I'd like to add a subpanel to add a couple circuits (Extra inverter Mini Split ACs, well pump, etc.) but my main interior panel is full and recessed in concrete. I added a manual transfer switch from the main breaker next to the electric meter (utility - power generator).
Can I tap the main lines with a polaris style 3 way splice insulated connector into a new sub panel box? If I splice this way, should I splice from the utility line or can I splice from the load line that goes into the interior service panel? Would I need to get a 125 amp sub panel for the new circuits I need and want to have spare for future use? Which would be better and cheaper? BR breakers are the most common here, but I keep hearing about Siemens.
Side note: My home uses 125 amp and noticed that the wire used from the main 125 amp breaker next to electric meter that goes to the interior service panel is 2 AWG. I had purchased 1/0 AWG wire. I'm not sure whether to change to 2 AWG wire or keep the 1/0 AWG for the transfer switch and subpanel.