A trench shield is planned for use in a trench with a 1:1 slope and a vertical lower side. According to OSHA, what is the minimum height of “A”? (This will be a diagram of a trench box with trench shield. A is the dimension of the box above the top of the trench.)

A trench shield is planned for use in a trench with a 1:1 slope and a vertical lower side. According to OSHA, what is the minimum height of “A”? (This will be a diagram of a trench box with trench shield. A is the dimension of the box above the top of the trench.)