when is a main breaker required for a sub panel????

smokey,
take a look at 384-16

Main OC protection is required for "Lighting and Appliance" panelboards (see 384-14a for a description of lighting & appliance panelboards, see 384-14b for power panelboards), however, it need not be integral to the panel. The panel may be considered protected by the OC protection of the feeder supplying the panel.