This is how it was explained to me, some folks may disagree but anyway. . .because flexwatt is resistance based heating, the length of the strip can affect performance, I was told that the middle of a length of flexwatt would tend to be the hottest when compared to the ends. This is why it was recommended to me that I run individual strips for each shelf, and it's also why I had some probe placement issues initially when I put the probe at the end right between the AC connections. Each of my shelves has a consistent reading at the middle. . .hope this helps and I hope it doesn't stir up too much trouble from the flexwatt experts. . .