Bava Metzia 80 - Transportation Rentals
If one rented a donkey on which to bring wheat and he brought barley instead - the renter is liable for the injury to the donkey. Barley is lighter than wheat, so the renter first loaded the same volume of barley as of wheat, but then loaded some more, to bring the weight up to the weight of wheat. The extra bulkiness of the load, however, is hard on the donkey, and that is why the renter is liable.
With the same type of load as mentioned in the rental agreement, what is the amount of overload at which the renter becomes liable for injury? 1/30, and this applies to animals, boats, and human porters.