Ken is right and there are specific labels for that too. But technically, if it is a control panel, Article 409 of the NEC would apply and 409.110 delineates the marking requirements that basically say you need BOTH things, the voltage of each service entering the box, an a label identifying that the source is from outside. However if the source is a control power transformer INSIDE of the same box, disconnected when the main 480V is disconnected, then you do not need to identify that at all on the outside.