Under What Conditions Is It Never Optimal to Exercise an American Call Option Early?

It is generally never optimal to exercise an American call option on a non-dividend-paying asset early. Exercising early means forfeiting the time value of the option.

The holder can always sell the option in the market for at least its intrinsic value plus the remaining time value. If the underlying asset pays dividends, early exercise might be optimal just before the ex-dividend date to capture the dividend.