I had a 100A service (MSP) with a 100A sub panel. I do not know the feeder size to the subpanel but it supports 100A but definitely not 200A.
Last year, the service (and MSP) got upgraded to 200A. The feeder to the sub panel of course stayed protected with a 100A beaker.
Last month, also the sub panel got upgraded to a 200A version. But since the feeder is still 100A, the sub panel stays a 100A panel, even though the panel itself would support 200A. The 200A main breaker got removed from the sub panel. The whole feeder and sub panel is protected by the 100A breaker in the MSP.
Can I put the 200A main breaker into the sub panel and use it as a “switch”? I would like to be able to turn off the sub panel without having to go outside to the MSP. Note: I would not operate this “switch” often; only very occasionally when I do work in the sub-panel.
Best Answer
There are two ways a breaker (main or branch) can be used as a "switch".
For this, any breaker will work just fine. There are plenty of times where the main breaker on a panel (whether technically main panel or subpanel is irrelevant) is turned off in order to safely work inside the panel. Some work can be done inside the panel without turning off the main breaker, but how much depends on the type of work, your skill level and safety concerns.
If you plan to turn a breaker on/off frequently, e.g., daily, then it should be switch duty rated, or SWD. This is typically on branch breakers, not main breakers, and often only on smaller ones. For example, according to manufacturer catalogs, the Eaton CH 15A - 20A single pole breakers, the Square D QO 15A - 20A single pole breakers and the Square D Homeline 15A - 20A breakers are all SWD rated. That doesn't mean you won't find some larger breakers that are SWD rated, but it gives you some idea of what look for. The larger the load, the more wear and tear involved and concerns about reliability with frequent switching.
So if the goal is "occasionally work on the panel without having to to the main panel to turn the subpanel off and back on" then yes, a main breaker (any size >= 100A) in the subpanel will do just fine. If the goal is "turn everything controlled by the subpanel on/off every day using one big switch" then this is not a good idea.