This paper presents a parametric study of the major factors affecting cable ampacity calculations. The current carrying capacity (or ampacity) of a cable depends on many of the installation properties and conditions. In this paper the effects on ampacity of conductor size, ambient temperature, bonding arrangement, duct size, soil thermal resistivity, resistivity and size of backfill (or duct bank) and depth of installation for underground installations are presented. For cables air the effects on ampacity of the intensity of solar radiation, the spacing from the wall and the grouping of cables are analyzed. For riser pole installations the effect of the solar radiation, wind speed, ventilation and diameter of the duct are shown.