Load and Maximum Demand Calculations
Maximum demand is the largest load the installation is expected to draw, and it drives consumer mains, main switch and protection sizing. AS/NZS 3000 provides diversity or assessment methods - either calculated diversity (applying factors to connected load by category) or a documented assessment based on known usage. You must know which method applies and show your reasoning.
Key concepts: connected load (sum of all loads), diversity factor (proportion assumed simultaneous), demand (the result), and coincidence across categories. Domestic calculations typically apply diversity to socket outlets, lighting, cooking and heating separately; commercial and industrial loads often use measured or declared demand. Three-phase balance matters: unbalanced single-phase loads across phases affect neutral current and protection.
Traps: applying domestic diversity tables to a commercial kitchen; forgetting fixed appliances that run simultaneously (e.g. air conditioning plus cooking); ignoring future load or a spa/pool addition; and mixing up kW and kVA - motors and welders have power factor, so kVA is the correct basis for cable and protection sizing where the load is reactive.
Worked example: A house has 12 kW of cooking, 6 kW of heating and 8 kW of socket outlets. Applying typical domestic diversity, you would not simply sum 26 kW; instead apply the standard factors per category, then add any load assumed continuous at full value. The reduced demand sizes the consumer mains, and the result must still satisfy the Ib ≤ In ≤ Iz chain with voltage drop checked over the actual run.