For Installers, Renewables, Solar

Does a smart home need a supply upgrade?

Considering whether to upgrade a consumer main is a tricky question becoming more common as households embrace electrification, writes Michael Shaughnessy, technical program specialist at the Clean Energy Council.

As electricity prices soar and the momentum of the energy transition increases, many consumers are busily electrifying their homes and vehicles and fuelling them with solar and battery storage. With this electrification comes increased demand on electricity distribution networks.

Much of the network was designed for lower consumption and this increase in demand can pose some curly questions around maximum demand for a domestic dwelling.

Here are questions consumers, electricians, electrical contractors, retailers and industry stakeholders need to get their heads around.

Matters of import: Understanding maximum demand

Maximum demand is the peak load current we expect to see on a cable (AS/NZS 3000:2018 cl. 1.6). Consumer mains and sub-mains are sized to carry this current, taking into account the way the cable is run. The consumer main runs from the point of supply, usually the network service provider’s fuse(s) to the main switch of the main switchboard. Sub-mains run from the main switchboard to any distribution boards that may be onsite.

AS/NZS 3000:2018 cl. 2.2.2 gives four possible methods for determining maximum demand: calculation, assessment, measurement and limitation.

Calculation: Calculation is one of the most common methods, with the process and tables given in AS/NZS 3000 Appendix C making a solid basis for explaining the value obtained. While much has changed in household appliances with the advent of inverter technology and the way houses are used as more people frequently work from home, the tables in Appendix C in AS/NZS 3000 haven’t changed in at least 22 years, barring the addition of electric vehicle (EV) supply equipment at 100 per cent, potentially resulting in an overly conservative outcome.

Assessment: Where the load is fluctuating, follows a definite duty cycle, is large and complex, or special types of occupancy exist, the maximum demand may be assessed. This method is not well defined, but the use of AC load analysis tables that consider various probable use cases would likely be of assistance.

Measurement: AS/NZS 3000:2018 states that where measurement is greater than the calculation or assessment methods, the measured value is the deemed maximum demand. It is carried out using an instrument over “30 minutes when demand is at its highest”. The standard does not go into what you have to do to push demand to “its highest”.

It is possible to be unrealistic about it and turn everything on, pitting the air-conditioner against the floor heating, but to replicate the most likely maximum demand situation, consider the most probable worst-case scenario and turn any solar and storage off for 30 minutes.

Any controls that would normally be in place such as home energy management systems, hot water solar diverters and smart EV charging operations should be left in place. With the solar and storage off, these controlled devices will behave as they would when there is no solar and the battery is fully discharged.

Limitation: Where the consumer main is concerned, the maximum demand will be the lower of the following two options. The first is where the maximum demand is simply the current rating of the circuit breaker protecting the consumer main. The second is where the maximum demand is the summation of circuit breakers protecting the associated final subcircuit(s) and any further sub-main(s).

The kicker with this final method is the installation still has to function correctly in line with section 1.6 of AS/NZS 3000 so if nuisance tripping is too frequent due to an overloaded consumer main circuit breaker, reparations will need to be made – and who pays for that?

Consider a dwelling with a modest supply, a consumer main good for little more than 50A with the way the cable has been installed. The owner is well advanced with electrification, including reverse cycle air-conditioning, induction cooktop and electric heat pump. They’ve recently purchased an EV and are considering installing a 7kW smart charger (30A) that they aim to charge mostly using excess solar (10kW system). Unfortunately, due to the intermittent nature of solar and battery storage, this generation will likely never be considered when determining maximum demand. Would this consumer require a consumer main upgrade?

When our dwelling owner goes out to market for their three (or more) quotes from electrical contractors/retailers for their EV charger installation, they may be surprised by some of the responses they get, ranging from quick installation with no consideration of maximum demand to quotes for thousands of dollars with seemingly very knowledgeable and authoritative advice that their supply will need to be upgraded beforehand.

So what is the answer?

In many cases, the requirement for costly and time-consuming supply upgrades can be staved off by control devices and limitation devices, at least in the short-term.

Matters of export

We are already seeing several devices vying for a slice of the export capacity on a consumer main and this will only increase in the future.

Most distribution network service providers (DNSP) have limits for the size of inverters that may be installed per phase with automatic approval. If you wish to install larger inverters than will be automatically approved, you need to apply to the DNSP for assessment.

Indicating the export limit you are willing to impose on the inverters can help. It is tempting to assume that upgrading the supply will ensure the inverters you wish to install will be allowed, but where the inverters in total will still be larger than the automatically approved value, this is not necessarily the case as there may be other network constraints in play. When a consumer wants to export more than the DNSP will allow, a consumer main upgrade could be considered.

Another consideration is voltage rise (Vr). When inverters push electricity out onto the grid, they do so at a slightly higher voltage than they see on the network to allow the current to overcome the impedance on the line. It is more complicated than that but suffice to say, many inverters attempting to export more than the cables were designed for – for example, voltage drop is allowed to be five per cent from point of supply to end of installation – can push the voltage too high.

AS/NZS 4777.1: 2016 states that Vr shall not exceed two per cent from the point of supply to the inverter AC terminals, with a recommendation of one per cent. The value of the current used for the Vr calculations shall be the rated current of the inverter energy system (IES).

The IES is defined as one or more inverters together with one or more energy sources and controls. Export limits are an example of controls, and therefore the reduced current value for the exported power may be used for the consumer main part of the Vr calculation. In many instances, this will help avoid any need for consumer main upgrade as the inverter cables and any intermediate sub-mains can be made larger instead.

Stakeholder responsibilities to their industry

Unnecessary upgrades to consumer mains slow the energy transition by adding cost and complexity to consumers and DNSP. Consumers suspicious of contractors and retailers looking to monetise the energy transition by performing questionable consumer main upgrades risk reducing social license and harming the industry’s good reputation.

As a result, consumers should always do their due diligence and get several quotes from reputable companies, while contractors and retailers should only offer upgrades where they are wanted or absolutely necessary.

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