300.3(C) Conductors of Different Systems.
(1) 600 Volts, Nominal, or Less. Conductors of circuits rated 600 volts, nominal, or less, ac circuits, and dc circuits shall be permitted to occupy the same equipment wiring enclosure, cable, or raceway. All conductors shall have an insulation rating equal to at least the maximum circuit voltage applied to any conductor within the enclosure, cable, or raceway.
Exception: For solar photovoltaic systems in accordance with 690.4(B).
FPN: See 725.55(A) for Class 2 and Class 3 circuit conductors.
Someone coming along later might not readily distinguish one wire from another and either get hurt or cause damage.
This is a prime example why untrained people should stay out of electrical components.