Unfortunately there are no specific laws (that I know of) prohibiting mandatory overtime.
Link to labor code - http://www.leginfo.ca.gov/cgi-bin/di...0&file=500-558
I would write in my own handwriting, "I am informing you (name of manager forcing overtime) that this is to confirm that for me to work overtime will place my patients, colleagues, and myself at risk. My fatigue may cause accidents an errors. As a result the hospital, not me, is responsible for any adverse effects on patient care. I will, under protest, try to carry out the assignment to the best of my ability."
Given to:_(supervisors name) at: _______ (time).
If lack of child care or elder care is a problem list that also.
They try it everywhere. Also claim your overtime.
When asked to work overtime I learned to say, "I would if I could, but I can't because........"
No insubordination there.