What is NOT a typical installation requirement for backflow preventers according to standard practice?

The option indicating that backflow preventers are typically placed horizontally is not a standard installation requirement. In practice, backflow preventers can be installed in various orientations, but it is most important for them to be installed in a manner that allows for proper operation and maintenance. Many backflow preventers are designed to be installed vertically or in a way that advances their functionality, such as enabling gravity drainage of accumulated water or contaminants.

On the other hand, the other options represent typical and important installation requirements for backflow preventers. Accessibility is crucial for maintenance and inspection, ensuring that personnel can easily reach the device. Location above the highest inlet is essential to prevent any potential back siphonage, thereby maintaining the integrity of the potable water supply. Additionally, having a drain option installed is a common practice that allows for safe drainage of the device without causing a hazard or backflow issue.