Quick Answer: Conductivity, Resistivity, and %IACS
Conductivity tells you how easily a material carries electric current. Resistivity tells you how strongly it resists current flow. %IACS compares a material’s conductivity against annealed copper, where 100% IACS is commonly treated as about 58 MS/m at 20°C. For busbars, terminals, grounding parts, and electrical contacts, these values help compare materials, but they do not replace full design checks for temperature rise, mechanical strength, plating, contact pressure, corrosion, and arc resistance.
The three measurements describe the same electrical behavior from different angles:
- Higher conductivity means easier current flow.
- Lower resistivity means easier current flow.
- Higher %IACS means the material is closer to or above annealed copper conductivity.
In practical electrical design, copper remains the baseline conductor, aluminum is used when weight and cost matter, silver is often used as plating or contact surface rather than bulk conductor, and tungsten or copper-tungsten is used where arc erosion resistance matters more than maximum conductivity.
Why This Matters in Electrical Components
Material conductivity affects heat, voltage drop, and current-carrying capability. If two parts have the same geometry, the lower-resistivity material will usually run cooler at the same current because it produces less Joule heating.
The relationship is:
พี = ไอ²อาร์ที่ซึ่ง:
พีis heat generated by resistanceฉันis currentRis electrical resistance
That is why conductivity matters in:
- copper and aluminum busbars
- MCB and MCCB conductive parts
- terminal blocks and grounding bars
- contactor and relay contacts
- silver-plated contact surfaces
- copper-tungsten arc contacts
- switchgear joints and bolted connections
For busbar-specific selection, see 10 ความแตกต่างระหว่างบัสบาร์ทองแดงและอลูมิเนียม แล้ว Busbar Selection Guide: Copper, Tin, and Silver Plating Compared.
What Is Electrical Resistivity?
Electrical resistivity is an intrinsic material property that describes how strongly a material opposes electric current. It is usually written as ρ and commonly expressed in:
Ω · m(ohm-meter)μΩ · cm(micro-ohm-centimeter)nΩ · m(nano-ohm-meter)
Lower resistivity is better for current-carrying conductors.
For example, annealed copper has a typical resistivity around 1.724 μΩ·cm at 20°C, while aluminum is typically around 2.7-2.9 μΩ·cm depending on purity and grade. This is why aluminum normally needs a larger cross-sectional area than copper to carry similar current at comparable temperature rise.
Resistivity is not fixed for every real-world part. It changes with:
- อุณหภูมิ
- material grade
- impurity level
- cold working
- heat treatment
- alloying elements
- plating and surface condition
That is why published values should be treated as typical reference values, not as final inspection limits unless tied to a specific material standard or purchasing specification.
What Is Electrical Conductivity?
การนำไฟฟ้า is the inverse of resistivity. It is usually written as σ and commonly expressed in:
- S/m (siemens per meter)
- MS/m (megasiemens per meter)
สูตรคำนวณคือ:
σ = (1 / ρ)Higher conductivity means the material carries current more easily.
Typical conductivity examples at 20°C:
- Silver: about 61-63 MS/m
- Annealed copper: about 58 MS/m
- Aluminum: about 35-37 MS/m
- Tungsten: about 17-19 MS/m
- 304 stainless steel: roughly 1.1-1.5 MS/m, depending on reference and condition
Conductivity is useful when comparing conductor materials, but it is not the only selection criterion. A terminal spring, for example, may need strength and elasticity more than maximum conductivity. A contact tip may need arc resistance more than pure copper conductivity.
What Is %IACS?
%IACS หมายถึง percent International Annealed Copper Standard. It expresses a material’s conductivity as a percentage of the International Annealed Copper Standard, where annealed copper is used as the reference.
In common engineering practice:
100% IACS ≈ 58 MS/m at 20°CSo:
- 100% IACS means roughly equal to annealed copper
- 60% IACS means about 60% of annealed copper conductivity
- 105% IACS means slightly higher than the IACS copper reference
%IACS is widely used because it lets engineers compare metals and alloys quickly without converting every value into resistivity or conductivity. It is especially common in copper alloys, aluminum alloy quality checks, conductor materials, and contact materials.
Important: %IACS is normally referenced at 20 องศาเซลเซียส. If the temperature changes, conductivity and resistivity change too.
Conversion Formula: MS/m, μΩ·cm, and %IACS
If conductivity is given in MS/m:
%IACS = (σ / 58) × 100ที่ไหน σ is conductivity in MS/m.
If resistivity is given in μΩ·cm:
σ(MS/m) = (100 / ρ(μΩ · cm))And:
ρ(μΩ · cm) = (100 / σ(MS/m))Quick Conversion Examples
| Given value | การแปลง | ผลลัพธ์ |
|---|---|---|
| Copper at 58 MS/m | 58 / 58 × 100 |
100% IACS |
| Aluminum at 36 MS/m | 36 / 58 × 100 |
About 62% IACS |
| Silver at 61.5 MS/m | 61.5 / 58 × 100 |
About 106% IACS |
| Resistivity 2.80 μΩ·cm | 100 / 2.80 |
About 35.7 MS/m |
| Conductivity 18 MS/m | 100 / 18 |
About 5.56 μΩ·cm |
These calculations are useful for quick material comparison. They are not a substitute for final thermal, mechanical, and standards-based verification.
Common Material Comparison Table
The values below are typical reference ranges at or near 20°C. Actual values depend on material grade, purity, processing condition, temperature, and measurement method.
| วัสดุ | Typical %IACS | Conductivity | Resistivity | Typical electrical use |
|---|---|---|---|---|
| เงิน | 105-108% | ~61-63 MS/m | ~1.59-1.64 μΩ·cm | Contact surface, plating, RF/high-performance surfaces |
| Annealed copper | 100% | ~58 MS/m | ~1.724 μΩ·cm | Busbars, terminals, conductors, grounding parts |
| ETP/OFC copper | ~100-101%+ | ~58-59 MS/m | ~1.70-1.72 μΩ·cm | High-conductivity electrical parts |
| อลูมิเนียม | 60-64% | ~35-37 MS/m | ~2.7-2.9 μΩ·cm | Lightweight busbars, conductors, power distribution |
| Tungsten | ~30-33% | ~17-19 MS/m | ~5.3-5.8 μΩ·cm | Arc-resistant contact materials, electrode applications |
| Copper-tungsten | varies widely | varies by W/Cu ratio | often ~3-6 μΩ·cm | Arcing contacts, breaker/contact applications |
| ทองเหลือง | varies widely | lower than copper | higher than copper | Terminals, connector parts where strength/formability matter |
| 304 不锈钢 | ~2-3% | ~1.1-1.5 MS/m | ~70-90 μΩ·cm | Structural parts, springs, corrosion-resistant hardware, not main conductors |
This table explains why material selection in electrical products is a balance. Pure conductivity matters, but so do strength, spring behavior, corrosion resistance, plating compatibility, contact pressure, manufacturability, and arc erosion.
For terminal-related applications, see วิธีการเลือก Terminal Block ที่เหมาะสม แล้ว Terminal Block Components Construction Guide.
Why Silver Conducts Better Than Copper but Is Not Always Used
Silver is the most conductive common metal. On the IACS scale, it can slightly exceed annealed copper. That raises a natural question: why not make every busbar and terminal from silver?
The answer is cost, mechanical behavior, and application need.
Silver is expensive compared with copper and aluminum. It is usually not needed as a bulk conductor because the conductivity improvement over copper is small compared with the cost difference. In many power-distribution parts, increasing copper cross-section, improving joint pressure, or using the right plating is more economical than replacing copper with silver.
Silver is valuable where the surface matters:
- contact faces
- sliding contacts
- plated conductor surfaces
- high-reliability connectors
- high-frequency or RF surfaces
In contact systems, silver and silver-based alloys are often used because surface conductivity, contact resistance, oxide behavior, and switching performance matter more than bulk conductivity alone.
For contact material context, see Contactor Contact Material Guide: AgSnO2 vs AgNi vs AgCdO.
Why Aluminum Needs Larger Cross-Section Than Copper
Aluminum is lighter and often less expensive than copper, but its conductivity is only about 60-64% IACS for typical high-conductivity aluminum. That means an aluminum conductor generally needs a larger cross-section than copper to achieve similar electrical resistance.
A simplified comparison:
- Copper gives high conductivity in compact space.
- Aluminum reduces weight and can reduce cost.
- Aluminum requires careful joint design because oxide layers, thermal expansion, and connection pressure affect long-term reliability.
In busbars, the decision is rarely "copper is better" or "aluminum is better." The correct decision depends on:
- available space
- allowable temperature rise
- การรองรับทางกล
- short-circuit strength
- plating or surface treatment
- joint design
- สภาพแวดล้อมในการติดตั้ง (Installation environment)
- total cost and weight
For a more application-specific comparison, see 10 ความแตกต่างระหว่างบัสบาร์ทองแดงและอลูมิเนียม.
Why Tungsten and Copper-Tungsten Are Used in Contacts
Tungsten is much less conductive than copper or silver, so it looks like a poor conductor if you only read the conductivity column. But contacts are not selected by conductivity alone.
Switching contacts must survive:
- arcing
- melting risk
- contact erosion
- welding tendency
- high local temperature
- mechanical impact
- repeated opening and closing
Tungsten has a very high melting point and strong arc erosion resistance. Copper-tungsten materials combine copper’s conductivity with tungsten’s arc resistance. As tungsten content increases, conductivity generally decreases, but arc resistance and high-temperature behavior improve.
That is why copper-tungsten and silver-tungsten type materials may appear in breaker contacts, arcing contacts, and severe switching applications. The goal is not maximum conductivity. The goal is a workable balance between conductivity, thermal behavior, arc resistance, and contact life.
Why Stainless Steel Is Not a Good Main Conductive Material
Stainless steel is useful in electrical products, but not because it is highly conductive. Austenitic stainless steels such as 304 have much higher resistivity than copper and aluminum. In %IACS terms, 304 stainless steel is often only a few percent of copper conductivity.
That makes it poor for main current-carrying paths such as busbars or primary terminals.
However, stainless steel can be useful for:
- screws and hardware
- springs
- brackets
- enclosure parts
- corrosion-resistant structural components
- non-primary conductive mechanical parts
The key is to use stainless steel where corrosion resistance or mechanical properties matter, not where low resistance is the main requirement.
How These Values Affect Busbars, Terminals, and Contacts
บัสบาร์
For busbars, conductivity affects temperature rise and voltage drop. Copper is compact and highly conductive. Aluminum can work well when designed with larger section, suitable surface treatment, and proper joints.
Key checks include:
- material conductivity
- cross-section
- temperature rise
- การทนต่อกระแสลัดวงจร
- joint resistance
- plating
- mounting insulation
- การระบายอากาศของตู้ควบคุม
For MCB busbar quality, see วิธีการตรวจสอบคุณภาพของบัสบาร์สำหรับ MCB แล้ว วิธีการเลือกบัสบาร์ที่เหมาะสมสำหรับ MCB.
บล็อกเทอร์มินัล
Terminal blocks need more than high conductivity. The terminal metal must also provide clamping strength, corrosion resistance, stable contact pressure, manufacturability, and compatibility with copper or aluminum conductors.
That is why many terminals use copper alloys or brass rather than pure copper. Pure copper is very conductive, but some alloys provide better stiffness, forming behavior, or screw-clamping performance.
หน้าสัมผัสไฟฟ้า
For contacts, the surface is often more important than the bulk conductor. A small contact area carries current through microscopic contact spots. Contact pressure, surface film, oxide behavior, plating, and arc erosion can dominate actual performance.
This is why silver alloys, silver plating, copper-tungsten, and other contact materials are used even when their bulk conductivity does not look ideal on a simple table.
Grounding Parts
Grounding parts need low impedance and mechanical reliability. Conductivity matters, but corrosion resistance, connection integrity, and long-term bonding are just as important. A ground bar or PE bar with poor joint contact can perform worse than the material table suggests.
For grounding component context, see บัสบาร์นิวทรัลเทียบกับบัสบาร์กราวด์ แล้ว ชุดฉนวนกราวด์บาร์คืออะไร.
Common Mistakes When Comparing Conductive Materials
Mistake 1: Treating conductivity as the only selection factor
High conductivity is valuable, but it does not solve mechanical strength, corrosion, arcing, spring force, plating, or manufacturing issues.
Mistake 2: Comparing pure metals to real alloys
Datasheet values for pure copper, pure aluminum, or pure silver may not match real stamped, plated, heat-treated, or alloyed components.
Mistake 3: Ignoring temperature
Conductivity and resistivity are temperature-dependent. A value stated at 20°C is not the same as behavior inside a warm distribution board or control cabinet.
Mistake 4: Using stainless steel as a current path
Stainless steel hardware can be mechanically useful, but it should not be treated as equivalent to copper or aluminum for primary current conduction.
Mistake 5: Forgetting contact resistance
In bolted joints and switching contacts, the interface can dominate the actual resistance. Plating, surface finish, torque, contact pressure, and oxidation may matter more than the bulk material number.
คำถามที่พบบ่อย
What does %IACS mean?
%IACS means percent International Annealed Copper Standard. It compares a material’s conductivity to annealed copper, where 100% IACS is commonly treated as about 58 MS/m at 20°C.
Is conductivity the same as resistivity?
No. They are inverse properties. Conductivity measures how easily current flows. Resistivity measures how strongly a material resists current flow. Higher conductivity means lower resistivity.
What is the formula between conductivity and resistivity?
The basic formula is σ = 1 / ρ. If conductivity is in MS/m and resistivity is in μΩ·cm, a convenient conversion is ρ = 100 / σ.
Why is copper used more than silver if silver is more conductive?
Silver is more conductive than copper, but it is much more expensive and not necessary for most bulk conductors. Silver is often used as plating or contact surface where contact resistance, surface behavior, or high-frequency performance matters.
Why does aluminum need a larger cross-section than copper?
Aluminum has lower conductivity than copper, typically around 60-64% IACS for high-conductivity aluminum. To achieve similar resistance, aluminum generally needs a larger cross-sectional area.
Is stainless steel conductive?
Yes, stainless steel conducts electricity, but poorly compared with copper and aluminum. It is useful for mechanical and corrosion-resistant parts, not for main current-carrying conductors.
Is tungsten a good conductor?
Tungsten conducts electricity, but not nearly as well as copper or silver. Its value in contacts comes from high-temperature and arc-resistance behavior, not maximum conductivity.
Does plating change conductivity?
Plating can strongly affect contact performance, especially at the surface. Tin, silver, and nickel plating may be used for corrosion resistance, solderability, contact resistance, or wear behavior. The best plating depends on the electrical and environmental duty.
สรุป
Conductivity, resistivity, and %IACS are three ways to compare how well a material carries current. For electrical products, the practical hierarchy is simple: silver is the most conductive common metal, copper is the main engineering reference, aluminum trades lower conductivity for weight and cost advantages, tungsten-based materials trade conductivity for arc resistance, and stainless steel is mainly structural rather than conductive.
For VIOX product applications, these values matter in busbars, terminal blocks, grounding components, contact materials, MCB/MCCB conductive parts, and switchgear joints. But the material table is only the starting point. Real electrical performance also depends on geometry, temperature rise, contact pressure, plating, corrosion, arc duty, and manufacturing consistency.
