08 — 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.
- —%
Please fill all fields with valid numbers.
- Opens the print dialog. Choose “Save as PDF” as the destination.
- Opens your email program with the values from this calculation. Nothing is sent to us until you send it yourself.
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.
Worked 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
- 53.7 °C
- Utilisation
- 68.3 %
- Temperature rise above ambient
- 18.7 K
- Resistance increase over 20 °C
- 13.2 %
Formula
- ϑ_conductor = ϑ_ambient + (ϑ_max − 30 °C) · (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.
- The reference is the 30 °C ambient for which the tabulated capacity applies.
- 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.
Frequently asked
What do I need the conductor temperature for?
Two things. First, for an honest voltage drop: 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.
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.
More tools
- 01Cable cross-section
- 02Voltage drop
- 03Cable resistance
- 04Cable losses
- 05Maximum cable length
- 06Current-carrying capacity
- 07Parallel cables
- 09Cable capacitance
- 10Short-circuit current
- 11Cable impedance
- 12Network impedance
- 13Short-circuit current at the far end
- 14Thermal short-circuit withstand
- 15Length and disconnection
- 16Motor current
- 17Torque
- 18Starting current
- 19Star-delta starting
- 20Soft starting
- 21Motor efficiency
- 22Speed control instead of throttling
- 23Speed and slip
- 24Setting the motor protection
- 25Single-phase motor
- 26Motor feeder
- 27Required motor rating
- 28PFD and SIL
- 29PFH and SIL
- 30Protective conductor size
- 31Earth rod
- 32Touch voltage
- 33Residual current protection
- 34Check discrimination
- 35Connecting a surge arrester
- 36Enclosure cooling
- 37Reference designation
- 38Reactive power compensation
Contact
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