On the EMB145, we would ususally take of using a reduced thrust setting, and packs on.
If we have to use TO (MAX) thrust, for performance reasons, then we cannot use the engines to drive the packs.
If we take off in icing conditions, we have to have the packs off.
If we take off in non-icing conditions, then we can use the APU for driving the packs.
If an engine fails, we advance the thrust levers to max thrust, and the FADEC's reschedule for TO (take off) thrust.
Both of these actions will shut off the packs automatically.