NEC 215.2, 220.31, 230.42 and 310.15(B)(7) state that the neutral conductor of a residential service or feeder can than the phase conductor, but only If it can be verified that the reduction in the size of the neutral will result in a conductor capable of safely carrying the load.

You throw a small rock straight up from the edge of a highway bridge that crosses a river. the rock passes you on its way down, 7.00 s after it was thrown. what is the speed of the rock just before it reaches the water 28.0 m below the point where the rock left your hand? ignore air resistance.