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.

;-)
\begin{table[!htb]
. You also have the[H]
(meaning: ‘here and nowhere else’), from thefloat
package, but it might yield huge vertical spacing. Other than this, could you post a compilable code?tabular
makes the layout you show and will never move anywhere. You may have put it in atable
environment. The only purpose of atable
environment is to specify that its content may be moved to help with page breaking.