Correcting copper conductor resistance for temperature — the 234.5 formula and why R20 makes readings comparable

The [guide on R1+R2 protective conductor continuity measurement](/guides/nen-3140/continuiteitsmeting-r1-r2-beschermingsleiding) and the guide on micro-ohmmeter contact resistance measurement both cover how a resistance reading is taken and what threshold or comparison it is checked against. Neither of those guides covers a subtlety that matters as soon as readings are compared over time or against a calculated value: the resistance of a copper (or aluminium) conductor is not a fixed number, it depends on the conductor's own temperature at the moment of measurement, and a raw reading taken on a warm day is not directly comparable to the same reading taken on a cold day without correcting for that difference first.

Why conductor resistance depends on temperature

The resistivity of copper, like that of most metals, increases approximately linearly with temperature over the normal range encountered in electrical installation work. A conductor measured at a higher temperature therefore reads a higher resistance than the identical conductor measured at a lower temperature, purely because of the temperature difference, with no change in the conductor's condition at all. This matters in two related but distinct situations:

Trending a measurement over time — for example comparing this year's R1+R2 or micro-ohmmeter reading on a specific connection to last year's reading, to detect a developing high-resistance joint before it becomes a thermal problem (see the [thermography delta-T guide](/guides/practical/thermografie-delta-t-beoordelingscriteria-urgentie)). If the two readings were taken at different conductor temperatures, a rising trend can be partly or entirely explained by the temperature difference rather than an actual degradation of the connection.
Comparing a measured value to a calculated or tabulated value — for example checking a measured conductor resistance against the reference resistivity figures in IEC 60228, which are themselves stated at a defined reference temperature (20 °C).

The 234.5 formula

To make readings taken at different conductor temperatures comparable, the measured resistance is converted to its equivalent value at a common reference temperature, conventionally 20 °C, denoted R20. For copper, this conversion uses the formula:

R20 = Rmeasured × (234.5 + 20) / (234.5 + t_measured)

where Rmeasured is the resistance actually read at the conductor's temperature at the time of measurement (t_measured, in °C), and 234.5 is the conventional constant (in °C) associated with copper's temperature coefficient of resistance in this linear approximation. This constant represents the (extrapolated, not physically real) temperature at which copper's resistance would reach zero if the linear approximation were extended down to that point — it is a mathematical convenience derived from copper's resistance-temperature coefficient, not a physical property observed directly. Aluminium conductors use a different constant in the same formula structure (commonly cited as approximately 228, though the exact value varies slightly between reference sources) — the constant is material-specific and must not be reused between copper and aluminium calculations. Because the exact reference constant can vary slightly between standards and reference tables, the specific constant used should always be confirmed against the source document being followed for a given calculation rather than assumed.

What "conductor temperature" actually means here

The temperature that belongs in this formula is the conductor's temperature at the moment of measurement, not necessarily the ambient air temperature reported by a thermometer nearby. For a conductor that has been carrying load current recently, or that is measured with a test current high enough to cause noticeable self-heating during the test itself, the conductor can be measurably warmer than the surrounding air, and using the ambient reading instead of the actual conductor temperature introduces an error into the correction. For a de-energised conductor that has been at rest (no load current) for long enough to reach thermal equilibrium with its surroundings, the ambient temperature is normally an adequate proxy for the conductor temperature.

Practical relevance

When trending resistance measurements on the same connection or conductor over successive inspection cycles — whether R1+R2 continuity values from periodic NEN 3140 inspection or micro-ohmmeter contact-resistance readings from switchgear maintenance — recording the conductor temperature at the time of each measurement, and correcting every reading to R20 before comparing them, is what makes the trend meaningful. Comparing raw, uncorrected readings taken on different days at different temperatures risks either masking a real developing fault (if the more recent reading happens to be colder) or flagging a false alarm (if the more recent reading happens to be warmer), neither of which reflects the actual condition of the connection.

Common mistakes

Comparing raw resistance readings across different seasons or different times of day without temperature correction — a rising trend can be partly or entirely a temperature artefact rather than a developing fault.
Using ambient air temperature when the conductor itself is warmer, for example shortly after the conductor carried load current, or during a test with a self-heating test current — this understates the correction needed and leaves the R20 result too high.
Reusing copper's 234.5 constant for an aluminium conductor — the reference constant is material-specific; aluminium requires its own constant in the same formula.
Treating the 234.5 figure as a directly measurable physical property of copper — it is a mathematical convenience from the linear resistance-temperature approximation, not a temperature copper actually reaches zero resistance at.