It is _______ in many jurisdictions for electrical wiring and plumbing pipes to be run inside return ducts.

In many jurisdictions, it is generally considered unacceptable for electrical wiring and plumbing pipes to be run inside return ducts. This is due to the potential hazards and code violations associated with having these systems in close proximity to the air return pathway. Return ducts are designed to channel air back to the HVAC system, and introducing electrical or plumbing fixtures can create several issues.

Having plumbing inside a return duct can lead to the risk of leaks or condensation, which can impact air quality and potentially lead to mold growth. Additionally, electrical wiring can create safety hazards due to the possibility of electrical shorts or interference with airflow, which affects system efficiency.

Furthermore, building codes often specify regulations concerning the routing of electrical and plumbing systems to ensure safety and functionality. Therefore, the correct understanding would lean towards the conclusion that such practices are unacceptable in most cases due to health and safety regulations.