Cover the potatoes with cold water in a large saucepan and cook until almost tender. Drain and dry the potatoes. Cut each potato lengthwise into quarters. Place the potatoes on a baking sheet and brush both sides of the potatoes with oil. Combine the chili powder, salt, and cumin in a small bowl, and sprinkle over the potatoes. Grill the potato wedges for 3 to 4 minutes per side until golden brown. Serve with ketchup.