  The Abelian $Z'$ boson decay width is calculated in a model-independent
approach. The analysis takes into consideration the special relations between
the $Z'$ couplings to the Standard model fields proper to the renormalizable
theories. The constraints on the $Z'$ couplings to the Standard model fermions
corresponding to the narrow width approximation are derived. The results are
compared with the benchmark $Z'$ models, such as $Z'_{\chi }$ and $LR$. It is
shown that the current experimental limits on the Standard model $Z$ width
impose no limits on the $Z'$ width, so that the ratio $\Gamma _{Z'} / m_{Z'}$
is allowed to be up to 100%. This fact is connected with the smallness of the
$Z-Z'$ mixing. So, the finite-width $Z'$ is admitted and cannot be excluded
from consideration. Also, the new constraints on the $Z-Z'$ mixing angle are
derived which agree with the corresponding earlier results of the LEP data
analysis.
