I'm working in this ciruit:

I googled how to make a box like this (Load box) but I didn't find any similar example. What I found of more similar was to use a nport but with no examples. Does someone has one good example or know how to make this circuit above?
Here what I've done:
\documentclass[11pt]{article}
\usepackage{circuitikz}
\usepackage{tikz} % for flowcharts
\begin{document}
\begin{center}
\begin{circuitikz} [american voltages, baseline=(current bounding box.center)]
\ctikzset { label/align = straight }
\draw (0,0)
to[V=$V_{Th}$] (0,2)
to[R=$R_{Th}$] (2.5,2)
to[short,i=$I$, -o] (4,2)
to[short] (4.5,2)
(0,0) to[short, -o] (4,0)
to[short] (4.5,0);
\end{circuitikz}
\end{center}
\end{document}


\node[draw,minimum width=2cm,minimum height=2.4cm,anchor=south west] at (4.5,-0.2){Load};to your code. – Apr 26 '15 at 14:07I added this way. Something wrong?
to[short] (4.5,0) \node[draw,minimum width=2cm,minimum height=2.4cm,anchor=south west] at (4.5,-0.2){Load}; \end{circuitikz} \end{center}
– Francisco Maerle Apr 26 '15 at 22:43to[short] (4.5,0);and then add the line. You are just missing the;is all. – cfr Apr 26 '15 at 22:46