I posted a reply earlier and I got to thinking, T310.15 is not a requirement. You dont have to use it if you choose not to. You can use T310.16 for your conductor size, but you still have to use T250.122 for your EGC. You have to increase the EGC when you increase the ungrounded conductors to compensate for voltage drop over long distances. That is a no brainer. Good question though.