infinite cardinal numbers

For every family of infinite cardinal numbers $\displaystyle (i \mapsto \kappa_i)$ on a non-empty index set $\displaystyle I$,

$\displaystyle \sum_{i \in I} \kappa_i =_c \text{max}(|I|, \text{sup}(\{ \kappa_i | i \in I \}))$.

I do not see how to prove this. We have proved a theorem that says that:

For every indexed family of sets $\displaystyle (i \rightarrow \kappa_i)_{i \in I}$ and ever infinite $\displaystyle \kappa$, if $\displaystyle |I| \leq_c \kappa$ and for each $\displaystyle \kappa_i \leq_c \kappa$, then $\displaystyle \sum_{i \in I} \kappa_i \leq_c \kappa$.

I am not sure if this theorem helps or not though. I need help on this one. Thanks.