Does a sub panel need a main breaker?

Answer for Canada, USA and countries running a 60 Hz, 120/240 volt, split supply service.

The main distribution panelboard has to have a main disconnect. This has been in the electrical regulations since the late 50's.

If the sub panel is in the same home under the same roof line, the sub panel enclosure does not need a main disconnect because it is just another branch circuit. Connecting a sub panel to a separate free standing garage or workshop is a different scenario and is governed by a different set of electrical code regulations.

What needs to be done to the panelboard is physically remove the neutral bonding screw. This is the screw that bonds the panelboard enclosure to the neutral bus. By doing this it stops a secondary ground pathway for a fault current from the sub panelboard to flow back to the main panelboard on the sub panel's neutral feeder.

The electrical code requires that there is only one position on an electrical distribution service where the distribution enclosure and the neutral bus join each other. That is at the main disconnect of the first over current device, which is the main service disconnect breaker or a fused disconnect switch whichever your system uses..