Passing a single wire through a hole in a metal cabinet should cause the metal to heat up.
Where a metal consumer unit or other enclosure is used, the line and neutral wires must pass through the same hole. If not, the magnetic field created by the current flow in the wire will create eddy currents in the metal, which can lead to overheating.
Passing both wires through the same hole avoids this, as the magnetic fields are cancelled out, the net current flow is zero.
In this experiment, a loop of wire is passed through two holes in a metal consumer unit box and the temperature is monitored.
Initially a current of 25A is applied. This does not cause any heating at all. Increasing the current to 125A causes the temperature to increase, but only by 2C.
At the domestic consumer unit scale (up to 100A), the overheating effect is negligible. However both wires should still be passed through the same hole, as this is required by BS7671.
The effect may be more evident at higher currents, however the equipment used in this experiment only goes up to about 140A, so any higher current tests will have to wait for another day.