I'm going to install a 4 meter pack panel on a 4 plex apartment building. For the house meter the AHJ says I have to install a commercial panel next to the 4 pack and feed it off of the meter side. He said he won't accept a 5 packer. I'm not sure why.

The 4 meter pack panel is rated at 400 amps. How do I size the feeder from it to the house panel?