I'm in the tool repair business. What size bit are you going to be using and what model of hammer drill is it you are renting along with what is the amp draw. Reason I ask is everybody will use a million miles of extension cord of too small of a gauge and they will totally and I'm mean totally burn out the tool. At a draw of 15 amp and assuming that your voltage is clean (110-115v) you will need a 10 gauge cord at a max length of 25 feet. If you don't follow this you will be buying yourself a burnt out hammer drill from the rental shop with holes still left to drill. A sharp bit is also essential.