Is teaching a graduate course with 30 students an unreasonable expectation for a PhD student with a TA contract? I am a PhD student in a social science field, and am on a teaching assistantship. My contract stipulates that TAs are to work no more than an average of 15 hours per week. 

For my particular department, the past protocol was that 10 hours were devoted to teaching duties (class time, office hours, prepping, and grading), while the remaining five hours are devoted to research with an assigned faculty member. Overall, I do not think that is unreasonable. 

However, that setup was determined when the PhD students mostly taught undergraduate courses. I have been assigned a graduate course, and have been told to expect an enrollment of "at least 30 students."

I have never taught before, at any level. I am also still taking a full-time course load of my own. 

Does this seem like an unreasonably high expectation from the Dean and program director? Or am I just being a wimp?

I definitely want to teach, and I want to make sure that my students receive the best quality education from me that I can provide. And I would like to do this without sacrificing my own academic progress, nor what is remaining of my sanity.

Just curious to know what other people's experiences in this area have been.
It is incredibly common for the number of hours devoted to teaching be woefully inadequate to deliver good instruction except possibly for highly seasoned instructors teaching the exact same course multiple times.

Unfortunately, I don't know any fair solution to this.  You'll probably spend two or three times longer than the expected number of hours in order to do even an adequate job.  From what I observe, people cope either by working far more than they're supposed to, or by delivering not-very-good instruction.  It's a difficult spot to be in.  (Alas, not the only difficult spot in academia.)

The bright side is that if you're really interested in teaching and put in way more time than you're "supposed" to, next time it won't be as bad (you'll only have to put in somewhat more than you're supposed to), and if you do well you will both help your students and be known as someone who is a good teacher (good if you want to do more teaching).