| Visual inspection | Dark oxide is easy to see but hard to judge | Discolouration is a clearer warning |
| Relative material cost | Lower | Higher |
| Best fit | Sealed, dry, permanent joints | Exposed, humid and serviceable joints |
Both finishes need a correctly sized terminal, the right torque and a joint that is protected from strain. The coating cannot compensate for a poor mechanical joint.
What to put on the drawing
A braid specification needs more than the word tinned. State the copper grade, the strand count and the individual wire diameter, the braid construction and the nominal cross-section. For a tinned braid, state the coating method and a minimum thickness, and reference a recognised wire standard.
Add the electrical requirement. A resistance per metre at 20 C is more useful than a vague statement about conductivity, because it captures the effect of strand count and braid angle. If the braid is part of a current path, ask for the resistance of the finished assembly, including the terminals.
For the joint, state the terminal material and finish, the bolt size, the washer type and the torque. If the braid will be soldered, state the solder alloy and whether the strands must be pre-tinned. If it will be crimped, state the crimp profile and the pull test.
Finally, state the environment. A braid for a sealed indoor cabinet and a braid for a coastal cabinet should not share a drawing. The finish, the terminal plating and the inspection interval all follow from that one sentence.
- Copper grade, strand count and wire diameter
- Braid construction, coverage and nominal cross-section
- Tin coating method and minimum thickness, if tinned
- Resistance per metre at 20 C
- Terminal material, plating and torque
- Solder or crimp process and the related test
- Service environment and inspection interval
Making the call
Choose bare copper when the joint is dry, protected, assembled with proper preparation and unlikely to be opened. The saving is real and the electrical performance is as good as tinned copper when the joint is new.
Choose tinned copper when the braid sees humidity, sulphur, salt or repeated handling, or when the joint will be re-terminated in the field. The coating buys time and tolerance to ordinary assembly variation.
In both cases, spend the effort on the joint. The braid is a flexible conductor, and its weakest point is almost always where it meets the terminal. A tinned braid with a poor crimp is worse than a bare braid with a good one.
Frequently asked questions
Does tinned copper braid carry less current than bare copper?
The difference is small at normal temperatures because the tin layer is thin and the copper cross-section does most of the work. At high temperature the coating sets a limit, and above roughly 150 C a silver-coated braid is the better choice. Size the braid on copper cross-section and derate for the ambient and mounting.
Can I join tinned braid to a bare copper terminal?
Yes, but prepare the bare copper side and use a joint that excludes moisture. The two finishes behave differently under clamping, so check the resistance after thermal cycling rather than only at assembly.
Why does my bare copper braid turn green?
The green colour is a copper salt formed by moisture and atmospheric gases. It is a sign that the surface is reacting, not proof that the braid has failed. Measure the joint resistance and inspect for strand loss before deciding to replace it.
Is tinned braid suitable for outdoor use?
It is better than bare copper in most outdoor atmospheres, but it is not a sealed system. In coastal or permanently wet locations, protect the joint with a closed connector or a suitable enclosure and inspect it on a defined interval.
Sources and further reading
- ASTM B33, Standard Specification for Tin-Coated Soft or Annealed Copper Wire for Electrical Purposes
- IEC 60228, Conductors of insulated cables
- IEC 60068-2-60, Flowing mixed gas corrosion test
Qumek Engineering Desk
Engineering team
The engineering team behind Qumek drawings, DFM notes, tolerances and inspection standards. Every guide is reviewed against the drawings and processes used on live programmes.
Specifying a braid and not sure which finish?
Send the drawing, the service environment and the terminal finish. The engineering desk will reply with a written note on the braid construction and the joint.
Send a drawing