26 mm 2 (10 AWG) ground wire must be used, and in all other markets a 6 mm 2 must be used. Specific attention must be paid to instrument grounding, especially where field instruments are connected to a computer or microprocessor based control system. this design is to prevent the secondary side from being damaged by the high voltage on the primary side, as the current will be conducted to the ground through the Grounding Wire to protect human lives. Each DISTRIBUTION BOX and controller must be grounded. Grounding of the units: Attach a ground wire from one of. For example, for High And Low Voltage Distribution Cabinets, the distribution box needs to be grounded regardless of size. There are several reasons for grounding.