Where can main condensate lines legally drain?

The correct answer is that main condensate lines can legally drain through an air break, plumbing fixture tailpiece, or to the exterior. This option adheres to plumbing codes that aim to prevent contamination of potable water systems while allowing for effective drainage of condensate.

An air break is a method used to prevent backflow, ensuring that any contaminated water does not siphon back into a clean water supply. This is crucial in maintaining safe drinking water. Connecting to a plumbing fixture tailpiece further allows for proper drainage without compromising the plumbing system's integrity. Draining condensate to the exterior is also acceptable as long as it is done in a way that does not create standing water or lead to drainage issues on the property.

The other options do not align with plumbing codes or best practices for condensate drainage. For instance, draining only into a sink can pose risks of backflow and contamination. Additionally, relying on a p-trap alone could lead to potential trap siphoning issues, where the water seal could be lost, allowing sewer gases to enter. Lastly, draining directly into the nearest floor drain may not provide the necessary protection against backflow and siphoning, and might not be compliant with local regulations.