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.