This study investigates the risks of non-conser-vative piping design according to ASME B31.1 for high-temperature piping subjected to long-term operation at high temperature in a creep regime based on a sensitivity analysis of the hold time. Design evaluations of high-temperature piping were conducted over a range of hold times in the creep regime according to ASME B31.1, which implicitly considers the creep effects, and the French high-temperature design code of the RCC-MRx, which explicitly considers the creep effects. Conservatisms were quantified among the codes in terms of the hold times. In the case of B31.1, the design evaluation results do not change depending on the hold time at high temperature, whereas in the case of RCC-MRx, they do. It was shown that the design limits of RCC-MRx were exceeded when the hold time exceeded certain values, whereas those of B31.1 were satisfied regardless of the hold times. Thus, the design evaluations according to B31.1 did not consistently yield conservative results and might lead to non-conservative results in the case of long-term operations in the creep range.