Maximum Demand for a Board with a Fixed-Capacity Supply
A licensed installer is adding a new 3-phase 32 A EV charger and a 4.8 kW single-phase instantaneous water heater to an existing single-dwelling installation. The existing consumer mains are 25 mm² copper, protected by a 63 A main switch and a 63 A service fuse, with a MEN link at the main switchboard. The current maximum demand, calculated from historical bills and connected load, is estimated at 48 A per phase. The supply authority's service fuse cannot be upgraded without a new connection application and a 6-week delay. The client wants the work completed this week. The installer has measured the existing load over 14 days with a data logger, recording a 30-minute peak of 41 A on the highest-loaded phase.
1. How should the installer determine whether the new load can be connected without exceeding the capacity of the existing supply, and what does the 14-day logger data change about the calculation?
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The installer must calculate maximum demand using a recognised method (AS/NZS 3000 maximum demand tables plus diversity, or a documented alternative), not simply add connected loads. The 14-day logger data is the strongest evidence available: a measured 30-minute peak of 41 A on the worst phase is a real maximum demand figure for the existing installation and should be used in preference to a theoretical table calculation that may over- or under-state the actual demand.
The new loads must be added with appropriate diversity. The EV charger is a continuous load and should generally be taken at full rating (32 A per phase, balanced across phases). The 4.8 kW water heater at 230 V draws about 20.9 A single-phase; because it is thermostatically controlled and short-duration in practice, some diversity may be defensible, but the conservative approach is to add it at full value on the phase it is connected to. The installer must check the worst-case phase, not the average.
If the calculated or measured demand plus new load exceeds 63 A on any phase, the installation cannot proceed on the existing service fuse without the supply authority's approval. The 6-week delay is a commercial constraint, not an electrical one, and does not justify overloading the supply. Options include interlocking the EV charger and water heater so they cannot operate simultaneously, using a load management device that limits total demand, or rescheduling to allow the upgrade. The installer should document the calculation and the client's instructions in writing.
2. The client insists the work proceed immediately. What should the installer do, and what records should be created?
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The installer should not connect loads that exceed the capacity of the supply protection, because doing so creates a real risk of nuisance tripping, overheating of the service fuse and mains, and non-compliance with AS/NZS 3000. The correct response is to explain the limitation in writing and offer compliant alternatives.
A compliant interim solution is a load management or interlock arrangement that ensures the combined demand cannot exceed the supply capacity. For example, a current transformer on the mains can signal the EV charger to reduce or stop charging when total demand approaches the service fuse rating, and the water heater can be interlocked so it does not run while the charger is at full output. This must be designed and documented, not improvised.
The installer should create a written record containing: the measured maximum demand data and its basis; the calculated demand with the new loads; the capacity limit of the existing supply; the options offered to the client; the client's decision; and the scope of work actually performed. A Certificate of Compliance/Electrical Safety Certificate should only be issued for work that is compliant. If the client refuses all compliant options, the installer should decline to proceed and record that decision.