When would you use single-phase versus three-phase power distribution?

Single-phase power is used for residential buildings, small offices, and loads up to about 5 kW where simplicity and lower cost are priorities. Three-phase power is required for larger commercial/industrial facilities, motors above 2-3 HP, and loads exceeding 5-10 kW. Three-phase offers more efficient power transmission, constant power delivery for motors, and ability to handle larger loads with smaller conductors.