We have a 200 amp service entrance. We are wanting to add to house, and wanted to see what to do about breaker box. Our 200 amp box is almost full now ( but there is room for additional breakers we need ) and we would have to add at least 1- 30amp for water heater, 3- 15amp for recepticles and lights, 1- no fault breaker, and two 60amp breakers? For heat pump for new addition ( addition is going to be 1500 square feet our old house is around 1200 ). Didn't know if we could change out 200amp breaker in box to 400amp breaker or have to change out 200amp breaker completely which was put in in 2005.

If you provide the total square foot of livable space, including a basement area that can be converted into livable space, and not including garage, attic, crawlspace, porches, plus a list and size of all major electrical appliances, hot tubs, wattage or amperage of cooling units, type of heat, wattage of any electric heat, I can do the calculations to size the rating of the service.

If you install a 400 amp panel with a 200 amp service entrance, all you have is a very expensive 200 amp panel.