In this paper a general method to determine the current on AC lines of high current multipulse transformer -rectifier is presented. This method is based on the Ampere's law application to the magnetic circuit of power transformer. It consider the current restrictions given by the rectifier topology. An analysis to determine AC currents is developed for a 12 pulses rectifier, which is formed by two 6 pulses rectifier connected in parallel, modified zig-zag + 22,5° and -7,5° (Group A). Results are compared with same AC lines current measured in a 36 DC load kA DC load, real case power rectifier of similar configuration. This Group A was a part of a 24 pulses power rectifier of a copper plant and it was connected in parallel with a 12 pulses 7,5° -22,5° (Group B). The method was also applied for an emergency connection that was used in the copper plant right after a transformer -22,5° failure, resulting a 24 non symmetric pulses controlled rectifier, 4 paralleled groups +22,5°, -7,5°, +7,5° and +15°. Results of the analysis are completely in agreement with real measurement.
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