I bought a 1600sf house built in 1970 a ranch w/basement converted to an apartment. And decided to map the breaker box and found all the upstairs lights & outlets (except -dryer and a bathroom ceiling heater). And all the downstairs lights and outlet (except kitchen outlets) are on 2- tied together 40amp breakers. They are in position 11&12 in the breaker box. If I flip these almost all of the house goes out. But I also have a combination of 15amp & 20amp breakers that will turn off different sections of the house when the two 40amp are on. Is this a common way they wire a house? Is there any thing I should know before I tie in to the 15 & 20amp breakers to add can lights other than their 80% capacity?