To determine the minimum panel amperage, you need to sum the calculated load of all the circuits and then apply any applicable demand factors or service sizing calculations as required by the electrical code. Based on the information you provided:1. Calculate the Total Connected Load

Add up all the loads you will connect in your panel.2.

Demand Factors

Demand factors allow you to reduce the calculated load based on the likelihood of all loads operating simultaneously.Without more information, it's impossible to definitively say the panel must be 400A.Here's what information I would need to give a more accurate response:

List of Loads

Provide a comprehensive list of all equipment connected to the panel, including: Individual circuit sizes (e.g., 20A, 30A, 50A). Voltage (e.g., 120V, 240V, 208V, 480V). The type of load (e.g., lighting, receptacles, HVAC equipment, motors, etc.). Wattage or VA rating (if known).

Equipment Details

Largest motor nameplate horsepower (HP). HVAC equipment: Specify whether it's heat pump, AC, electric heat, etc., and their respective ratings (Amps, Watts, VA). Number and type of electric vehicle (EV) chargers (if applicable).

Location

NEC edition adopted by the local jurisdiction.*

NEC Demand Factors to apply: If you know what section of the electrical code applies to your project. Flag for review