What issue arises from using two conductors under a screw rated for only one?

Using two conductors under a screw that is rated for only one can lead to a potential overload situation. This occurs because each conductor carries its own current, and with both connected to a single point that is not designed to handle the combined load, the screw connection may not be able to manage the total amount of current flowing through it.

When a connection is overloaded, there is an increased risk of overheating, which can lead to insulation failure, sparking, or even fire. Additionally, the physical connection may not be secure, potentially causing intermittent contacts that can lead to arcing and further hazards. Therefore, the issue of overload is a significant safety concern when improperly connecting multiple conductors, emphasizing the importance of adhering to the manufacturer's ratings and electrical code requirements.