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Uncertainty is a major issue facing electric utilities in planning and decision making. Modeling uncertainty can be based on the approach called “Unknown but Bounded” in which upper and lower limits on the uncertainties are assumed without probability distributions. Interval mathematics provides a tool for the practical implementation and extension of the unknown but bounded concept. The calculation of transmission line sag and inductance are used as examples to illustrate the use of interval mathematics. The purpose of sag calculations is to determine the change in cable length, given a fixed span and an interval of cable sag values, for numerous variations in temperature. The resulting change in cable length will therefore be an interval corresponding to the interval of cable sag values. Inductance values are also calculated using the traditional single point numbers as well as interval numbers. The values from these two methods are compared to prove the validity of interval analysis.