[Minimum] sizing of main breakers and feeders are done through Article 220 calculations under Patrs III, IV, and/or V. There are quite a few factors which impact the minimum sizing, including continuous and non-continuous loads. The calculations are the only NEC-compliant method.
Originally Posted by KHall
In determining size for imbalance, your loads, as distributed on the phases, will have to be converted to line current and weighed against the minimum size as calculated to determine if larger sizing is required.
Whomever says or otherwise portrays "I am there" is a liar.