Whenever the commission shall determine to hold a hearing or conduct an investigation upon a complaint made and filed as in this article provided, it shall fix a time and place for public hearings thereupon; and whenever the commission shall determine to conduct a hearing or investigation upon specifications filed by it, it shall fix a time for public hearings of the matter [matters] under investigation; and a copy of the complaint or a copy of the specifications filed by the commission as basis for said hearings shall be served upon the person or utility interested. All hearings shall be held at the offices of the commission in Santa Fe, or at such place in the state as the commission may designate.