Electric wire is a generic term for metal wires that transmit electric power and signals. Electric wires can be categorised into power line wires and communication wires according to the intended use. This section provides an overview of electric wires and introduces examples of how to observe and measure them using a digital microscope.

Differences between Electric Wires and Cables

Electric wire is a generic term for metal wires that conduct electricity. Some wires are categorised as cables based on structural differences.

Electric wire (isolated wire)
An electric wire is a conductor, which transmits electricity, that is covered with an insulator, which does not transmit electricity.
A cable is a bundle of electric wires covered with a sheath.
  1. A: Conductor
  2. B: Insulator
  3. C: Sheath
  4. D: Filler
  5. E: Binder tape

Conductor Materials and Characteristics

Almost all conductors used for electric wires and cables are made from copper or aluminium.
Based only on conductivity, silver and gold are also good materials. However, these materials are rarely used because they are expensive.

Conductor materials and their conductivity and resistivity
Material Conductivity (% according to the International Annealed Copper Standard (IACS)) Resistivity (10-6 Ωm)
Ag (silver) 106.4 0.0162
Cu (copper) 100 0.0172
Au (Gold) 71.8 0.024
Aluminium (Al) 61.7 0.0275

Characteristics of copper and aluminium

Copper and aluminium conductors used for electric wires and cables have the following characteristics.

  • Copper conducts electricity very easily with its very high conductivity.
  • Copper basically does not oxidise in dry air at room temperature.
  • Copper is generally used as the conductor in commonly used cables.
  • The density of aluminium is lower than copper, with a weight of one-third that of copper. This light material is suitable for long-distance applications such as power lines.
  • When aluminium oxidises, the surface is covered with an alumina layer, which is resistant to corrosion.
  • Aluminium is cheap and available at around one-third to half the price of copper.

Differences in Regulations for Electric Wire Cross-sectional Areas between Japan and the United States

In Japan, the cross-sectional areas of electric wires (stranded wires) are regulated with the Japanese Industrial Standards (JIS). The unit is SQ, which originated from an English measure for cross-sectional areas, square millimetre. The UL standards in the United States use American wire gauge (AWG). The table below shows conversions between AWG (UL) and SQ (JIS).

Conversion table between AWG (UL) and SQ (JIS)
Wire gauge (UL standards) Outer diameter (mm) Cross sectional area (mm2) Corresponding SQ size (JIS)
AWG 4/0 11.68 mm 107.2 mm2 100 SQ
AWG 3/0 10.40 mm 85.03 mm2 80 SQ
AWG 2/0 9.266 mm 67.42 mm2 60 SQ
AWG 1/0 8.254 mm 53.49 mm2 60 SQ
AWG 1 7.348 mm 42.41 mm2 38 SQ
AWG 2 6.543 mm 33.63 mm2 38 SQ
AWG 4 5.189 mm 21.15 mm2 22 SQ
AWG 6 4.115 mm 13.30 mm2 14 SQ
AWG 8 3.264 mm 8.37 mm2 8 SQ
AWG 10 2.588 mm 5.26 mm2 5.5 SQ
AWG 12 2.052 mm 3.31 mm2 3.5 SQ
AWG 14 1.628 mm 2.08 mm2 2 SQ
AWG 16 1.290 mm 1.31 mm2 1.25 SQ
AWG 18 1.024 mm 0.823 mm2 0.75 SQ
AWG 20 0.8128 mm 0.519 mm2 0.5 SQ
AWG 22 0.6426 mm 0.324 mm2 0.3 SQ
AWG 24 0.5105 mm 0.205 mm2 0.2 SQ
AWG 26 0.4039 mm 0.128 mm2 0.12 SQ
AWG 28 0.3200 mm 0.0804 mm2 0.08 SQ
AWG 30 0.2540 mm 0.0507 mm2 0.05 SQ

Cross-sectional Areas and Allowable Currents of Electric Wires

The larger the cross-sectional area of an electric wire, the larger the allowable current.
The following table shows the allowable currents of typical copper wires.

Allowable current of a single wire
Diameter (mm) Allowable current (A)
1 mm 16
1.2 mm 19
1.6 mm 27
2 mm 35
2.6 mm 48
3.2 mm 62
4 mm 81
5 mm 107
Allowable current of a stranded wire
Cross sectional area (mm2) Allowable current (A)
0.9 17
1.25 19
2 27
3.5 37
5.5 49
8 61
14 88
30 139
50 190
100 298
200 469
400 745
600 930
800 1080
1000 1260

Electric Wire Observation and Measurement Examples Using Digital Microscopes

These are the latest examples of observation and measurement of electric wires using KEYENCE’s VHX Series 4K Digital Microscope.

Observation of electric wire surfaces
VH-Z20, 100×, ring illumination + diffuse attachment
The diffuse attachment allows for observation with no glare.
Depth composition of a cable cross-section
VH-Z20, 30×, ring illumination + HDR
Lower: Normal image, Upper: Depth composition + HDR image
The HDR function allows for detailed observation of cable cross-sections.
Observation of cracks on a metal electric wire surface
VHX-E100, 100×, ring illumination
Normal image
VHX-E100, 100×, ring illumination
Optical Shadow Effect Mode image
Fine surface cracks can be visualised using Optical Shadow Effect Mode.
Observation of a peeled copper wire
VHX-E500, 2000×, coaxial illumination
Observation of electric wire defects
VH-Z500, 3000×, coaxial illumination
The 3D depth composition function allows for accurate observation of defect shapes.
Profile measurement of welded parts on an electric wire
ZS-20, 100×, ring illumination
The weld metal that rises above the surface can be quantified using the 3D measurement function, which allows for accurate OK/NG judgement.
Thickness measurement of copper electric wire sheaths
VHX-E500, 500×, coaxial illumination
Quantification of air bubbles in an electric wire sheath
VH-Z500, 500×, coaxial illumination, before measurement
VH-Z500, 500×, coaxial illumination
Automatic area measurement image
The automatic area measurement function allows for accurate area measurement of each air bubble.
Observation of an electric wire sheath surface
VHX-E500, 1000×, ring illumination
Differences of surface conditions caused by differences in materials and manufacturing conditions can be visualised using the 3D depth composition function.