In which situations does the basic rule of 300.15 require a box or fitting to be installed?

The basic rule of 300.15 in the National Electrical Code (NEC) primarily addresses the requirements for installing boxes or fittings where conductors are connected or routed. This rule is critical for ensuring the safety and functionality of an electrical system.

When multiple conductors are used, consolidation and proper routing can lead to potential overcrowding or stress on the conductors, making it necessary to utilize a box or fitting to provide adequate space and support for the connections.

Additionally, when a fitting is required for connection, this also necessitates the use of a box or fitting as it ensures that the connections are protected and meet safety standards.

Therefore, the correct answer encompasses both scenarios: the requirement for a box or fitting arises not just when multiple conductors are present but also when a fitting is needed for making secure electrical connections. This comprehensive understanding of the rule ensures that electrical installations are conducted safely and in compliance with the NEC.