  We have calculated lower limits for the global effective thermal conductivity
in a sample of 165 Abell clusters. We assumed that cluster X-ray luminosity is
compensated by a conductive heat flux which we then compare with an upper limit
to the temperature gradient inferred from the cluster temperature and radius.
This gives a lower limit to the thermal conductivity and therefore to the
relative suppression from the Spitzer conductivity, kappa_{Sp}. Not a single
cluster requires super-Spitzer values of thermal conduction to balance the
observed X-ray luminosity. The suppression coefficient f=kappa/kappa_{Sp} is
clustered in a range 10^{-2} < f < 0.4. A weak dependence of $f$ versus $z$ is
observed over $ 0 < z <0.41$. Possible biases and/or selection effects are
discussed.
