New whole house generator and ATS

Yeah I had to do that recently and I just took all the breakers out with the wires still connected. Not the cleanest job but got the ground bar in.
 
Another possibility is to reroute the SE cable to the ATS inside, above the panel.
Replacing the SE cable with SER cable isn't going to be that difficult, it's only about 30' from where the ATS is going to be installed to where the house panel is located and it's all open.
 
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