This paper investigates the dependency of the critical content for electrical conductivity of carbon powder-filled polymer matrix composites with different matrixes as a function of the carbon powder content (volume fraction) to find the break point of the relationships between the carbon powder content and the electrical conductivity. The electrical conductivity jumps by as much as ten orders of magnitude at the break point. The critical carbon powder content corresponding to the break point in electrical conductivity varies according to the matrix species and tends to increase with an increase in the surface tension of the matrix. In order to explain the dependency of the critical carbon content on the matrix species, a simple equation (Vc * = [1+ 3(γc 1/2 . γm 1/2)2/(ΔqcR].1) was derived under some assumptions, the most important of which was that when the interfacial excess energy introduced by particles of carbon powder into the matrix reaches a universal value (Δqc), the particles of carbon powder begin to coagulate so as to avoid any further increase in the energy and to form networks that facilitate electrical conduction. The equation well explains the dependency through surface tension, surface tensions between the particles of carbon powder.