500 watts is the heating power of PTC, if the box is well insulated and heat accumulates for a long time, the temperature will rise "infinitely"!
However, PTC elements have a "constant temperature" characteristic, that is, when a certain temperature is reached, their own resistance will increase, so that the heating power will decrease, so as to maintain a temperature equilibrium point.
What is this temperature equilibrium point? This should be informed by the PTC manufacturer.
