I have a customer that wants to install a single phase 120/208v coffee brewer to a 3 phase panel. Can this be done safely? The unit uses 220v for heating elements and 120v for control board.
Single phase power is just one (or two) poles of a 3 phase system, ALWAYS. So no, there is no "safety" issue, it is done all the time. The only issue is that it MIGHT be that the heating elements inside of the coffee brewer are configured to be connected Line-to-Line, instead of Line-to-Neutral. If so, then they will get 208V instead of 240V. Since the elements have a fixed resistance, the wattage output of the elements will drop to 75% of what they are rated for at 240V (P = V squared / R). That then will mean that it will just take longer for it to heat up the water.