26 mm 2 (10 AWG) ground wire must be used, and in all other markets a 6 mm 2 must be used. Each DISTRIBUTION BOX and controller must be grounded. Grounding of the units: Attach a ground wire from one of. Today, we're diving deep into the world of distribution box grounding, breaking down the standards, and shining a light on those sneaky mistakes that even experienced electricians sometimes make. This helps to reduce the potential difference that exists between conductive parts and the earth. These locations are usually marked with grounding symbols for easy cable crimping.