I have a 200amp main panel and a 200amp sub panel, a foot apart in my house. Concern I have is from the main panel a 100amp breaker feeds the 200amp sub panel. Is this correct?
There are 5 factors in subpanel sizing:
- Feed Breaker
OK, because that's the point of this question. But including for completeness for future readers.
The feed breaker must be greater than or equal to the amount of power you want to supply, including any continuous use derates. If you expect to use anything up to 80A continuous or 100A peak (or an appropriate mix - e.g., 40A continuous plus 50A peak) then you are OK. If you expect to use more then you need a larger feed breaker.
- Wire Size/Type
Almost certainly OK, but doesn't hurt to double check.
The wire size must be greater than or equal to the feed breaker size. Wire size ampacity varies depending on: type of cable or type of individual wires, copper vs. aluminum (copper can carry more current at a given physical wire size) and, in very long installations (hundreds of feet) voltage drop adjustment. Ignoring voltage drop (which you can normally safely do until you get to a couple hundred feet), that means consulting an ampacity table. 100A typically requires 3 AWG copper (individual wires or SE or USE cable) or 1 AWG aluminum. For a few feet between panels you can use copper or aluminum. If the panels are more than a few feet apart, aluminum is the standard because of costs. Using larger wire than is needed is always OK, but watch out for maximum size on breakers and lugs.
- Subpanel Bus Rating
OK, since a 200A panel by definition has at least a 200A rating, and 200A > 100A.
This is usually not an issue. But in theory you could have a small panel with perhaps 100A bus rating originally with a 60A feed. If you add a new large load (e.g., oversized EV charging) and increase the feed breaker and wire size you could end up with a subpanel that can't handle the load even though it has space for the breakers. That is relatively uncommon, and most of the time you run out of breaker spaces (and need a new panel) long before you overload the bus.
- Feed Panel Load Calculation
Need to check this!
This is where it gets tricky. When a panel is initially installed, and when major changes are made, you need to a Load Calculation. This determines how much current the panel will draw under normal circumstances. It includes the size of the area served, certain required standard circuits (kitchen, bathroom, laundry), cooking equipment (special rules apply which can sometimes make that less than you might expect) and any other fixed loads (HVAC, water heater, etc.). If you have 200A service and 150A load calculation then you can't feed 100A to the subpanel. Or you probably could have that as the feed breaker, but you couldn't actually plan to make use of more than 50A.
- Subpanel Load Calculation
Need to check this!
A similar load calculation needs to be done on the subpanel. Using the above 200A/150A example, if you calculate the subpanel load to be 50A or less then you're fine. But if you calculate the subpanel load to be 75A then you have to figure out what to change to increase the available feed or to lower the subpanel load, or use some sort of load management to dynamically adjust load when necessary.