What is the most important reason for not having a power line splice in a conduit run between boxes?

The most significant reason for not having a power line splice in a conduit run between boxes is that it is an unsafe practice. Splices can create potential failure points in electrical systems, which may lead to overheating, arcing, or electrical shock hazards. In a conduit, the airflow is typically restricted, and the heat generated by a splice could escalate, increasing the risk of fire or damage to insulation and nearby wiring.

Maintaining the integrity of power lines is essential for both safety and reliability, so keeping splices to a minimum and ensuring they are made in appropriate settings — such as within junction boxes designed for that purpose — is crucial. This enhances safety and ensures compliance with electrical codes designed to minimize risks.

While other options may hint at issues related to practices or effects on communication lines, the primary concern always stays rooted in safety, making it the most critical aspect in electrical work.