Under which condition can circuit breakers be installed in parallel for a single load?

Circuit breakers can be installed in parallel for a single load when they are factory assembled. This condition ensures that the breakers are designed and tested together, which helps maintain coordinated performance and ensures that they share the load equally. Factory-assembled breakers are built to specific standards that take into account the characteristics of the individual breakers, such as their trip curves and operational parameters. This coherence prevents potential issues such as one breaker taking on more load than the other, which can lead to overheating or failure.

The other conditions listed do not meet the necessary requirements for safe and effective parallel installation. For instance, older models may not comply with current safety standards, and installing small loads might not justify the need for parallel setups. Additionally, installing circuit breakers separately would not result in the synchronized tripping and load distribution that factory-assembled versions ensure.