Electrical Distribution Board Sizing and Layout

What You Need to Know

Every commercial building needs at least one distribution board. It splits the incoming power supply into separate circuits for lights, power outlets, HVAC, and other loads. Get the sizing wrong, and you run out of circuit breaker slots before the fit-out is done. Get the layout wrong, and the board fails its AS/NZS 3000 clearance requirements.

• Distribution boards must comply with AS/NZS 61439 for construction and type testing (AS/NZS 3000, Cl 2.10.3.2)
• Calculate maximum demand using AS/NZS 3000 Appendix C — connected load multiplied by diversity factors gives the after-diversity maximum demand (AS/NZS 3000, Appendix C)
• Main switchboards need 1.0 m minimum clearance from all accessible faces in commercial buildings (AS/NZS 3000, Cl 2.10.2.2)
• All switchboards need 0.6 m unobstructed space with doors open or switchgear fully racked out (AS/NZS 3000, Figure 2.19)
• Access openings and doorways must be at least 0.9 m wide by 2.2 m high (AS/NZS 3000, Figure 2.20)
• Boards rated 800 A per phase or over 3 m long need two emergency exit paths - one exit is acceptable if there is 3 m clear space in front (AS/NZS 3000, Cl 2.10.2.2)
• Exit doors must open outward without keys or tools from inside
• The main switchboard must be easily accessible and near a building entrance - in multi-tenancy buildings, it must not sit within any single tenancy (AS/NZS 3000, Cl 2.10.2.2)
• Circuit breakers arranged so mechanisms open in the general “off” direction (AS/NZS 3000, Cl 2.10.3.4)
• Phase sequence must follow L1–L2–L3 - single-phase loads must be balanced evenly across all three phases
• Enclosures rated IP42 for indoor dry locations, IP66 for outdoor or wet areas - all live parts shielded (AS/NZS 3000, Cl 1.5.4)

Take a small commercial building with 200 kVA of connected load. After applying diversity factors from AS/NZS 3000 Appendix C, the maximum demand might come down to around 140 kVA. At 400 V three-phase, that is about 200 A. You would size the main distribution board (MDB) at 250 A or 400 A to allow for future growth.

A typical MDB for a commercial building sits between 400 A and 1,600 A. Sub-distribution boards on each floor usually range from 100 A to 400 A. The board must handle not just the normal load but also the prospective fault current at that point in the network - the busbars and circuit breakers must withstand the thermal and mechanical stress of a short circuit.

Phase balancing matters. If you put all the lighting on one phase and all the power outlets on another, one phase runs hot while the others sit light. Spread single-phase loads across L1, L2, and L3. A common method alternates lighting circuits between phases.

For switchboard room lighting, you need 160 lux for general use, 400 lux for maintenance tasks, and at least 1 lux from emergency lighting.