I am about to install a photocell to control my lighting. But I am not sure with the sizing of the contactor I am about to use. The lighting panel is fed from a transformer. Shall I put it in the line side or at the load side of the transformer?

 

I would put it on the load side to avoid the inrush current of switching transformer. The contactor should be rated at least as high as the protection fuses or to match the transformer, I'm sure you can look that up in the code book for your location.

Roy