Georges,

The main breaker is described in ABYC E-11. It also allows easy isolation of the branch circuits and multimeter at the panel.

11.12.1.6. Panelboards and Switchboards - A tripfree circuit breaker or a fuse shall be installed at the source of power for panelboards and switchboards, and shall not exceed 100 percent of the load capacity of that panel, or 100 percent of the current carrying capacity of the feeders.

EXCEPTION: The trip-free circuit breaker or fuse at the source of power may be rated at up to 150 percent of the conductor ampacity if there is a submain circuit breaker or fuse in the panelboard or switchboard that is rated at no more than 100 percent of the load capacity, or the feeder ampacity, whichever is less.