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Modern commercial direct-current current transformers (DCCT) can measure currents up to the kA range with accuracies better than 1 × 10 −5 . We discuss here a DCCT calibration method and its implementation with commercial instruments typically employed in low resistance calibration laboratories. The primary current ranges up to 2 kA; in the current range below 100 A the calibration uncertainty is better than 3 × 10 −7 . An example of calibration of a high-performance DCCT specified for primary currents measurement up to 900 A is discussed in detail. INTRODUCTIONDirect-current current transformers (DCCT) are the most accurate dc high-current sensors commercially available [1], reaching specified relative accuracies in the 10 −5 range and integral nonlinearities below 10 −6 . The verification of such high performances and the calibration of the DCCT ratio require metrological facilities capable of handling high currents, with high accuracy and automated operability [2][3][4][5]. Ultimate current ratio accuracy is achieved in cryogenic current comparators (CCC) [6]. In a CCC, ratio accuracy is obtained by constraining the magnetic flux (generated by the current being compared) within superconducting shields. An extremely high sensitivity is achieved with a superconducting quantum interference device (SQUID) flux sensor. Even though CCCs capable of handling currents up to 100 A have been realized [7], these devices are research instruments not available in calibration laboratories. Ferromagnetic-core, room-temperature current comparators (CC) are current ratio devices which can achieve ratio errors lower than 10 −7 [8], and can be self-calibrated through step-up procedures [9, 10] with similar levels of uncertainty. Thus, a CC can be employed as current ratio standard in a DCCT calibration setup. Although complex and expensive instruments, high-current CC are common in electrical calibration laboratories, since they are part of commercial resistance ratio bridges employed for the measurement of low-value resistors. These instruments include also current sources, detectors, and firmware for automated operation. The calibration of the DCCT ratio with a reference current ratio standard (possibly having a different nominal ratio) can be performed by different methods. Recent papers [11,12] describe a method based on the comparison of the voltages developed by the secondary currents of the devices being compared on calibrated resistance standards. Here we present a simple method that allows the calibration of the ratio of a DCCT by using commercial components, originally designed for the calibration of low-value resistors. This method does not require calibrated resistance standards; the accuracy, dependent on the primary current, is better than 3 × 10