Key takeaways
- ETP copper is the default for busbars: it conducts at or near 100 per cent IACS and is widely available.
- Oxygen-free copper matters most where the part is brazed or used in a reducing or high-vacuum atmosphere.
- The conductivity gain from oxygen-free copper is small; the process benefit can be large.
- Specify the grade and the standard on the drawing, and do not accept a certificate that names only copper.
What the two grades are
Electrolytic tough pitch copper, usually C11000 under ASTM or Cu-ETP under EN, is the standard busbar material. It is refined electrolytically and then cast, and it contains a small but deliberate amount of oxygen, typically a few hundred parts per million, present as copper oxide. That oxygen is part of the grade, not a defect.
Oxygen-free copper is made so that the oxygen content stays very low. C10200 is the common oxygen-free grade, and C10100 is the higher purity oxygen-free electronic grade. Both are refined to remove oxygen rather than to change the copper content in a dramatic way. The difference in conductivity between ETP and oxygen-free copper is small, often a fraction of a per cent.
Both grades are supplied annealed or cold worked, so temper is a separate choice. A hard oxygen-free bar and a hard ETP bar behave similarly in a press brake. The grade controls chemistry and high-temperature behaviour; the temper controls strength and springback.
Conductivity, resistance and the real gain
ETP copper is specified at a minimum conductivity of 100 per cent IACS in the annealed condition under the common standards. Oxygen-free grades are typically specified at 101 per cent IACS. The difference in resistivity is around one per cent, which is smaller than the effect of a section tolerance or a joint with poor contact.
That matters for busbar sizing. If a design is marginal on temperature rise, changing from ETP to oxygen-free copper will not rescue it. A small increase in section, a better joint face or a lower ambient will do more. The grade is not a thermal solution.
Conductivity is also affected by temper and by annealing. A hard-drawn bar has slightly lower conductivity than an annealed one because of the cold work. The effect is small but it is one reason to check the certificate against the temper that was ordered, not just the grade.
A one per cent conductivity gain is not a section change
Check the joint, the plating and the ambient before upgrading the copper. The biggest resistance in a busbar system is often the interface, not the bar.
Oxygen, hydrogen and brazing
The important difference appears at temperature. ETP copper contains copper oxide particles at the grain boundaries. If the bar is heated in a reducing atmosphere, such as hydrogen or a cracked ammonia atmosphere, the hydrogen can diffuse in and react with the oxygen. The reaction produces steam inside the metal, and the pressure can open voids along the grain boundaries. The result is hydrogen embrittlement, and it can make a brazed or annealed part crack.
Oxygen-free copper removes the oxygen that feeds that reaction, so it is the usual choice for parts that are brazed or sintered in a reducing atmosphere, used in vacuum devices or subjected to a hydrogen furnace. The grade is specified for the process, not for the conductivity.
This is why a busbar that is induction brazed to a terminal in air may be perfectly sound in ETP, while the same joint brazed in a hydrogen furnace needs oxygen-free copper. The atmosphere and the temperature decide the risk. Ask the fabricator how the joint will be made before choosing the grade.
If brazing is needed but the atmosphere is not reducing, ETP is usually acceptable. The filler metal and the flux then become the controlling variables. A hydrogen atmosphere is the specific condition to design around.
Forming, annealing and grain structure
Both grades bend well in the annealed condition. In hard temper, both resist bending and both spring back, and the minimum bend radius depends more on temper and thickness than on the oxygen content. A fabricator who can bend ETP can generally bend oxygen-free copper with the same tooling.
The difference is in the anneal. Oxygen-free copper can be annealed in a reducing atmosphere without the internal oxidation risk, which is one reason it is used for parts that need a soft condition after forming. ETP can also be annealed, but the furnace atmosphere and the cycle have to be controlled to avoid the same hydrogen problem.
Cost, availability and lead time
Oxygen-free copper costs more than ETP because the refining and casting route is more controlled and the volume is smaller. The premium varies with the market and with the product form, so it should be quoted rather than assumed. For a large programme, even a small premium per kilogram becomes a real number.
Availability is the other factor. ETP bar is stocked in many sizes and is easy to source in cut lengths. Oxygen-free bar may need to be ordered in a mill quantity, which affects the minimum order and the lead time. If the part is a one-off, the availability difference can matter more than the price.
| Property | ETP, C11000 | Oxygen-free, C10200 or C10100 |
|---|---|---|
| Oxygen content | A few hundred ppm, present as oxide | Very low, typically below 10 ppm |
| Conductivity | About 100 per cent IACS minimum | About 101 per cent IACS |
| Brazing in hydrogen | Risk of embrittlement at temperature | The usual choice for reducing atmospheres |
| Bending and forming | Good in annealed temper | Good in annealed temper |
| Plating | Standard pre-treatment works | Standard pre-treatment works |
| Availability | Widely stocked in bar and strip | Narrower stock, often mill order |
| Relative cost | Lower | Higher, quoted by form and volume |
Typical values only. Confirm the grade, temper and property limits against the standard and the mill certificate for the actual order.
When ETP is the right call
ETP is the right call for most busbars. Distribution bars, panel bars, links, earth bars and battery interconnects are usually cut, punched, bent and plated, with bolted joints. None of those steps exposes the copper to a reducing atmosphere at high temperature, and the conductivity is already at the top of the practical range.
It is also the right call when the joint is the limiting feature. A bolted joint with a sound surface, correct torque and compatible plating will dominate the resistance, and the grade of the bar will be lost in the noise. Spending the premium on a better joint face or a thicker plating is usually a better use of the budget.
ETP is easier to source in the exact section and temper, which shortens the route from drawing to part. For a programme with several sizes, that availability is worth more than a fraction of a per cent of conductivity.
When oxygen-free copper earns the premium
Oxygen-free copper earns its place when the process or the environment demands it. The clearest case is a hydrogen or reducing atmosphere during brazing, annealing or sintering. The second is a high-vacuum or sealed device where outgassing and internal voids matter. The third is a customer specification that names the grade for a physics reason, such as a magnet or a radio frequency component.
It also appears in parts that are welded or brazed and then bent, where a crack at the joint would be a reliability failure rather than a rework. In those cases the cost of the grade is small compared with the cost of a field failure.
How to specify the grade
Name the grade and the standard, not just the colour of the metal. Write C11000 to ASTM B187, or Cu-ETP to EN 13601, or the equivalent your supply chain uses. Add the temper, the conductivity minimum and the certificate requirement. If oxygen-free is needed, say which grade and why, so the supplier does not substitute a lower-cost oxygen-free variant with different limits.
Put the process reason in the notes. A note such as oxygen-free copper required for hydrogen brazing tells the supplier and the inspector what the grade is protecting. It also makes a substitution easier to challenge.
Check the certificate against the order at goods in. The certificate should show the grade, the temper, the lot and the conductivity. If the part is brazed, add a check for voids or cracks at the joint, because the grade alone does not prove the process was right.
Frequently asked questions
Is oxygen-free copper more conductive than ETP copper?
Slightly. Oxygen-free grades are typically specified at about 101 per cent IACS against about 100 per cent for ETP. The difference is around one per cent in resistivity, which is usually smaller than the effect of section tolerance, joint quality or plating. The main reason to choose oxygen-free copper is process behaviour, not conductivity.
Why does ETP copper crack after brazing?
ETP copper contains oxygen as copper oxide. In a reducing atmosphere at high temperature, hydrogen can react with that oxygen and form steam inside the metal, which opens voids at the grain boundaries. The part can then crack. Oxygen-free copper avoids that reaction and is the usual choice for hydrogen brazing or annealing.
Can oxygen-free copper be used for ordinary busbars?
Yes, it can, but it usually adds cost and lead time without a functional gain. For cut, punched, bent and bolted bars, ETP is normally sufficient. Use oxygen-free copper where the process or the application needs it, and record the reason on the drawing.
Which standard should I call out for copper busbar?
ASTM B187 and EN 13601 are common for copper bar and busbar. Name the grade, the temper and the conductivity minimum, and ask for a mill certificate linked to the lot. If the part is for a regulated assembly, check whether the end product standard adds its own material requirements.
Sources and further reading
- ASTM B187, Standard specification for copper bar, bus bar, rod and shapes
- EN 13601, Copper and copper alloys, copper rod, bar and wire for general electrical purposes
- Copper Development Association, copper and copper alloy properties and applications
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.
Not sure which copper grade the busbar needs?
Send the drawing and describe the joining process. The engineering desk will say whether ETP is enough or whether the atmosphere calls for oxygen-free copper.
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