Why Common-mode current is the portion of current that flows in the same direction, in phase, on every conductor of a cable, returning through ground or stray capacitance rather than through the other conductor. Because the currents do not cancel, the cable radiates like an antenna and also picks up RF, which is why common-mode current is a major cause of RFI and noise coupling. Ferrite chokes and common-mode chokes work by presenting high impedance to this in-phase current while leaving the wanted signal untouched.