IEC 61439 is the governing standard for low-voltage switchgear and controlgear assemblies, and it sets verified limits on how a panel can be modified or extended without voiding its compliance basis. Its design must account for transformer capacity, available fault current, and the true demand of downstream loads. This discussion covers the last three. LV distribution boards, part of the electrical distribution system, securely distribute low-voltage power to facility circuits. You can find here a step-by-step guide to help you through the process. Determine the voltage level (e., 230V single-phase or 400V three-phase).