Copper Resistivity
Copper resistivity is the resistance of a copper conductor 1 m long and 1 mm² in cross-section, and it is about 0.0175 Ω·mm²/m at 20 °C. It is the constant in all cross-section and voltage drop calculations. The value rises as temperature increases, meaning a hot cable drops more voltage.
The resistance of a conductor is found with R = ρ·L/A. The resistance of a 2.5 mm² copper strand 10 m long is 0.0175·10/2.5 = 0.07 Ω. This small number corresponds to a loss of 1.4 V at 20 A; it shows why the milliohm range has to be taken seriously in low-voltage DC systems.
- Temperature dependence: the resistance of copper rises as it heats, so a loaded and hot run drops more voltage than it does when cold.
- The aluminium difference: the resistivity of aluminium is markedly higher; a larger cross-section is needed for the same current.
- Joints and terminals are not included: the formula gives only the plain conductor; a loose terminal and an oxidised lug add extra resistance on top of it.
In practice keeping this constant in mind lets you make quick checks in the field: you can work out the resistance of a run of known cross-section and length in your head and compare it with the voltage drop you measure. If the measured value is markedly above the calculated one, the fault is not in the cable but at the connection points.
Context in which this term is used: Installation, redundancy and field continuity