Wall-mounted boxes should be 4. This height makes it easy to reach without bending or stretching. Adhering to these guidelines during the installation of a distribution box ensures. This document replaces PG&E Document 051122, Rev. #21 For a description of the changes, see Page 28. PRINTED COPIES MAY NOT INCLUDE THE MOST UP-TO DATE STANDARDS, REFERENCES, OR REQUIREMENTS. TO EVERY CIRCUMSTANCE OR ELECTRICAL SYSTEM. SRP ENCOURAGES EACH USER TO CONSULT WITH ITS OWN TECHNICAL ADVISOR CONCERNING THE APPLICABILITY OF THESE TANDARDS TO. secondary unit substation is a close-coupled assembly consisting of enclosed primary high voltage equipment, three-phase power transformers, and enclosed secondary low-voltage equipment. Residential installations typically follow recommended heights between 1.