AbstractThis paper describes a method for calculation of the effective relative dielectric constant εs of VVF cable. For power line communications (PLC), VVF cables in homes are used as 2‐ to 30‐MHz high‐frequency transmission media. As they are not designed and installed for such transmission media, there are many branches and impedance mismatch points that cause PLC signal reflections and standing waves. In order to estimate such standing wave generation and estimate undesirable PLC signal radio wave leakage, the length of the cable and the relative dielectric constant εs are important. However, the insulator of VVF cable is thin and not symmetric, so that it is not easy to calculate effective εs analytically. In this paper, the effective εs is calculated numerically using the PC software EXCEL by drawing the electrical fluxes and analyzing the effect of the insulator for each flux. The calculation result for VVF cable considering the limited insulator area around the conductors shows that the effective εs is about 13% less and the transmission speed is 7% faster than when the insulator occupies the full space around the cable. © 2009 Wiley Periodicals, Inc. Electr Eng Jpn, 167(4): 24–31, 2009; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/eej.20862