The difference between a designed and actual transmission-line conductor sag value due to conductor properties' dispersion and measurement errors is estimated. Total error of sag calculation is presented as a weighted sum of partial errors of six measured/specified parameters involved in the state equation. The resulting formula for weighting factors for partial error is obtained by differentiating the state equation and enables analysis of the contribution of each error. It is shown that the factors for error in elasticity modulus or in creep values are small, so these errors have a weak effect on final result error. In contrast, weights for errors in sag and temperature measurements during stringing operation are large, and these errors contribute significantly to final sag error. Derived formula for weighting factors provides the tool to evaluate the total error of sag calculation and enables establishment of the possible limits for transmission-line thermal rating. Absolute error of sag calculation for the ACSR conductor is about 1.5 m for a 350-m span and about 2 m for a 500-600 m span. Absolute sag error for the AAAC Al59 conductor is 25% less.
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A. Polevoy (2014) studied this question.