An authority expressed in general terms, however broad, does not authorize an agent to do any of the following:
(a) Act in the agent’s own name, unless it is the usual course of business to do so.
(b) Define the scope of the agency.
(c) Violate a duty to which a trustee is subject under Section 16002, 16004, 16005, or 16009 of the Probate Code.