Method for auto-aligning parts of complex numbers in table.

\documentclass[a4paper]{article}

\usepackage{booktabs}
\usepackage{array}
\usepackage{xstring}
\usepackage{collcell}

% see http://tex.stackexchange.com/a/83639/14685
\newlength{\WidestRealNum}
\newcommand*{\ApplyNumFormatting}[1]{%
    \StrBefore{#1}{+}[\RealPart]%
    \StrBehind{#1}{j}[\ImagPart]%
    $\makebox[\WidestRealNum][r]{$\RealPart$} + \text{j}\ImagPart$%
}%
\newcolumntype{N}[1]{>{\settowidth{\WidestRealNum}{$#1$}\collectcell\ApplyNumFormatting}l<{\endcollectcell}}

\begin{document}
\begin{table}[h!]
    \caption{Bus Loads}
    \label{fig:figurename}
    \centering
        \begin{tabular}{l|N{999}}
        \toprule
        \textbf{Bus} & \multicolumn{1}{c}{\textbf{Bus Load (MVA)}} \\
        \midrule
            b1 &  50 + j 30.99 \\
            b2 & 170 + j105.35\\
            b3 & 200 + j123.94 \\
            b4 & 150 + j49.58 \\
        \bottomrule
        \end{tabular}
\end{table}
\end{document}