Perovskite-type metal oxides are being used in a wide range of technologies, including fuel cells, batteries, electrolyzers, dielectric capacitors, and sensors. One of their remarkable structural properties is cationic ordering in A or B sites, which affects electrical transport properties under different gaseous atmospheres, and chemical stability under CO2 and humid conditions. For example, a simple-perovskite-type Y-doped BaCeO3 forms BaCO3 and ((Ce,Y)O2-delta) under CO2 at elevated temperature, while B-site-ordered double-perovskite-type Ba3Ca1.18Nb1.82O9-delta remains chemically stable under the same conditions. Early structural studies on Ba3Ca1+xNb2-xO9-delta (BCN) showed that the B-site ordering (1:1) is sensitive to the Ca content. However, ambiguity rises, as 1:2 B-site ordering was not observed in the parent and doped analogues when x was varied, which motivated us to revisit the complex oxides BCN (x = 0-0.45) to determine the atomic structure by a mean of combined synchrotron X-ray and neutron diffraction methods. Surprisingly, the B-site ordering increases with increasing Ca/Nb mixing in the B-sites in BCN. In addition, the electrical conductivity of BCN was found to be the highest at x = similar to 0.18, and it decreased as the Ca/Nb ratio further increased in BCN. Such a result was very similar to that for the Y-doped BaZrO3, where the mobility of proton carriers was found to decrease as the dopant (Y) increased. A higher Ca/Nb ratio also promotes the growth of grain size, as Ca ions could serve as a sintering aid, improving the structural integrity.