A setup based on a direct current (dc) comparator, typically employed in high-current low-resistance calibrations, has been developed for the accurate calibration of dc current ratios for dc currents up to 600 A. For primary currents above 100 A, multiple turns of the primary conductor are made through the device under test. A full uncertainty analysis reveals that the 1 part in 10⁶ total expanded uncertainty (k = 2) in a calibration of a zero-flux device is limited by the behavior of the device, in particular the instability and sensitivity to the position of the primary current conductor.