How to ditch gas for induction cooktops without frying your apartment building’s electrics

Apartment dwellers sometimes seem convinced that it is impossible to electrify their buildings without constructing a new power station next door. I hear a number of recurring misconceptions and one of these is that the building can’t support induction cooktops.

This article shows that the switch is likely to be more straightforward than most expect, and it certainly won’t require a power station next door.

Gas cooktops can still be found in recently built apartment buildings, but are particularly common in older buildings. For the same reasons I described in a previous article on induction cooktop wiring, there is a misconception that induction cooktops are power guzzling appliances that will cause the building’s grid connection to melt down at dinner time.

A recurring concept in calculating the maximum demand of anything in an apartment building, be it electricity, gas, or water, is diversity. This is the idea that the use of appliances or fixtures in the building is scattered across time and location. Correctly accounting for diversity is incredibly important. Otherwise, you could end up spending far too much money on infrastructure to handle a peak demand level that never happens.

Diversity comes to the rescue again with cooking appliances in apartment buildings in two ways.

First, cooking tends to be a fairly short activity and not everyone cooks dinner at the same time. Families with young kids may be cooking by 5.30pm. Others might not start cooking until 8.30pm. Some residents might choose to eat out. Second, as I discussed in a previous article, it is extremely rare that every ring on an individual cooktop is run at full power simultaneously.

Both these factors combine to help shrink the maximum demand to a much smaller number than multiplying the power of an induction cooktop by the number of apartments. Maximum demand is calculated at every place where there is a main switch, so we have several levels in an apartment building to consider.

Maximum demand for multi-unit buildings

For the purpose of illustration, I will use an example apartment building with 96 apartments over eight floors (12 units per floor). The building has a three-phase supply and each unit is connected to a single phase. That means there are four units connected to each phase on each floor.

At the level of individual apartments, each unit has its own internal switchboard distributing circuits internally and a main switch (typically rated at 40A or 63A). Maximum demand has to be checked for this main switch.

Each unit is connected to a distribution board, one per floor. This distribution board has its own 3-phase main switch rated to cover the maximum demand of the floor. On each floor, four units will be connected to each phase. That’s the second level of circuit aggregation where maximum demand has to be checked.

Each distribution board is then connected to the main switchboard, which has another 3-phase main switch rated to carry the maximum demand of the whole building. That’s the third level of aggregation where 32 units (four per floor) are connected to each phase. At this level, the diversity is much greater.

The aim is to design a system that is not unduly expensive, but minimises the likelihood of nuisance tripping in day-to-day use. If the current through any one of these circuit breakers is exceeded, be it in an apartment, on one floor, or in the main switchboard, the world won’t end, but a number of residents ranging from one to the whole building are going to be mightily annoyed.

AS/NZS 3000 (the wiring rules) provides Table C1 for calculating the maximum demand for different classes of electrical loads (known as “load groups”) in either single buildings or multi-unit buildings. Within each load group, four levels of aggregation are considered: one living unit per phase, 2–5 units per phase, 6–20 units, and 21 or more units.

The good news is that the distribution boards and main switchboard are likely to already accommodate electric cooktops. That’s because Table C1 says that where two or more units are connected, and there are any cooking appliances (load group C) present, the maximum demand is the same whether the units contain an electric cooktop, an electric oven, a tumble dryer, or some combination. It is very likely that the units would have been constructed with at least one of those.

For 2–5 units (e.g. at the floor distribution boards), the maximum demand is 15A. For 21 or more units (e.g. at the main switchboard), the maximum demand is 2.8A multiplied by the number of units.

As you move closer to the main switchboard and have more units connected at each level of distribution, the diversity among connected cooktops becomes greater.

If the apartments have an electric oven and a gas cooktop (often the case), the wiring between the distribution boards and the main switchboard, and the main switchboard and the electricity network should already be sufficient to accommodate the electric oven with an electric cooktop instead.

Maximum demand for individual units

The story gets a little bit more complicated at the level of individual apartments. A circuit needs to be installed for the new cooktop from the kitchen bench to the switchboard. It’s not far, but it is not necessarily a simple job in some buildings.

For units with an existing electric oven, one solution may be to pull the oven circuit cable and run a new, heavier gauge cable that can handle both appliances on one circuit. This also overcomes a potential problem of not having any spare slots in the switchboard.

An electrician needs to ensure that there is sufficient ‘headroom’ between the current maximum demand of the unit and the rating of the main switch in the unit switchboard.

To calculate the additional demand for the cooktop in an individual unit, the maximum current (the so-called “connected load”) of the new cooktop is multiplied by 50% (see load group C in Table C1 above).

A 32A induction cooktop will therefore add 16A to the maximum demand at the unit switchboard. If the additional demand exceeds the available headroom, a wiring upgrade will be needed between the unit and the distribution board it is connected to. This is where things could get expensive, although these costs would be paid by each unit owner at the time they choose to get off gas.

There are options, though, if the addition of the cooktop exceeds the available headroom. One simple solution is to select “low current” or “power lite” induction cooktops.

Some cooktops allow the installing electrician to hardwire the maximum wattage and the induction cooktop will strategically operate the cooktop zones to remain below this limitation. There are several 60cm four-zone “low current” induction cooktops on the market that internally manage power consumption so that their connected load never exceeds 20A.

This is achieved, for instance, by limiting the power available on zones 3 and 4 if you were to pair zones 1 and 2 and run them at high power. With a connected load of 20A, the maximum demand will be 20A 0.5 = 10A, which is 6A lower than otherwise and may keep maximum demand below the main switch rating. That’s a very low cost solution to a potentially expensive problem.

Next steps

The next step for owners corporations is to establish what main switches are installed in apartments and how much headroom they have. An electrician can help with this. This will help guide owners corporation decisions such as allowing residents to proceed to install a cooktop and what the maximum permitted wattage would be. An electrician can also assist with the details of how to practically run cooktop cabling within the apartments.

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This article has hopefully made it clear that induction cooktops can be readily supported in apartment buildings without large building-level network upgrades.

The most likely problems lie at the switchboards of individual apartments where the addition of the cooktop takes the maximum demand of the apartment over the rating of the main switch. Should this occur, new “power lite” cooktops represent a potentially easy way of installing these appliances in apartments.

An important conclusion is that none of this requires much coordination by an owners corporation. Each unit owner can move to electric cooking at their own pace without a large capital expenditure by the owners corporation.

Ben Elliston is an independent energy researcher and modeller. He is also the chair of the ACT branch of the Australian Electric Vehicle Association. Ben thanks Mark Hemmingsen for reviewing this article.