, a junior electrical engineer at a bustling firm. Leo just landed his first big project: designing the electrical system for a new community hub that features a cafe, a workshop, and a small office space. The project lead, Sarah, gives him a critical task:
Calculating Maximum Demand (MD) is essential for sizing electrical components like cables and circuit breakers without over-engineering the system. It represents the highest expected peak load, rather than just the sum of all connected equipment. Core Calculation Methods Electrical standards like AS/NZS 3000 (Australia/NZ) BS 7671 (UK) define four primary ways to determine MD: Calculation (Using Diversity Factors):
Apply standard factors (based on local electrical codes) to each category.
A crucial refinement is the distinction between kW and kVA demand. For purely resistive loads (heaters, incandescent lights), kW equals kVA. However, for inductive loads (motors, transformers, discharge lighting), the power factor (PF) is less than 1. Utilities often measure MD in kVA because that reflects the total current—and hence the total strain—on their infrastructure. The relationship is: maximum demand calculation
Demand charges are typically expressed as (or $/kW) per month. For example, in a steel factory with a maximum demand of 1,500 kVA and a demand charge of $75/kVA/annum, the annual demand charge is 1,500 × 75 = $112,500. Many utilities also apply a demand ratchet —a provision that allows the customer to be billed based on the greater of the current month’s peak demand or a fixed percentage (e.g., 80%) of the peak demand during previous months, often the past 12 months. This ratchet discourages customers from having a single month of very high demand because the higher value persists.
Sarah explains the "story" of the building: "The workshop saws run intermittently. The cafe oven is on in the morning, but the office ACs don't peak until the afternoon. The building never uses its capacity all at once." This reality—the Maximum Probable Load Maximum Demand The Calculation Strategy
Divide the sum of your modified loads by the overall diversity factor to find the true peak operational requirement. Practical Calculation Example , a junior electrical engineer at a bustling firm
Sum the diversified currents calculated for each group to determine the total estimated peak current. Step 4: Determine the Final Service Rating
I=P3×VLcap I equals the fraction with numerator cap P and denominator the square root of 3 end-root cross cap V sub cap L end-fraction = Current in Amperes ( = Power in Watts ( VLcap V sub cap L = Line-to-line voltage ( Step 2: Apply Diversity Factors
"Then add it up!" Henderson snapped.
Scenario:
To calculate maximum demand accurately, several factors must be considered:
is defined as the highest average load (in kilowatts, kVA, or amperes) that occurs within a specified time interval over a billing period. The interval is typically 15, 30, or 60 minutes, depending on the utility company. It represents the highest expected peak load, rather
This report details the preliminary maximum demand calculation for the proposed development at . The assessment ensures the electrical infrastructure is appropriately sized to handle the peak anticipated load while allowing for future expansion.