These custom ferrites are made of a special high-frequency material and uniquely modified to help prevent high-current saturation. A ferrite is clamped around the insulated incoming power wires in your breaker panel. Ferrite dimensions: Inner Diameter 0.72″, Outer Diameter 1.5″, Length 1.9″. MADE IN THE USA These custom ferrites are made of a special high-frequency material and uniquely modified to help prevent high-current saturation. A ferrite is clamped around the insulated incoming power wires in your breaker panel. Most homes require three ferrites, but a few homes may require four ferrites