In this case, if the internal cash use to finance the project, internal cash-carrying 15% cost because, internal cash belongs to the equity shareholder only. So, if internal cash is idle so can only finance for the new project. As it is mentioned, debt finance is cheap so raising the maximum fund by debt mode only to achieve optimal capital structure. If the additional capital raised by issue of equity share, it would lead to an increase in the number of equity shareholders. And it will decrease the EPS (earning per share). And the result will affect the market value of the firm as well. As a true supporter of the company, I would only suggest issuing further equity share only to the extent of balance the optimal capital structure. And balance capital will raise only by debt mode