What is the most common coupling size for both wye and Siamese fittings?

The most common coupling size for both wye and Siamese fittings is 2.5 inches. This size is prevalent in fire service operations due to its balance between adequate flow capacity and manageable weight, making it effective for both supply and attack lines. Wye and Siamese connections enable firefighters to split or combine water flows, necessitating a coupling size that can sustain the pressure and volume required in fire fighting scenarios. The 2.5-inch size meets the needs for larger volumes of water while fitting standard equipment and hoses used in the field, allowing for efficient connections and operations.