Temperature

Prof. Dr. Thorsten Jungmann (last updated 2026-05-04)

Temperature (German: Temperatur) is one of the seven base quantities of the SI. In electrical engineering, it matters mainly because the electrical properties of materials and components depend on temperature. In electrical engineering, it is common to assume a temperature of 20°C for ‘room temperature’.

1 Quantity symbol and unit

The quantity symbol for temperature is $T$ . Occasionally, the Greek lower-case letter $ϑ$ (‘theta’) is also used as the quantity symbol.
The unit of temperature is the degree Celsius with the unit symbol $° C$ .

[T] = ° C

Alternatively, especially for temperature differences, the SI base unit kelvin ( $K$ ) is used:

[Δ T] = K

2 Practical and worked examples

The relationship between the two temperature scales:

T [K] = T [° C] + 273.15

0 K = - 273.15° C

(absolute zero)

0° C = 273.15 K

2.1 Temperature coefficient of a copper conductor

The resistance changes with temperature according to:

R (T) = R_{0} \cdot (1 + α \cdot Δ T)

Example: copper line with $R_{0} = 1.0 Ω$ at $20° C$ , $α_{C u} = 3.9 \cdot 1 0^{- 3} K^{- 1}$

At $T = 70° C$ : $Δ T = 70° C - 20° C = 50 K$

R (70° C) = 1.0 Ω \cdot (1 + 3.9 \cdot 1 0^{- 3} K^{- 1} \cdot 50 K) = \underline{1.195 Ω}

2.2 Power loss and heating

With a power loss of $P_{loss} = 50 W$ and a thermal resistance of $R_{t h} = 2.5 K/W$ , the excess temperature is:

Δ T = P_{loss} \cdot R_{t h} = 50 W \cdot 2.5 K/W = \underline{125 K}

At an ambient temperature of $T_{amb} = 25° C$ : $T_{co m p o n e n t} = 25° C + 125 K = \underline{150° C}$

2.3 Conversion kelvin ↔ Celsius

Physical calculations: $T = 300 K = 300 - 273.15 = \underline{26.85° C}$
Temperature difference: $Δ T = 80° C - 20° C = \underline{60 K}$

3 Typical orders of magnitude

Reference temperatures (German: Bezugstemperaturen):

Room temperature (electrical engineering standard): $20° C$ or $293.15 K$
Room temperature (physics): $300 K$ or $26.85° C$

Operating temperatures (German: Betriebstemperaturen):

Household appliances: $- 10° C$ to $+ 40° C$
Industrial electronics: $- 25° C$ to $+ 70° C$
Power electronics: $+ 85° C$ to $+ 125° C$ (junction temperature)
Transformers: up to $+ 105° C$ (winding temperature)
Incandescent bulbs: up to $2500° C$ (tungsten filament)

Limit temperatures:

PVC insulation: max. $70° C$
Silicone insulation: max. $180° C$
Semiconductor junction: max. $150° C$ (typical)

4 Example phrasings

‘The measurement is carried out at an ambient temperature of $23° C$ .’
‘With a temperature increase of $50 K$ , the resistance rises by 19.5 %.’
‘The maximum permissible operating temperature is $85° C$ .’
‘All material properties refer to the reference temperature of $20° C$ .’

Resistance with temperature coefficient:

R (T) = R_{0} \cdot (1 + α \cdot Δ T)

Thermal resistance and power loss:

Δ T = P_{loss} \cdot R_{t h}

Conductivity:

σ (T) = σ_{0} \cdot (1 - α \cdot Δ T)

Temperature coefficient:

α = \frac{1}{R _{0}} \cdot \frac{Δ R}{Δ T}

Resistor: temperature stability and temperature coefficient are important for precision
Conductor: the operating temperature determines the current-carrying capacity
Transformer: the winding temperature limits the continuous power
Semiconductor: the junction temperature determines service life and power

Wattsinside

Temperature

1 Quantity symbol and unit

2 Practical and worked examples

2.1 Temperature coefficient of a copper conductor

2.2 Power loss and heating

2.3 Conversion kelvin ↔ Celsius

3 Typical orders of magnitude

4 Example phrasings

Table of Contents

Backlinks

Wattsinside

Temperature

1 Quantity symbol and unit

2 Practical and worked examples

2.1 Temperature coefficient of a copper conductor

2.2 Power loss and heating

2.3 Conversion kelvin ↔ Celsius

3 Typical orders of magnitude

4 Example phrasings

5 Related quantities

6 Related components

Table of Contents

Backlinks