I am going to be installing a charger for a client for their Tesla. Do I need to ha e a separate shut off box for a double gang 100amp breaker for the charger?
Receptacle plugs can serve as a means to disconnect an appliance.
If there is no receptacle, but is instead direct-wired, appliances rated over 300 volt-amperes require a disconnect. If the breaker box is within sight and is 50 ft or less the breaker can serve as the disconnect. If it is not within sight, then a means to lock the breaker in the off position can suffice as a disconnect.
Addendum: There is a small section in the code that talks about car charging disconnects. As mentioned by another post, 2011 625.23 says greater than 60A or 150 volts to ground requires a disconnect -- That section was moved to 625.42 in the 2014, and to 625.43 in 2017.
That being said, 60A or less can use a receptacle plug as a disconnect if listed by the manufacture, and with residential 150V to ground is not an concern.