Understanding Maximum Demand vs Maximum Load
NECA Victoria are often approached by members regarding clarification the requirements and differences between Maximum Demand and Maximum load within an electrical installation.
Electrical installations are designed around two related but different concepts: maximum load and maximum demand.
Maximum load is the total connected electrical load that could operate if all appliances, items of equipment, circuit or load point were switched on and drawing their rated current at the same time. The resultant current drawn is based on installed capacity, not necessarily on normal operation.
Maximum demand is the greatest load an installation is expected to draw from the supply at one time under realistic operating conditions. It recognises that not all loads operate together. For example, a house may have lighting, socket outlets, cooking appliances, air conditioning, hot water and an electric vehicle charger installed, but these loads will usually cycle on and off or be used at different times.
As maximum demand accounts for diversity, it is normally lower than maximum load, and in many cases making it a more a beneficial method for selecting consumer mains, main switches, switchboards, protective devices and supply capacity.
AS/NZS 3000:2018 allows that maximum demand may be determined by calculation, assessment, measurement or limitation, depending on the installation and the available information. Diversity factors are commonly applied to groups of loads so that the installation design reflects realistic use, rather than the simple sum of nameplate ratings.
Understanding the distinction helps avoid either under-sizing or over-sizing of the required supply. If maximum demand is underestimated, cables or protective devices may be overloaded, creating overheating, nuisance tripping or safety risks.
If maximum load is used without diversity, the installation may lead to increased costs as cable and equipment is oversized.
In short, maximum load describes total connected capacity, while maximum demand describes the realistic peak demand the electrical supply must safely support.
Impact on Electrical Installation Design
Maximum demand is one of the main inputs used in electrical installation design. How much current the installation is expected to draw, influences the selection of consumer mains, submains, switchboards, main switches, protective devices and upstream supply arrangements.
- Supply capacity: The calculated maximum demand determines whether the existing supply is adequate or whether consumer mains, network upgrade or demand limitation arrangement is required.
- Cable sizing: Consumer mains, submains and final sub-circuits must be selected to carry the expected demand safely, while also allowing for installation conditions, and future load capacity.
- Switchboard and busbar ratings: Main switchboards and distribution boards must have ratings suitable for the expected demand, available fault current and connected load arrangement.
- Protective device selection: Circuit-breakers, main switches need to be coordinated so they can carry normal demand, interrupt faults and avoid unnecessary tripping during normal operation.
- Voltage drop: Higher demand increases voltage drop along cables, so the design requires that voltage at the load remains within acceptable limits, especially allowing for long cable runs.
- Thermal performance: Incorrect maximum demand calculations may cause cables, terminals, switchgear and enclosures to operate at higher than intended temperatures
- Future expansion: A well-considered maximum demand assessment should allow practical spare capacity for future expansion for additional equipment such as EV chargers, heat pumps, A/C units.
- Diversity factors: Calculation, assessment, measurement or limitation may be used to determine maximum demand.
Common Mistakes to Avoid
- Confusing maximum load with maximum demand: Adding every connected load together without applying diversity can greatly overstate the supply required.
- Applying one generic diversity factor to everything: Different load types, such as lighting, socket outlets, cooking equipment, motors, air conditioning and EV charging, may need different assessment rules.
- Ignoring loads that run continuously: Some loads, such as refrigeration, safety systems, process equipment or essential ventilation, may need to be treated as operating for long periods or continuously.
- Overlooking phase balance: In three-phase installations, the maximum demand should be reviewed per phase because one overloaded phase can limit the installation even if the total kW appears acceptable.
- Relying only on nameplate ratings: Nameplate values are important, but measured demand, operating patterns, control systems and realistic duty cycles should also be considered where available.
- Not allowing for future growth: A design that only just meets the present demand may be difficult or costly to upgrade when new equipment is added in the future.
- Ignoring supply authority requirements: Network service providers may have particular requirements for connection applications, demand assessment and load limits.
- Using outdated assumptions: Modern installations may include EV chargers, solar inverters, batteries, heat pumps or controlled loads that change the demand profile compared with older installations.
- Failing to document assumptions: Diversity allowances, operating assumptions, power factor values and calculation methods should be recorded so the design can be checked, reviewed and updated later.
Summary
The impact of installation design is a balance between safety, reliability and cost. If maximum demand is underestimated, the installation may suffer from overloaded equipment, nuisance tripping, poor voltage performance or unsafe operating temperatures. If it is overstated, the result may be oversized cables, switchgear and higher installation costs.
A good design therefore uses realistic demand assumptions, appropriate diversity, phase-by-phase checking and clear documentation.