Skin effect arises because the changing magnetic field inside a conductor induces eddy currents that oppose current flow at the centre, so AC flows mostly within one skin depth of the surface. In copper the skin depth is about 9.2 mm at 50 Hz, 8.4 mm at 60 Hz and only about 65 micrometres at 1 MHz. It raises AC resistance, which is why large busbars are made as hollow tubes or spaced flat bars and high-frequency windings use Litz wire.
No point going to a solid 100 mm round bar, skin effect means the middle carries almost nothing, run two flat bars with a gap.
Related terms
Impedance
Total opposition a circuit offers to alternating current, combining resistance and reactance, in ohms.
Bus
Rigid conductor or common node in a substation or switchboard where several circuits connect and share power.
Harmonics
Voltage or current components at whole multiples of the supply frequency, produced mostly by nonlinear loads.