If you run a 100 amp line or even a 60 amp line, what ever. the breaker in the main box would be sized for protecting the wire size to the sub panel. If you need a 15 amp or 20 amp ckt from the sub panel, you just put that size breaker and wire in for that. A 12 to 16 ckt panel is only a couple bucks. That way if you want a dedicated line for an upstairs a/c unit you are well covered. Instead of a main breaker in the sub panel, you can just use a lug connection terminal. It is quite easy. The main thing is to make sure you use 4 wire conductor cable. Check at what breaker size and wire size the prices are. A lot of times it is much cheaper to go larger with a sub panel. You could do it yourself and get someone to make the actual connection to the live main panel. All connections in the sub panel can be done with the power off. ..............bob
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