26 mm 2 (10 AWG) ground wire must be used, and in all other markets a 6 mm 2 must be used. 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. Each DISTRIBUTION BOX and controller must be grounded. I realize that common trip is not required, although handle ties might be a good idea, 3 phase breakers seem like an. There are several factors that make substation grounding absolutely necessary.