Each DISTRIBUTION BOX and controller must be grounded. 26 mm 2 (10 AWG) ground wire must be used, and in all other markets a 6 mm 2 must be used. Grounding of the units: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. The grounding "bus" (grounding bus, PE bus) in the box is directly connected to the power ground wire or grounding system; 2. This helps to reduce the potential difference that exists between conductive parts and the earth.