The system (20) according to the invention for calculating an electric quantity relative to an electrical installation comprising several secondary electrical conductors (42A, . . . , 48C) electrically connected to a primary electrical conductor (34; 36; 38). This system comprises a first module (60) including a wireless transmitter (70) and a plurality of second modules (62A, 62B, 62C). Each second module includes first means (84A, 88A; 84B, 88B; 84C, 88C) for receiving a first synchronization message (M1) and second transmission means (84A, . . . , 88C) for sending a second message (M2A, M2B, M2C) containing at least one intensity value measured by the corresponding current sensor to a third module (63). The intensity values are measured quasi-simultaneously and the third module includes a unit (104) for calculating the electric quantity from intensity values received via said second messages.