Must coils be custom fitted to workpieces?

Things to consider heating different-sized workpieces

A coil used to heat a 30in workpiece can certainly be used to heat a 10 in workpiece given the following two criteria. First there should be enough spare capability in the power supply to overcome the coupling loss to the 10 in workpiece in the larger coil. Second, the time to heat the smaller workpiece may be much longer given the poor coupling to the part.