Most people run a continuous length of flexwatt tape from the bottom shelf to the top shelf in the rack. If I am reading you correctly, you have plans to cut the flexwatt tape to fit every shelf and wire each shelf with it's own electrical plug.

If this is the case, I would recommend that you don't do it your way. Each shelf will have a potential to have a different resistance due to many technical reasons. With that, you could have a different temperature for each shelf. Your thermostat will not be accurate for all shelves either. Then think of the possible points of failure if you are wiring each shelf independently.

Your best bet all around, would be to run one continuous length of flexwatt tape. Remember to cut the end of the flexwatt tape properly, There are a couple of different styles and each has to be cut properly to make it work properly. Also remember to tape the very end of the flexwatt tape. You will have 110 volt AC at the end. That could be dangerous.

Good Luck!
Jim Smith