Hi, there has been a thread on this before,and there are so many variables, I don't think the one answer would apply to all thoroughbreds,or all draughts.depends on the condition of the horse ,thetype of ground you would be going on ,hilly ,flat etc
Our shetland pony, Cowboy, is 370 pounds give or take a bit (he's quite the chunk though.) and he can comfortably carry two adults in a lightweight (45 lb) cart. We never make him pull more than 290-300 pounds, and his average is usually about 250. This goes along with what Corporal said, what they can ideally carry (20% body weight) times four.
Being a shetland, I'm sure he could carry more for short distances, but it's really not a good idea.