Can Residential Generator be Smaller than Service Size?


New England
I am an electrical designer in Mass, and I am seeing a lot of existing conditions at homes where, for example, a residential house has a main service of 400 amps, and the main disconnect is a service rated 400 amp ATS, but the generator is only a 200 amp generator with no generator panel. So basically it's a whole-house backup, but with only half the available amperage as the main service. I know that most residential houses do not use nearly as much amps as we have to design for, but is it a code violation to have a setup like I mentioned? I was brought on to a design/build project to review the installation where the EC is doing something similar. Where in the code does it say he can or can't? Any help would be appreciated.


Senior Member
Tampa, FL, USA
Electrical Contractor
To add to what @Electromatic said, a load calculation or study has to be performed to determine the actual load. If that load is less than or equal to what the generator can handle, then the situation is code compliant.