How to Choose the Right Copper Tungsten Grade for Vacuum Interrupters

The CuW grade selection question, CuW70 vs CuW75 vs CuW80, or whether to go higher to CuW85 or CuW90, comes up in nearly every new VCB design project and every refurbishment program. The grades look similar on paper, the names follow a simple pattern (CuW70 = 70 wt% tungsten), and the trade-offs are straightforward in principle. In practice, the choice has cost, service life, and assembly implications that aren't always obvious to engineers new to the material family.

This guide walks through how to choose, what the trade-offs actually look like in service, and where the cost crossover points sit. References are to GB/T 8320-2025 (the current Chinese national standard, effective 2026-05-01) and equivalent ASTM specifications.

The Core Trade-Off

Tungsten content drives every important CuW property, and the directions are consistent:

As tungsten content goes up:	What changes
Conductivity	Decreases
Density	Increases
Hardness	Increases
Bending strength	Increases
Arc erosion resistance	Improves
Material cost	Increases (tungsten more expensive than copper)
Machinability	Slightly harder, slower feed rates

The first point, lower conductivity at higher tungsten content, is the trade-off that drives most engineering decisions. CuW exists because pure copper can't survive arc erosion at MV breaker currents; CuW80 exists because CuW70 still wears too fast in some applications. The question is always "how much arc resistance do I need vs how much conductivity am I willing to lose."

Minimum Property Values by Grade

The numbers below come from GB/T 8320-2025 Table 2 (vacuum switch grades), the most relevant table for vacuum interrupter and VCB applications. These are minimums any factory selling material as a given grade must meet; actual lot properties usually run modestly above the minimums.

Grade	Tungsten	Conductivity ≥ (% IACS)	Density ≥ (g/cm³)	Hardness ≥ (HB)	Bending Strength ≥ (MPa)
CuW70	70 wt%	42	13.80	175	790
CuW75	75 wt%	38	14.50	195	885
CuW80	80 wt%	34	15.15	220	980
CuW85	85 wt%	30	15.90	240	1080
CuW90	90 wt%	27	16.75	260	1160

Additional requirement for vacuum switch grades per GB/T 8320-2025: gas content O ≤ 0.008%, N ≤ 0.002%. This matters because residual gases trapped in the CuW material can outgas during the vacuum interrupter sealing process or during operation, compromising the vacuum.

For 40.5 kV-and-above apparatus, the standard tightens the requirements further, see GB/T 8320-2025 Table 3. CuW70 for 40.5 kV+ requires density ≥ 13.90, hardness ≥ 180, bending strength ≥ 800. Higher voltage class means stricter minimums.

Grade Selection by Application

CuW70: The Default

For most MV vacuum interrupter contact applications at 12 kV and 24 kV, CuW70 is the right starting point. The 42% IACS minimum conductivity is high enough to keep continuous-current heating manageable in standard MV breaker duty. The arc erosion resistance is adequate for typical breaker cycle counts over a 30-year service life. The cost is the lowest in the practical grade range.

CuW70 covers roughly 60–70% of new VCB designs and the majority of refurbishment orders. If your application doesn't have a specific reason to push higher, CuW70 is the answer.

CuW75: Mid-Range

Specify CuW75 when:

• The breaker design is at 24 kV or 40.5 kV and the arc duty is on the higher end of typical
• Your application sees more switching cycles than standard distribution duty (high-cycle motor breakers in industrial settings, for example)
• An existing OEM specification calls for CuW75 (some VCB manufacturers standardized on 75 for their full product line)

The conductivity drop from CuW70 (42%) to CuW75 (38%) is modest, about 10% relative. For most applications, the continuous-current heating impact is small enough that the longer arc-erosion life is worth the trade.

CuW80: Heavy Duty

Specify CuW80 when:

• The application is 40.5 kV and the arc duty is severe
• The breaker cycle count is high (industrial process applications with frequent switching, traction substations with continuous operation)
• The application has documented arc-erosion issues with CuW70 or CuW75
• An OEM specification requires it

The conductivity at CuW80 (34% IACS minimum) starts to matter for continuous-current heating in larger breaker designs, at high continuous currents, the I²R heating in CuW80 is measurable compared to CuW70. Sizing the contact cross-section may need to be slightly larger to compensate.

CuW85 and CuW90: Specialty

These grades appear in the GB/T 8320-2025 standard but are uncommon in general MV switchgear. Where they show up:

• High-voltage class apparatus (above 40.5 kV) with severe arc duty
• Specialty vacuum switches for industrial or transportation applications
• OEM-specific designs that have standardized on these grades

The conductivity drop becomes meaningful (≥30% IACS for CuW85, ≥27% for CuW90), and the material cost is significantly higher than CuW80. For most engineers, the question of CuW85 / CuW90 will only arise if an OEM specification explicitly calls for it.

Cost Considerations

CuW pricing increases with tungsten content because tungsten is more expensive than copper as a raw material. CuW75 typically costs slightly more than CuW70 per piece; CuW80 noticeably more; the higher-tungsten specialty grades (CuW85, CuW90) more again. The exact pricing depends heavily on order volume, dimensional complexity, plating requirements, and current market conditions for raw materials.

The service-life advantage of higher grades has to be weighed against the cost. Under more severe arc duty, higher-tungsten grades extend service life, but quantifying that advantage for a specific application requires looking at your duty cycle, expected switching frequency, and field replacement cost. For most standard MV applications, CuW70 is the cost-effective default; stepping up makes sense when the application specifically calls for it.

For a quote tailored to your application, send the drawing, target quantity, and intended duty cycle.

Other Selection Factors

Compatibility with Existing Designs

For refurbishment, match the original grade. The breaker mechanism was designed and tested with the original CuW grade in mind; switching to a different grade changes the contact mass, the heating characteristics, and potentially the breaker's certified ratings.

For new designs that are similar to an existing reference design, starting with the reference grade and adjusting if specific reasons emerge is usually the right approach.

Ni-Containing CuW

GB/T 8320-2025 Appendix A documents the effect of nickel additions on CuW conductivity: roughly 1% nickel reduces conductivity by 30–35% across the grade range. Ni-containing CuW shows up in specific applications where the nickel improves machinability or interface behavior, but it's a specialty choice. Standard CuW without nickel is the default.

Surface Treatments

CuW contacts can be supplied as machined, silver plated, or nickel plated. Plating is on non-arcing surfaces. The contact face stays bare CuW. Silver plating lowers contact resistance at the carrier interface; nickel plating provides corrosion resistance. Specify per drawing.

Whole Contact (整体电触头) Bonding

When the CuW contact is brazed to a copper or copper-alloy conductive end as a whole contact (整体电触头), GB/T 8320-2025 requires the bonded joint to meet minimum tensile strength: ≥185 MPa for Cu conductive end, ≥226 MPa for CuCrZr or similar alloy ends. This is the joint strength at the CuW-to-conductive-end interface, not the CuW material itself.

How to Decide: A Practical Path

If you're starting fresh on a new VCB design:

1. Start at CuW70 unless there's a specific reason to go higher. This is the default for most 12 kV / 24 kV applications.
2. Check OEM references. If your design is based on or similar to an existing OEM VCB, use the same grade.
3. For 40.5 kV+, consider CuW75 or CuW80. The stricter density / hardness / bending strength requirements at higher voltage class often align with the higher-tungsten grade specs.
4. Look at your duty cycle. High-cycle industrial applications justify higher grades; standard distribution doesn't.
5. Cost vs life trade-off. Run the numbers on field replacement cost and downtime cost vs the per-piece price delta. For most utility applications, CuW70's lower cost wins.

If you're sourcing for a refurbishment, the answer is almost always "match the original", unless the original grade has documented failure issues, in which case stepping up one grade is the standard fix.

What to Send When Sourcing

When you contact us for a CuW contact quote, include:

• Grade and tungsten content target (CuW70, CuW75, etc.)
• Form factor: static arc contact, moving arc contact, contact pair, shielding cap, rod stock, or custom geometry
• Dimensions per drawing (DXF / PDF / STEP)
• Voltage class (12 / 24 / 40.5 kV+)
• Continuous current rating
• Plating requirements (silver, nickel, or as-machined)
• Quantity (helps with quoting and lead time)
• Whole-contact requirements (if brazed to a conductive end, specify the carrier material)

We work to GB/T 8320-2025 minimums by default and can provide material certification per lot documenting actual properties on request.

Summary

For 12–24 kV vacuum interrupter contacts in standard MV breaker duty, start with CuW70, best conductivity in the grade family, lowest cost, adequate arc-erosion resistance for typical service life. Step up to CuW75 for higher-cycle or higher-voltage-class applications, CuW80 for severe arc duty or 40.5 kV+ where the stricter standard requirements come into play. CuW85 and CuW90 are specialty grades; specify only when an OEM design calls for them or when documented arc-erosion issues drive the choice.

When in doubt: send the drawing and the application duty cycle. We can recommend a grade.

Related Reading

• Copper Tungsten Properties Guide: Conductivity, density, hardness, arc behavior, deeper material reference
• Copper Tungsten (CuW) Series: Full CuW product range
• CuW Static Arc Contact: Standard static arc contact in CuW70 / 75 / 80
• AgW vs CuW: When to Use Silver Tungsten vs Copper Tungsten: Sister-material comparison for LV vs MV applications