Consider that there may be as much as 5% (or even more) variability in manufacturing tolerance, as well as 5% variability in your supply voltage. If the element is exactly 4500 watts and the voltage is exactly 240 then your have 18.75 amps (1.25x = 23.4375). Now let's raise the voltage by 5% (252 volts). The same resistance (12.8 ohms, not accounting for change due to heating) gives you 19.6875 amps (1.25x = 24.609375). Then if the element itself is 5% lower in resistance (12.16 ohms) you'd have 19.7368421 amps at 240 volts (1.25x = 24.671052625) and at 252 volts you'd have 20.7236842 amps (1.25x = 25.90460525). You may need that 30 amp breaker. And even the breakers themselves have some variability (why the trip curves have some width to them).
