Final answer:
A main breaker is not required in a sub-panel if the sub-panel is in the same building as the main service panel with its own main breaker, and it's subject to local electrical codes.
Step-by-step explanation:
A main breaker is typically not required in a sub-panel if the panel is located in the same building as the main service panel, which already has a main breaker to protect the entire electrical system. The main service panel's breaker effectively acts as the overarching protective element, ensuring that the maximum current flowing to the sub-panels does not exceed safe limits. However, it's important to note that this is subject to local electrical codes and regulations, which can vary, so it's always best to consult with a licensed electrician or local building code authority.