Eddy currents flow not only in each individual lamination, but also within the core body across lamination surfaces causing extra power loss in the core. In order to control and minimize the inter-laminar losses, it is necessary to provide adequate insulation between laminations.

In cores where the volts per turn, which is the effective potential in producing eddy currents is relatively low; the inter-lamination resistance need not be very high. In small cores where volts-per-turn is low and stack< pressure is not excessive, oxide blue coating produced on a rough material surface such as CRML steel is sufficient insulation to effectively limit the inter-laminar eddy current losses.

In applications using wide laminations operating at high induction or high frequencies, where volts per turn are high, additional insulation may be necessary. There are several types of insulation such as C5, C4, and C3 that generally prove to be adequate for a variety of applications. For grain oriented materials a C5 coating is applied on top of the C2 coating for high insulation resistivity.

In general, the type of insulation and its method of application depends on various design factors such as method of holding the laminations in a core assembly, the mechanical pressure applied to the core, the type of core material and frequency and induction level.