Our Voltimum Experts answer your questions on a daily basis in our Technical Expertise area. This Question of the Day, about tests and a calculation concerning a cable installation in the ground, is answered by SELECT:
The installation concerned has cable that is 16mm directly in the ground, the length is 560m and the current is about 13A with 243V at source.
The calculation I am using is 2.9 x 13 x 560/1000 = 21.1V. This must be wrong. I have test results back: Ve = 236V, a change of 7V which is under 3%. I must have missed something out, as the calculated and actual results are miles apart.
If somebody could explain my mistake, I would appreciate it, and if you could also show how to calculate Ze for this, I would be grateful. Also, I know that Zs = 0.24 ohms. Does Ze = Zs+(R1=R2)?
Answer: Assuming that the cable is an SWA type (non-thermosetting), using Table 4E4, a 16mm cable will drop between 2.9 and 2.5mV/A/m. Therefore, 2.9 x 13 x 560 / 1000 = 21.12V. You were right!
Therefore, a much larger cable is required. The main problem here is the distance of the run. Remember that measurements should be carried out with the load connected.
Generally speaking Zs values = Ze + (R1 + R2) approximately.
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