You can use siunitx and, possibly, booktabs.
\documentclass{article}
\usepackage{siunitx,booktabs} % beautiful tables
\DeclareSIUnit{\rpm}{rpm}
\begin{document}
\begin{table}[htp]
\centering
\begin{tabular}{
S[table-format=1.3]
S[table-format=1.2]
S[table-format=3.0]
S[table-format=1.2]
S[table-format=1.3]
S[table-format=3.0]
}
\toprule
\multicolumn{3}{c}{$I_L=\SI{0.8}{\ampere}$} &
\multicolumn{3}{c}{$I_L=\SI{1.5}{\ampere}$} \\
\multicolumn{3}{c}{$T=\SI{3.46}{\newton\metre}$, $\omega=\SI{1500}{\rpm}$} &
\multicolumn{3}{c}{$T=\SI{4.35}{\newton\metre}$, $\omega=\SI{1500}{\rpm}$} \\
\cmidrule(lr){1-3} \cmidrule(lr){4-6}
{$I_A$ (\si{\ampere})} &
{$I_f$ (\si{\ampere})} &
{$P_{\mathrm{in}}$ (\si{\watt})} &
{$I_A$ (\si{\ampere})} &
{$I_f$ (\si{\ampere})} &
{$P_{\mathrm{in}}$ (\si{\watt})} \\
\midrule
1.84 & 0.3 & 378 & 2.2 & 0.38 & 532 \\
1.54 & 0.4 & 380 & 1.9 & 0.45 & 527 \\
1.25 & 0.5 & 375 & 1.65 & 0.55 & 527 \\
1.14 & 0.6 & 384 & 1.57 & 0.65 & 530 \\
1.145 & 0.65 & 384 & 1.63 & 0.75 & 530 \\
\bottomrule
\end{tabular}
\caption{Load $V$-curves test}
\end{table}
\end{document}
Note how the units are specified, in a uniform style which is consistent with BIPM rules for the International System.

There's generally no need to print headers boldface, they're already prominent enough.
Avoid worksheet-like rules: they're good for presenting data on a computer screen, when one has to work with them. For handouts and print, they're just of a hindrance.

;-)– egreg Oct 03 '17 at 20:53\begin{table[!htb]. You also have the[H](meaning: ‘here and nowhere else’), from thefloatpackage, but it might yield huge vertical spacing. Other than this, could you post a compilable code? – Bernard Oct 03 '17 at 20:55tabularmakes the layout you show and will never move anywhere. You may have put it in atableenvironment. The only purpose of atableenvironment is to specify that its content may be moved to help with page breaking. – David Carlisle Oct 03 '17 at 21:03