What is used to connect lengths of rigid conduit together to create a long run?

In the context of creating a long run of rigid conduit, couplings are the correct choice because they are specifically designed to join two lengths of conduit together. Couplings provide a secure, stable connection that maintains the integrity of the overall electrical system by ensuring that the electrical pathway remains continuous. They are essential in maintaining the structural and electrical properties of the conduit system.

While adapters, inserts, and connectors are all components used in electrical installations, they serve different purposes. Adapters are typically used to connect conduit of different sizes or types. Inserts, in some contexts, can refer to components used to reinforce or facilitate the attachment of additional components, whereas connectors generally relate to devices that connect conductors rather than conduit sections. In this instance, couplings are the recognized standard for extending rigid conduit runs, which is why they are the correct answer.