sum sign in conditional expression [duplicate]

Question

I would like to use the sum sign in an conditional expression. There is no error message but the Fz is supposed to be under the sum sign but instead its a subscript. Can someone help me solving my problem?

\begin{equation}  
X = \left\{  
\begin{array}{ll}  
q_{B} = \sum_{Fz}\left(WT_{Fz}  & , \textnormal{ if } q_{B} > 1800 * N_{FS} \\  
q_{S} = \sum_{Fz}\left(WT_{Fz}  & , \textnormal{ if } q_{B} \leq 1800 * N_{FS}\\  
\end{array}  
\right.  
\end{equation}

Answer 1

Crystal ball suggests that you might be looking for this:

% My standard header for TeX.SX answers:
\documentclass[a4paper]{article} % To avoid confusion, let us explicitly 
                                 % declare the paper format.

\usepackage[T1]{fontenc}         % Not always necessary, but recommended.
% End of standard header.  What follows pertains to the problem at hand.

\usepackage{mathtools}



\begin{document}

Some text.
\begin{equation}
    X = \begin{dcases*}  
        q_{B} = \sum_{Fz} WT_{Fz},  & if $q_{B} > 1800 * N_{FS}$; \\  
        q_{S} = \sum_{Fz} WT_{Fz},  & if $q_{B} \leq 1800 * N_{FS}$ \\  
    \end{dcases*}  
\end{equation}

\end{document}

Output:

Answer 2

This works:

\documentclass{article}

\usepackage{amsmath}

\begin{document}

\begin{equation}  
X = \begin{cases}   
q_{B} = \sum\limits_{Fz} WT_{Fz}  & , \textnormal{ if } q_{B} > 1800 * N_{FS}, \\  
q_{S} = \sum\limits_{Fz} WT_{Fz}  & , \textnormal{ if } q_{B} \leq 1800 * N_{FS}.\\  
\end{cases}  
\end{equation}


\end{document}