Good question. The answer isn’t cut and dry, but it does let us share some fun facts about how it might have come into common practice.
- Historically, during the early days of inside plumbing, only a single cold-water pump/faucet was the norm. It was placed on the right side of the sink at that time because most folks are right-handed. When the option for hot water came about — it seemed logical that it would be placed on the left side of the sink.
- Today, the Uniform Plumbing Code (UPC) specifies that hot water should correspond to the left side of any fittings. For safety reasons, always having hot water on the left and cold-water access on the right helps prevent accidental scalding or injuries.