The industry seems to be sanguine about the performance of dc power-flow models, but recent research has shown that the performance of different formulations is highly variable. Considering their pervasive use, the accuracy of dc power-flow models is of great concern. In this paper, three dc power-flow formulations are examined: the classical dc power-flow model, dc power-flow model with loss compensation and the so-called a-matching dc power-flow model. These three models are tested in three systems of different sizes, ranging from 10 buses to 62,000 buses. By comparing the dc power-flow results with the ac power-flow results, the paper concludes that the a-matching formulation has the highest accuracy among three dc power flow formulations.