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Ever since it was demonstrated that installed extruded dielectric underground distribution cables lose significant dielectric strength due to formation of electrochemical trees in the insulation users have sought a means to determine the remaining useful service life. Subjecting miniature and full size crosslinked polyethylene (XLPE) insulated cables to accelerated aging tests and comparing the results to aging of I5kV cables removed from service the authors show how the remaining service life of installed cables can be estimated from dissipation factor measurements. This non-destructive test is made utilizing the available system voltage and would require the cable to be disconnected only several hours from service. The method devised to estimate service life is applicable to both polyethylene (PE) and XLPE insulated cables and its accuracy will be increased as more dissipation factor data is taken on cables now in service.