Conductor Facts: Electrical Resistance

Electrical resistance of conductors is usually expressed in terms of ohms per unit length. In the English system, it is Ω/mft (ohms per 1000 feet), in metric Ω/km (ohms per 1000 meters) at a standard temperature of 20ºC (68ºF).

Measurement

Standard procedure measures the DC resistance of a 5 foot minimum length (1.5 meters) and converts into the units Ω/mft or Ω/km. Utilizing a resistance bridge prevents resistance heating of the sample, especially in smaller gauges.

Definitions

• Electrical Resistivity
• A material’s electrical resistance per unit volume. Resistivity is a material property and is independent of its geometry (cross-sectional area and length). High resistivity designates the material as a poor conductor of electricity. Electrical resistivity is expressed in Ω-inch (or Ω-cm) etc.
• Electrical Conductivity
• The inverse of resistivity. It is a measure of a material’s ability to conduct electric current, usually compared to copper, and is generally stated in terms of %IACS (International Annealed Copper Standard).
• Temperature Coefficient of Resistance
• A constant which reflects the change in a material’s electrical resistance (resistivity) due to a change of one degree in temperature. It is expressed in unites per ºC (or units per ºF).

Conductor Resistance

R = ρL/A

Where R is resistance in ohms, ρ is volume resistivity, L is length of specimen and A is the cross-sectional area of the specimen.

Conductivity and Resistivity (ρ) of Common Conductor Alloys

Temperature Correction

Ambient temperature affects the electrical resistance of most metals. In general, a higher temperature will increase the resistance. Readings must be corrected to a standard reference temperature, generally 20ºC (68ºF), for proper interpretation.

The formula for temperature correction is:

RT = Rt/[1+α(t-T)]

Where RT is resistance at reference temperature T, Rt is resistance measured at temperature t, α is temperature coefficient of resistance, T is the reference temperature (normally 20ºC (68ºF), and t equals the measurement temperature.

Temperature Correction Factors (α) for Common Conductor Alloys at 20ºC (68ºF)

Note: The coefficient α varies with the material, the conductivity, and the temperature range. The value for 100% IAC conductivity copper at 20ºC (68ºF) is 0.00393. Values for coefficients of other materials, conductivities, and temperatures can be found in NBS Handbook-100, table 2.

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