If two dwelling units are supplied by a single feeder or service, and the calculated load under Part III (Standard Method) exceeds that for three identical units calculated under 220.84, the lesser of the two loads may be used. This method provides a code-compliant and efficient approach to sizing the service or feeder for the entire building.
So in accordance with NEC 220.85, the calculation starts with the load of the larger unit, considered as the “third unit” for demand factor purposes.The demand factor for a three-unit dwelling (45%) is applied to estimate the total service load for a duplex.

NEC Load Calculator (Multifamily and Apartment)

Building Load Calculation

⚠️ Do not select Three-Phase when Voltage is 240 V

House load refers to the electrical demand for common areas and shared systems in a building, such as lighting,irigation system, elevators, and fire alarm.