Calculators
Estimate conductor temperature in service
The permissible conductor temperature is only reached at full utilisation. Running at half the current means a markedly cooler conductor and a smaller voltage drop than a calculation at 70 °C suggests. This estimate gives both.
Result
- Expected conductor temperature
- —°C
- Utilisation
- —%
- Temperature rise above ambient
- —K
- Resistance increase over 20 °CBy this much the actual voltage drop exceeds the tabulated figure.
- —%
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A guide, not a design to standard. We give no warranty for the correctness of the results. The applicable standards and case-by-case verification govern.
The grey values are an example. Simply enter your own.
Matching services
Calculating is the easy part.
A formula gives you a number. Designing a plant also demands installation method, grouping, discrimination, the standards in force and a look at the installed base. That is what we take on.
Method and questions
Conductor temperature
How warm the conductor actually gets at partial load.
Discuss a projectWorked example
The default case the calculator starts with, worked through once. Change the values above and it recalculates immediately.
Inputs
- Operating current
- 28 A
- Current-carrying capacity as installed
- 41 A
- Ambient temperature
- 35 °C
- Permissible conductor temperature
- 70 °C · PVC
Result
- Expected conductor temperature
- 51.3 °C
- Utilisation
- 68.3 %
- Temperature rise above ambient
- 16.3 K
- Resistance increase over 20 °C
- 12.3 %
Formula and standards
Formula
- ϑ_conductor = ϑ_ambient + (ϑ_max − ϑ_ambient) · (I_B / I_z)²
- Resistance increase = α · (ϑ_conductor − 20 °C)
- α = 0.00393 /K for copper
Assumptions and standards
- The temperature rise grows with the square of the current. This applies to the steady state, not to short peaks.
- At I_B = I_z the conductor reaches exactly its permissible temperature. That is how capacity as installed is defined. The temperature rise therefore counts from the actual ambient.
- Enter the capacity as installed, that is, already corrected for grouping and temperature.
- An estimate, not a thermal calculation. Solar gain, neighbouring heat sources and varying load are not included.
What do I need the conductor temperature for?
Two things. First, for an honest voltage drop, because resistance depends on temperature, and calculating at 70 °C is too pessimistic at half load. Second, for the heat balance in an enclosure or a cable duct.
Why the square?
Because power loss grows with the square of the current, and in the steady state the temperature rise is proportional to the power loss. Half the current means a quarter of the heating.
Does this hold under strongly varying load?
Only to a limited extent. Depending on cross-section a cable needs minutes to hours to reach its final temperature. Under short peaks it stays cooler than calculated. Under long full-load periods it reaches the value.
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