Maximum Demand Calculator

The maximum demand calculator estimates the peak electrical load that an installation will draw simultaneously, which determines the required supply capacity, transformer rating, and main cable size.

The calculation begins by categorising all connected loads — lighting, power points, cooking appliances, air conditioning, motors, and other equipment. Each category has its own demand factor (diversity factor) that accounts for the statistical likelihood that not all loads operate at full capacity simultaneously.

Under AS/NZS 3000:2018 Section 2 and Tables C1-C5, demand factors are applied per load type. For example, general power outlets use a demand factor based on the number of points: 10A at the first point plus 5A for each additional point in a domestic installation. Lighting loads apply the connected wattage with diversity from Table C3. Cooking appliances use Table C4 which reduces demand based on the number of appliances.

BS 7671:2018+A2 Appendix A provides current demand values per point and diversity allowances in Table A1. The approach groups loads into categories (lighting, heating, cooking, motors, etc.) and applies percentage-based diversity to each. IEC 60364-1 Clause 311.1 establishes the general principles for assessing maximum demand through diversity.

The total maximum demand is the sum of all category demands after applying their respective diversity factors: MD_total = SUM(P_category x DF_category) / (V x PF x sqrt(3)) for three-phase supplies, yielding the demand in amperes. From this, the calculator determines the required supply phase configuration, minimum main switch rating, transformer kVA, and main cable size.

Results include a per-category demand breakdown, applied diversity factors with clause references, total maximum demand in kW and amperes, recommended supply capacity, and a visual stacking chart showing the contribution of each load category.

## Maximum Demand Calculation for 3-Phase vs Single-Phase Supplies

For single-phase installations (typical residential in UK and Australia), maximum demand is calculated as MD = Total_kW / (V x PF), where V is 230V (UK/AU) or 120V (US). For three-phase supplies (commercial and industrial), the formula becomes MD = Total_kW / (V_LL x PF x 1.732), where V_LL is the line-to-line voltage (400V UK, 415V AU, 480V US). The calculator automatically adjusts for phase configuration and regional voltage standards. Balanced loading across all three phases is assumed unless specific per-phase loads are entered.

## Maximum Demand Tables by Standard

Each standard provides its own maximum demand tables with distinct diversity allowances. AS/NZS 3000 Tables C1 through C5 cover domestic, commercial, and industrial installations with specific rules for cooking appliances, air conditioning, and EV charging. BS 7671 Appendix A (Table A1) uses a percentage-based diversity system per circuit category. NEC Article 220 provides demand factors in Tables 220.42 (lighting), 220.44 (receptacles), and 220.55 (cooking equipment). IEC 60364 Clause 311.1 establishes principles but defers detailed diversity factors to national annexes. Understanding which maximum demand table to use for your jurisdiction is critical — applying AS/NZS diversity factors to a BS 7671 installation can result in undersized or oversized supply equipment.

## Residential vs Commercial Maximum Demand

Residential maximum demand calculation follows the domestic tables in each standard, where loads are relatively predictable: lighting, socket outlets, cooking, water heating, and increasingly EV charging and heat pumps. Diversity is high (typically 40-60%) because residents use appliances intermittently. Commercial maximum demand is more complex because load profiles vary dramatically by building type. An office building with predominantly lighting and computing loads has different diversity from a restaurant with continuous cooking and refrigeration. Industrial maximum demand requires consideration of motor starting diversity, process load duty cycles, and the coincidence factor for large equipment. The calculator supports all three building types with appropriate diversity tables.

## Maximum Demand Formula: Step-by-Step

Step 1: List all connected loads by category (lighting, socket outlets, cooking, heating/cooling, motors, special loads). Step 2: Determine the connected load in watts or amperes for each category. Step 3: Apply the diversity factor from the appropriate standard table (AS/NZS 3000 Table C1, BS 7671 Table A1, or NEC Table 220.42). Step 4: Sum all diversified loads to obtain the total maximum demand in kW. Step 5: Convert to amperes using MD(A) = MD(kW) x 1000 / (V x PF) for single-phase, or MD(A) = MD(kW) x 1000 / (V x PF x 1.732) for three-phase. Step 6: Select the next standard main switch rating (63A, 80A, 100A, etc.) and size the supply cable accordingly. The calculator performs all six steps automatically with full standard clause references.

## Transformer Sizing from Maximum Demand

Once maximum demand is known in kW, transformer sizing follows: kVA_required = MD_kW / PF, where PF is typically 0.85 to 0.95 depending on the load mix. Add 15-25% growth margin per AS/NZS 3000 Clause 2.2.2 and IEC 60076-1. Select the next standard transformer rating: common sizes are 100, 160, 200, 315, 500, 630, 800, 1000, 1250, 1600, and 2000 kVA. Oversizing wastes capital and increases losses; undersizing risks overloading and premature failure. The calculator provides a direct transformer size recommendation cross-linked to the transformer calculator for detailed impedance and short-circuit analysis.