An energy-saving device (1) inserted between a three-phase power supply (A) and a three-phase load (L), comprising a three-phase electrical transformer (10), each phase of which includes a transformation assembly (1 1) with a primary winding (2) connected at a first end (5) to one phase of the power supply (A) and electromagnetically coupled to a secondary winding (3) connected at its second end (S1) to one phase of the load (L). The device (1 ) involves the primary winding (2) comprising two portions (21, 22), where a principal portion (21) extends between a first point (PO) and a second point (P1), and the second portion (22) extends from the second point (P1) to a third point (P2). The device also involves each of the transformation assemblies (11) being dimensioned so that the value of the voltage (Vpo-p2) established between the first point (P0) and the third point (P2) of the primary winding (2) is in the range defined by the voltage (Vkvp), applied to the principal portion (21) multiplied by the coefficients 1.2043 - 2% and 1.2043 + 2%; the value of the voltage (Vso-si) between the first end (SO) and the second end (S1) of the secondary winding (3) is in the range defined by the voltage (Vkvp), multiplied by the coefficient 0.1021 - 5% and 0.1021 + 5%; the value of the current (IPO-PI) flowing through the principal portion (21) is in the range defined by the current (Ikas) flowing through the secondary winding multiplied by the coefficients 0.1133 - 5% and 0.1133 + 5%; the value of the current (lPi.p2) flowing in the second portion (22) is in the range defined by the current (Ikas) multiplied by the coefficients 0.0940 - 5% and 0.0940 + 5%; the value of the magnetic induction relating to the configuration defined by the first point (P0) and the third point (P2) of the primary winding (2) and of the secondary winding (3) is in the range defined by the coefficient of magnetic induction (Ckim) for the configuration defined by the principal portion (21) of the primary winding (2) and of the secondary winding (3) multiplied by the coefficients 0.9965 - 0.03% and 0.9965 + 0.03%.