A.The overflow drain on a tub will not prevent an overflow of the tub if the tub or tub/shower valve is flowing water faster than 2 or 3 gallons per minute. The main purpose of the overflow drain on a tub is to have a place for the water to exit as water is displaced when someone enters the tub. Most people fill up the tub and then turn off the water before entering the tub. The normal tub overflow is 1 1/2" diameter which can normally handle 2 to 3 gallons of water per minute if the piping is clean and free of any debris or hair. Many tub or tub/shower combination valves in use today were installed before water conservation laws were passed and are capable of allowing water flow rates of between 6 and 35 gallons per minute depending on the type of valve installed. As you can see, a normal, gravity fed, tub overflow will not be able to keep up if the faucet is running faster than 2 or 3 gallons per minute. To prevent a tub overflow we suggest you keep watch while your tub is filling with water.