A couple of years ago I had a licensed electrician add a second 100 amp load center to my home (the original 100 amp load center, still in use, could accept no additional breakers).
He fed the new load center from the meter. The meter is CL200 rated.
Conductors for each panel come from the meter base. I pulled the new panel cover and see conductors connected to the main lugs going into a conduit connecting the new panel and the meter base box. Both panels are in the same structure, a single family house. The original panel is in the garage; the new panel is just outside the side door to the garage – say 5ft apart. Both panels are main breaker types. The new is a Siemens PW2020B1100 w/copper bus in type 3r enclosure. The wire size between the new panel and the meter base is 3/0.
Can I assume that the electrician verified that the meter and base could support two load centers?
Best Answer
100 + 100 ≠ 200
First off -- you do not need a 200A service to feed two 100A panelboards, provided the total load as determined by NEC Article 220's calculations does not exceed the service ampacity. This is a consequence of the how 230.90(A) Exception 3 interacts with 230.40 Exception 2 and your specific setup (bold running text mine for emphasis, italics theirs):
(Note that exception 5 has to do with the 310.15(B)(7) allowances for residential service conductor sizing -- they need to be taken into account when determining the total ampacity of your service and the ampacity of your individual sets of service entrance conductors, but stop there.)
A tale of a misplaced panelboard
But, there's more! While what the electrician did (putting a second main panel on your service) would have been OK when done properly as it would fall under 2014 NEC 230.40, exception 2 (bold running text mine for emphasis, italics theirs):
However, your electrician screwed up when he put the second panel on the outside of your garage, as that ruins the grouping required by 230.40, exception 2 and 230.72(A):
So, in any case, you'll need to have the electrician move the second panelboard inside to a spot next to where the first one lives, or replace the first panelboard with an exterior unit that can be mounted next to where the new one lives.