  Power-like loop corrections to gauge couplings are a generic feature of
higher-dimensional field theories. In supersymmetric grand unified theories in
d=5 dimensions, such corrections arise only in the presence of a vacuum
expectation value of the adjoint scalar of the gauge multiplet. We show that,
using the analysis of the exact quantum effective action by Intriligator,
Morrison and Seiberg, these power corrections can be understood as the effect
of higher-dimension operators. Such operators, both classical and quantum, are
highly constrained by gauge symmetry and supersymmetry. As a result, even
non-perturbatively large contributions to gauge coupling unification can be
unambiguously determined within 5d low-energy effective field theory. Since no
massive hypermultiplet matter exists in 6 dimensions, the predictivity is
further enhanced by embedding the 5d model in a 6d gauge theory relevant at
smaller distances. Thus, large and quantitatively controlled power-law
contributions to gauge couplings arise naturally and can, in the most extreme
case, lead to calculable TeV-scale power law unification. We identify a simple
5d SU(5) model with one massless 10 in the bulk where the power-law effect is
exactly MSSM-like.
