\documentclass[border=10pt]{standalone}
\usepackage[siunitx, RPvoltages]{circuitikz}
\newcommand{\myflow}[3][above]{%arguments: [options (default above)] {component name} {label to add}
  \draw [-{Triangle[open]}] (#2-Ffrom) -- (#2-Fto) node[pos=0.5, #1]{#3};
}

\begin{document}
\begin{circuitikz}
  \draw (0,0) to[short, f, name=S1] ++(3,0);
  \myflow{S1}{$I$}
\end{circuitikz}
\end{document}
