What can a grad student do when their assistantship requires more hours than specified in their contract? I'm currently finishing my second year as a STEM PhD student at a fairly large research university. As a grad student, I am expected to fulfill 20 hours of extra duties each week. Typically this involves working as a TA for two classes and then completing a few hours a week in the tutoring center or helping out in large lecture classes. This has gone fine for me during that last three semesters. However, this semester I was only scheduled for one TA section. So this left about 10 hours in the tutoring center for my leftover duty. 

Over the course of the semester the department has been giving me fairly large grading assignments for upper-division classes. The first was given about three weeks in and required some shuffling of my schedule to accommodate (I'm getting ready to take my quals and am taking multiple core classes this semester). This replaced 7 hours of my tutoring center time.  Now toward the middle of the semester they've given me another fairly large grading assignment, but are only counting it for 3 hours so as not to go over my 20 hour limit. 

Is this typical behavior for a department? I feel like having my schedule disrupted like this is very unprofessional. Also, I know there are other grad students in my department who could more readily take on this assignment. This new assignment is for a class outside of my field, and I have zero experience with the material. It just seems like there's no consistency to these work expectations as many other students have duties that count for their full time, but are much easier and require fewer hours in practice.

Is this something that I should bring up with my department? All of these duties are starting to take a toll on my work. I appreciate any advice.
It is fairly common for graduate students to have TA assignments that are expected to take up about half of their time, as you have stated is the case for you. In this case, you should not really think of yourself as a full-time student, but as a half-time student, half-time employee of the department, since your teaching work is what you are being paid to do. Moreover, a healthy department will typically have its expectations of students adjusted accordingly.

There is, of course, a huge amount of variance in the actual time required for a given class, based on the particular students, professor, assignments, etc.
It sounds like this is what's being the problem for you: since you only got assigned one class, you were expecting to have a light load, but the department is trying to "get its money's worth" from you, and is doing so in a manner that you find difficult to plan for.

I would think it is entirely reasonable to ask for notification farther in advance, so that you can plan better.  It's also entirely possible that you will not get it, since the person making the assignments is likely an overloaded professor who is dealing with requests that are being given to them at the last minute as well.

Ultimately, my recommendation would be discussion with your advisor so that you can:


make them aware of the impact last-minute requests are having on you,
enlist them to help in either improving the situation or alleviating its impact on you, and
prevent it from happening to you again in the future.