According to NEC Section 300-7 (b), cable trays must be designed to accommodate the thermal expansion and contraction of the cables they support. The metal gets longer, and the heat becomes excessive. As cables and trays expand or contract, they can cause stress on the structure, leading to potential damage or misalignment. To determine the number of expansion splice plates you need, decide the length of the straight cable tray runs and the total difference between the minimum winter and. All materials expand and contract due to temperature changes.