C101 vs C110 vs Tellurium Copper: Conductivity Against Machinability
September 21, 2026
Four copper grades cover most machined parts. They look identical on a drawing and they behave nothing alike at the machine. Picking the wrong one is the most expensive mistake on a copper RFQ, and it is made before anyone gets a quote.
The numbers side by side
| Grade | Conductivity %IACS | Machinability | Typical use |
|---|---|---|---|
| C101 / C10100 OFE | 101 | 20 | Ultra high vacuum, hydrogen brazing, EDM electrodes |
| C110 / C11000 ETP | 100 | 20 | Busbars, terminals, general electrical work |
| C145 tellurium | 93 | 85 | Threaded parts, connector bodies, high volume turning |
| C147 sulphur | 95 | 85 | Same as C145, slightly better conductivity |
| C182 chromium | 80 | 20 | Resistance welding electrodes, hot stiff parts |
| C360 brass (reference) | 26 | 100 | Not copper, shown for scale |
Machinability is rated against free-cutting brass C360 at 100. These are published typical values for the alloys and not a commitment from any shop. Full alloy data sheets are available from the Copper Development Association.
Read the table once and the shape of the decision is obvious. Moving from pure copper to a free-machining grade costs about 7 points of conductivity and buys roughly four times the machinability. Moving to brass costs three quarters of the conductivity.
C101 when oxygen free actually matters
C10100 is refined to 99.99 percent copper with the oxygen removed. That matters in three situations:
- Hydrogen brazing or hot hydrogen service. Oxygen in the copper reacts and the part becomes brittle. This is the classic reason to specify C101.
- Ultra high vacuum. Oxide inclusions outgas. Vacuum chambers, flanges and seals are normally specified as OFE for this reason.
- Sinker EDM electrodes, where structural uniformity affects burn quality.
If none of those apply, C101 is usually paying for a certificate you do not need. The conductivity gain against C110 is about one point.
C110 as the default
C11000 electrolytic tough pitch copper is the everyday electrical copper. Around 100 percent IACS, widely stocked in plate, bar and rod, and cheaper than C101. It machines exactly like C101, which is to say slowly, so the choice between them changes your material cost and paperwork rather than your cycle time.
For busbars, terminals, lugs, cold plates and most heat sinks, C110 is the right default. Specify C101 only when you can name the reason.
C145 and C147 when the part has features
Tellurium copper and sulphur copper contain a small addition (around 0.5 percent tellurium, or about 0.4 percent sulphur) that breaks the chip. Nothing else about them is exotic, and that one change transforms how they cut:
- Threads come out clean instead of torn
- Cross holes and slots need far less hand deburring
- Cycle times drop several times over on turned parts
- Surface finish improves without a separate finishing operation
The cost is 6 to 8 points of conductivity and a thinner supply chain. Stock is less common than C110, so lead time on the raw material can become the long pole on a small order.
These grades suit connector bodies, terminals, valve parts, fittings and anything produced in volume on a lathe. They are not for ultra high vacuum.
C182 is a different animal
Chromium copper is age hardened. It is far stronger and harder than pure copper and holds its shape when hot, which is why resistance welding electrodes are made from it. It sits near 80 percent IACS and it does not machine quickly. Choose it for mechanical and thermal reasons, never to make machining easier.
How to decide in one pass
- Does the part go into ultra high vacuum, or get brazed in hydrogen? Specify C101.
- Does it need to stay hard and stiff while hot, or is it a welding electrode? Specify C182.
- Is there an electrical or thermal number that genuinely needs 100 percent IACS? Specify C110.
- Anything else, especially with threads, cross holes or volume? Ask for C145 or C147 and compare the quotes.
If you are not sure which bucket you are in, write the requirement on the drawing instead of the grade. Shops quote the requirement, not the label.
What picking wrong costs
Over-specifying C101 on a part with twenty tapped holes is the common version of this mistake. You pay pure copper machining time plus OFE material plus the deburring that free-machining copper would have avoided, and the finished part performs the same in the assembly.
Under-specifying is rarer and worse. A C145 part in a vacuum chamber can pass inspection and fail in service.
Why copper is difficult to machine explains what happens at the tool, which is where these differences come from.
Frequently asked questions
Is C101 better than C110?
Only for specific jobs. C101 is oxygen free, so it survives brazing in hydrogen and suits ultra high vacuum work. For conductivity alone the difference against C110 is about one point of IACS, which almost never changes a design. C110 is easier to buy and costs less.
How much conductivity do I lose with tellurium copper?
C145 is rated near 93 percent IACS against about 101 for C101, so roughly 7 to 8 points. In exchange the machinability rating goes from about 20 to about 85. For most terminals, connector bodies and threaded parts that trade is worth taking.
Can I let the shop choose the grade?
Yes, if you write the requirement instead of the grade. Put the conductivity, current rating or thermal figure on the drawing and mark the grade as a suggestion. A shop can then offer the cheapest alloy that still meets the number.
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