NEC 725.121(B) prohibits connecting transformers in _______ unless specifically listed for interconnection.

The correct answer, parallel, pertains to the specific requirements set forth in NEC 725.121(B) regarding the interconnection of transformers. This code section is designed to ensure the safe and effective operation of electrical systems, focusing on the potential hazards that can arise from improperly connected transformers.

When transformers are connected in parallel, several critical factors must be properly managed, including their phase relationships, voltage levels, and impedance characteristics. If transformers are not specifically listed for parallel operation, they can lead to issues such as circulating currents, overheating, and even equipment failure. This can also pose safety risks, including fire hazards and electrical shock.

By restricting the connection of transformers in parallel unless they are specifically listed for such configurations, the NEC aims to promote safety and reliability in electrical installations. This ensures that only those transformers designed to operate together under the specified conditions can be interconnected, thus minimizing the risk of serious electrical problems.

Options that suggest other forms of connection, such as series or combination, may be permissible under different rules, or may have their own sets of requirements to ensure safe operation. However, the critical focus of NEC 725.121(B) is directly on the conditions and safety associated with parallel connections.