While most pre-installed ceiling electric boxes should support a weight of 50 pounds, you should be sure before you attempt to install it. Remove the existing chandelier and examine the existing box. If it is plastic, the weight and volume should be stamped somewhere inside it. If it is the older metal variety, verify that it is securely fastened to a solid frame. If there was an existing chandelier, look on the mounting strap assembly for the weight. This will give you an estimate of what size will work.