Cable temperature

[Prospero]

How can I find out the average conductor temperature of a cable, if I know the th external temp of the cable, the ambient temp/humidity and the cable type/construction?

I have got some 4 core (TP&N) XLPE SWA cu conductor cables in the range 35-300mm, which are quite warm to the touch.

 

[RajT]

A typical XLPE cable will have a max continous conductor temp of 85deg C at 40 deg ambient.This rating is for a fully loaded of the cable.

 

[jghrist]

Not easily, especially with 4-core cable. Without access to the conductor itself, you would have to calculate the temperature using thermal modelling techniques such as the Neher-McGrath or IEC 287 methods.

Basically, you calculate the heat generated by I2R losses in the conductor and shields, then calculate the temperature rise through the thermal resistance of the cable insulation and jacket.

Rating of Electric Power Cables by George J. Anders, IEEE Press, 1997 gives the theory and equations for calculating ampacity. Calculations for temperature rise could be developed using the same theoretical methods. Good luck.

On second thought, you might be able to calculate the resistance if you could measure sending end and receiving end voltage and current (including phase angles). Then measure the resistance of an unenergized cable at a known temperature. You could find the temperature with the equation:

T2 = R2*(T0 + T1)/R1 - T2

Where R2 is the resistance of the energized cable, R1 is the resistance of the unergized cable, T0 is 228 degC for aluminum or 234.5 degC for copper, T1 is the temperature of the unenergized cable, and T2 is the temperature of the energized cable.

You would have to use the resistance, not the impedance of the cable, thus the need to take phase angles into account in measuring voltages and current.

 

[RajT]

Jghirst,
I beg to defer with you in that the query is about the cable getting warm due to conductor temp.No need to get get therotical as the cable manuf data defines the max temp and that is all is required for application engineering.

 

[jghrist]

RajT,
But this does nothing to answer Prospero's question. How does knowing the maximum permissible conductor temperature help in finding out what the actual temperature is?

 

[Prospero]

Also, having an idea of how the external temp relates to the conductor temp and hence current, could this be a method to determine the loading on the cable?

 

[RajT]

All one wants is an indication to the state of the cable.
So if the ambient is 40deg or less and if the cable is not overloaded at its 40deg ambient rating factor then it does not matter how hot it feels as the conductor temp would not have exceeded 85 deg.