We are constructing an addition to the house. The addition requires a new meter base location. The plan was to install a meter base with breaker spaces. Most of the big circuits would be moved to the meter base. What is left in the existing 200 amp breaker box will be run as a 125 amp sup-panel. Do I need to replace the 200 amp breaker box or can I run the 125 amp wire to the 200 am circuit breaker in the box which powers the box?? The 125 amp breaker will be at the meter base.

 

The existing main breaker can just be a redundant means of disconnect.

Does your power company allow meter mains?

The grounds will need to be separated in the old panel and the bond screw will need to be removed. You will also need a new 4 wire feeder to the panel.

 

You would need to verify the minimum wire size allowed on the lugs in the existing 200A panel and use wire at least no smaller than that for the new 125A feed. Why not just make the feeder 200A? You could get a meter/main with 200A feed-through lugs.