Tub Spout Below Flood Rim

Definition of plumbing cross connection: A cross connection is defined as any actual or potential physical connection between a public water system or the consumer's water system and any source of non-potable liquid, solid, or gas that could contaminate the potable water supply by backflow.

The concepts behind the codes in regards to cross-connection is to prevent  possible contamination of the water supply. When it comes to bathtubs, the codes concerns themselves with proper fixture installation as well as minimizing the risk of “Back-siphonage” and cross connection. Back-siphonage is created by a difference in water pressures which causes water to flow back into the water supply  pipes (potable water supply).

How likely is this, probably not very likely but that is not the question, it could happen and our water supply is at risk.

To prevent the potable water supply system from being contaminated, the supply lines or fittings are required to be installed in a manner that will prevent Back-siphonage.  So you thinking how can that occur?

Let use a hypothetical scenario, you were bathing with a little fragrant bubbles and the spout of your tub faucet under the flood level (Flood Rim), such as in this photo, and there happens to be a break in the water main or there is a great demand due to a fire.

In this case the water supply pressure could quickly drop and possibly cause suction due to loss of pressure that could now draw your bubbly and dirty bathwater back into the faucet, through your supply lines and into the public water supply. Then now your neighbor can now enjoy your suds in their glass of ice water.