I have a home built in the 50's with a 100 amp main panel. I will be building a workshop in the back that will need at least a 60 amp sub panel but I was thinking of just going to a 100 amp subpanel. Now the question is, Can I install a new 200 amp main with upgraded service entry from the power company next to my 100 amp main? Had an electrican come buy to quote me on just upgrading original 100 amp to 200 amp but he does not want to mess with the old wiring and certain other items that are not up to code today. So to summarize all this up, I would have a whole new 200 amp panel with new service entry, meter, weather head etc. Feeding my 100 amp panel for the house and 100 amp panel for the shop.

  • Can you post photos of your existing main panel please? Mar 24, 2021 at 3:11

1 Answer 1


Count the amps on the breaker handles in your existing panel. If you're extra skilled, figure out which pole they are on also. You'll find you have well in excess of 100A of breakers on each pole, and well over 200A of breaker handles. Already.

This kind of oversubscription is allowed because not everything will be going full-blast at once.

As such you can have the existing 100A panel plus another 100A panel at the house, plus a 100A panel in the shop.

You can have a new 200A main panel in the house that feeds the existing 100A panel as a subpanel, and a 100A panel in the garage also.

The way you avoid overloading that is called a "Load Calculation" which takes into account general house loads on average (3 VA/watts per square foot of house), certain circuits (e.g. 1500W per kitchen general recep circuit), and certain large appliances (either their actual nameplate or a stock formula e.g. for ranges). A garage subpanel doesn't go into the load calculation by its breaker trip, but rather, by which tool(s) you're likely to use simultaneously with the other loads. If your house is legal for a 100A load calculation right now, adding a "60A" panel in the garage isn't going to make you over-top a 200A service.

While you're doing the upgrades, consider a "Meter-Main" which is a combination meter and main breaker. That will be Code when NEC 2020 is adopted in your state. However it also provides ease of maintenance later.

They also make "Farm panels" which combine a meter-main with a miniature 8-space panel, to allow you to tap the 200A service to feed various subpanels and very large loads, while still sending the full 200A onward to a 200A subpanel. . So for instance the farm panel might have a 100A breaker to feed an existing panel, another 100A breaker to feed a shop, and the straight-thru feed goes to feed a 200A subpanel for large or future loads.

Again, the amp rating of the subpanels is irrelevant, what matters is whether the service size is sufficient for the loads to be served based on the Load Calculation.

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