Για να εμβαθύνουμε λίγο στο θέμα παραθέτω τα κάτωθϊ :
TREFOIL FORMATION
For short cable lengths single core cables may be laid in flat formation and it is fairly common to use wooden blocks to clamp the cable (metal loops e.g. clamps and bolts around individual cores shall be avoided).
For long cable runs the inductance of flat formation becomes too high and it is necessary to run cables in trefoil formation.
Trefoil formation is the preferred method of installing single core cables
και εδώ:
http://users.ntua.gr/stpapath/Paper_2.48.pdf
και λίγο "πιο βαθειά" εδώ :
http://tkne.net/downloads/power/cables/Basic Elect[1].Mod.E-...