Understanding the scope and limitations of non-destructive testing procedure is essential for selecting the appropriate test parameters for material inspection. This paper presents the scope of material (δ s ) and probe dependent (δ t ) penetration depths for determining the optimal test frequency (f opt ) for detection of sub surface defects in electrically thick conducting specimens. Numerical modelling is carried out for a pancake coil above an electrically thick aluminium plate, t/ δ t > 1, to study the influence of the EC probe and defect location (t df ) on the test frequency for near and deep sub surface defects. The study concludes that the optimal test frequency, f opt for detection of deep sub surface defects (t df / t≈ 1) is determined by the probe dependent skin depth, δ t , and the plate thickness is related to f opt by, t∝1/fopt. The numerical observations were experimentally validated for machined sub surface notches on a 10 mm thick (t) aluminium plate. © 2018, Springer Science+Business Media, LLC, part of Springer Nature.