The Difference Between Connected Load and Maximum Demand

One of the most common mistakes in electrical design is confusing Total Connected Load with Maximum Demand. If a designer simply adds up the rating of every MCB in a distribution board, the resulting figure (the Total Connected Load) will be massively oversized. This leads to unnecessarily thick cables, oversized switchgear, and exorbitant connection upgrade fees from the DNO (Distribution Network Operator).

• Total Connected Load: The theoretical maximum current if every single light, socket, and appliance in the building was operating at 100% capacity simultaneously.
• Diversity Factor: The mathematical percentage applied to specific circuits, acknowledging that not everything is on at once.
• Maximum Demand: The actual, realistic load expected under normal operating conditions. This is the figure (Calculated in Amps and kVA) used to size the incoming supply and main protective devices.

What is the Usage Factor (UF)?

The standard IET On-Site Guide diversity multipliers assume a typical, concurrent usage of a building. However, professional designers must often account for temporal diversity—meaning the time of day a load is active. This is where the Usage Factor (UF) is critical.

For example: An industrial plant might have a total OSG Maximum Demand of 200A. However, if the plant operates in two distinct shift patterns where the heavy manufacturing machinery (120A) runs during the day, and the high-intensity cooling systems (80A) only run at night, those two loads will never draw power concurrently. The designer can apply a custom Usage Factor (e.g., 75%) to safely lower the Final Design Demand to reflect the actual operational habits of the client, saving immense costs on supply sizing.

Future Growth Allocation (Regulation 311.1)

BS 7671 requires the designer to assess the maximum demand, but best practice dictates that a commercial installation should never be designed to sit at 100% of its capacity on day one. By utilising the Future Growth input in this calculator, you can instantly add a safety margin (typically 10% to 20%) to the Final Design Demand to accommodate future circuit additions without requiring a supply upgrade.

Commercial Kitchen & Catering Rules (Table A2)

Commercial kitchens in hotels, restaurants, and schools require a unique calculation method. Unlike standard commercial premises, catering equipment is diversified using a "staggered" logic to account for high-intensity service periods:

• The Largest Appliance: Calculated at 100% of its full load.
• The Second Largest Appliance: Calculated at 80% of its full load.
• All Remaining Appliances: Calculated at 60% of their full load.

Three-Phase Balancing and Neutral Currents

In a three-phase (400 V) system, an unbalanced load creates current in the neutral conductor and can lead to voltage fluctuations. Our engine actively monitors the Maximum Demand across L1, L2, and L3. If it detects a variance of more than 20% between the highest and lowest loaded phases, it triggers a warning, prompting the designer to re-allocate single-phase loads across the distribution board.