It is never optimal to exercise an American call option early. Since the price of the option is $$ C = \text{Payoff} + C_{E} $$ where $C_E > 0$ is the extrinsic value, also known as time-value of the option - a premium on top of the payoff. Extrinsic value reflects the additional value that market participants place on the option for the possibility of the payoff increasing before expiry, and is always greater than zero. Therefore, it will always be more valuable to sell the option rather than exercise it.