Water is drawn in to a pump due to presence of atmospheric pressure. A pump can only pressurize a liquid while creating a low pressure at the inlet area. The liquid is pushed into the inlet area due to atmospheric pressure. The value of atmospheric pressure is equivalent to 33.5 feet of water column. Therefore the maximum depth a pump draw is 33.5 feet. However, due to existence of losses, vapour pressure etc., the depth a pump can safely draw water is limited to 25 feet.