On a three-phase single-core run, the arrangement of the three cables is an electrical decision, not a matter of appearance. Each core carries alternating current and produces a magnetic field. Grouped tightly in a trefoil triangle, the three fields largely cancel outside the group; laid flat or spaced apart, they do not, and the difference shows up as heat and reduced capacity.
Trefoil
Trefoil is the three cables clipped together in a symmetrical triangle. With the phases at equal spacing, the mutual inductance between them balances and the net external field is small. This keeps induced currents low in nearby steelwork, cable glands and any single-core armour, the places where stray fields cause unwanted heating, and it keeps the three phase impedances equal so the load shares evenly. Trefoil is the default for most MV and HV single-core runs.
Flat
Flat formation places the three cables side by side. Touching-flat gives an unbalanced field and unequal impedances, so it is less satisfactory electrically. Spaced-flat, with a gap of roughly one cable diameter between phases, restores neither balance but provides air around each cable and better heat dissipation, which can raise the current rating on a heavily loaded run, and it eases the pull on a wide tray.
Selecting a formation
Trefoil suits balanced impedance, a low external field and neat cleating in ducts and on walls. Spaced-flat suits runs that need every available amp and have the width for it. In either case single-core cables must be cleated firmly, since a short circuit forces the phases apart with considerable energy, and the current rating must be re-checked whenever the formation changes, because the derating differs. Heavily loaded feeders are also where MV and HV XLPE cables and 105°C high-ampacity XLPE cables are specified.