From a 120/240-volt, 3Ø, 4-wire panelboard, the maximum size motor branch circuit time-delay fuses allowed is:

In determining the maximum size of motor branch circuit time-delay fuses for a 120/240-volt, 3-phase, 4-wire panelboard, it's essential to reference the National Electrical Code (NEC) guidelines, specifically those related to motor circuits.

The maximum fuse size allowed for a branch circuit supplying a single motor is typically based on the motor's full-load current (FLC). For a motor running at full load, the NEC indicates that time-delay fuses can be sized at a certain percentage of the motor's FLC. Generally, this corresponds to 250% of the full-load current for motors larger than 1 horsepower.

In practice, for common motor sizes and configurations, the 40 ampere fuses conform precisely to the recommended sizing for many applications, as it complies with NEC Table 430.52, which outlines protective device sizes for motors. This choice ensures proper protection while allowing for potential overload conditions without nuisance tripping.

In summary, for a 3-phase motor branch circuit derived from a 120/240-volt panelboard, 40 ampere time-delay fuses are deemed appropriate and compliant with industry standards aimed at protecting the motor and associated wiring from overcurrent situations.