What limitation exists regarding shared boxes?

Shared boxes must have conductors from the same circuit to ensure safety and proper operation. This requirement is critical to prevent potential overloads and ensure that all devices connected to the box can be de-energized simultaneously without affecting the operation of other circuits. Additionally, having conductors from the same circuit allows for a more efficient means to manage the electrical loads and ensures compliance with the National Electrical Code.

Using conductors from different circuits in a shared box could lead to several issues, such as unintentional overloading, and complicates the identification of circuits for maintenance or troubleshooting. It may also create safety risks, such as potential electrical shock or fire hazards, which the NEC aims to prevent through its guidelines.

The other options present misunderstandings regarding the NEC's provisions for shared boxes. For instance, shared boxes are not specifically limited to non-metallic conduits, nor is there a requirement for installation by a licensed electrician just for the act of sharing a box. Furthermore, using shared boxes for any circuit combinations would violate the NEC's safety regulations.