Understanding Maximum Demand, AS/NZS3000

Introduction

According to the Australia wiring rules book AS/NZS3000 the maximum demand is the maximum current which the consumer mains or submain conductor can carry for a sustained period without overheating.

Why is it important?

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Energy experts, engineers, and financial analysts invest much time in understanding a system’s maximum demand, and they have good reasons. Some of these include:

Sizing Loads

Understanding maximum demand helps in sizing electrical infrastructure appropriately. It ensures that the capacity of generators, transformers, cables, and other components matches the actual peak load, preventing under- or over-sizing.

Resource Planning

Utility companies use maximum demand data to plan and allocate resources effectively. This helps ensure a stable and reliable power supply to consumers during peak usage times.

Load Management

Knowledge of maximum demand allows for better load management. Businesses can schedule non-essential activities during off-peak times to reduce demand charges and minimize operational costs.

Energy Efficiency

Monitoring and managing maximum demand can lead to improved energy efficiency. Identifying peak demand patterns can help implement energy-saving strategies during high-demand periods

Grid Stabilty

Excessive demand can strain the power grid, potentially leading to voltage drops or even outages. Managing maximum demand helps maintain grid stability and prevents overloading.

Renewable Integration

Maximum demand data assists in integrating renewable energy sources like solar and wind. Aligning peak energy generation from renewables with peak demand helps optimize energy usage.

Sizing Backup Systems

Emergency backup systems like generators or batteries are often designed based on maximum demand. Accurate knowledge ensures backup systems can handle peak loads during power outages

Infrastructure Investment

Utility companies use maximum demand data to plan infrastructure upgrades and expansions, ensuring the grid can handle future growth and changes in consumption patterns.

How do we determine maximum demand?

AS/NZS3000 states that the maximum demand of an electrical installation shall be determined, taking account of the capacity, physical distribution and intended use of electrical equipment in the electrical installation and the manner in which the present requirements might vary.

Following through on this definition the maximum demand can be determined using the following methods

Assessment method
Measurements
Limitations, and
calculations

Lets go through each of these methods in turn.

By Assessment

The assessment method is used on sites where electrical equipment operates under fluctuating or intermittent loads or a defined duty cycle, the electrical installation is large or complex as well as if special types of occupancy exist.

This method involves going through the installation, looking at nameplate data of equipment and just trying to figure out how things work.

Its not the best way of determining maximum demand and there are better methods of doing it.

By Measurement

The other method of determining maximum demand is by measurement.

The maximum demand may be determined by the highest rate of consumption of electricity recorded or sustained over a period of 30 minutes when demand is at its highest by a maximum demand indicator or recorder.

To get the best out of this method its better to measure across seasons so that seasonal; loads are also captured.

Done correctly this the best method for determining maximum demand.

By Limitation

This method is very easy as the maximum demand can be obtained immediately from the circuit protection device.

In your home if the main circuit breaker is 60A then the maximum demand is taken as 60A, if the main circuit breaker has adjustable trip settings, then the current setting at that time is taken as the maximum demand.

Alternatively, if drawings of the site are available then a quick check of the circuit main breaker or isolator will yield the maximum demand.

This method normally gives misleading results as the circuit will normally operate at currents way less than the main switch setting.

Also, to achieve protection grading with upstream and downstream protective devices the main switch might be set at higher current settings than normal.

The drawing below shows a situation where a Main Switch can be used to determine the maximum demand of an installation.

In this case the installation has a maximum demand of 160A.

By Calculation

The last method which we would look at is by is by calculation.

The maximum demand may be calculated in accordance with the given standards for the appropriate type of electrical installation and electrical equipment supplied.

Calculations done according to table C1 for domestic installations and Table C2 for industrial installations.

Diversity Factors

This is the ratio of the sum of the individual maximum demands of the various subcircuits of a system to the maximum demand of the whole system.

When we apply diversity factor to maximum demand calculation, we get what we refer to as an after diversity maximum demand.

Diversity factors applied depend on:

Type of installation residential Vs Industrial.
Size and type of loads
Operating conditions of loads eg tropical conditions for air conditioners
Quantity and types of power outlets.