Megabytes of data are often required for today's detailed CAD models. Users are often willing to sacrifice some of this detail for efficient rendering, transmission over networks, and robot motion planning. For example, many robotics and automation applications such as collision checking require only the simple bounding envelope of a complex set of nested CAD models. In contrast to approximate graphics rendering, these applications require that the simplified envelope be conservative: that the true CAD model be strictly contained inside it. One conservative bounding envelope is the bounding box. Another is the convex hull. Generally each conservative bounding envelope can be "trimmed" by a conservative bounding contour (CBC) of its 2D projection. This yields a hierarchy of conservative bounding envelopes of the original CAD model. In this paper we describe a method for generating the CBC at different levels of simplification. We give an algorithm that computes the CBC of a given 2D projection of a wireframe model. Since the wireframe requires neither boundary representation (BREP) nor constructive solid geometry (CSG) our method is compatible with almost all existing CAD standards.