Maximum Demand Calculator

If you added up the nameplate rating of every light, socket outlet, and appliance in a house, you would get a number far bigger than what that house actually draws at any one moment. Not everything runs at full load at the same time — the cooktop cycles on and off, the air conditioner is not always on maximum, and every light in the house is rarely switched on together. Sizing the consumer mains and main switch off the raw connected total would mean paying for cable and switchgear the installation will never actually use.

AS/NZS 3000 Appendix C solves this with load groups and diversity factors. Every load in the installation gets sorted into a group — lighting, socket outlets, fixed appliances, cooking, hot water, heating and cooling, EV charging, and so on. Each group has its own diversity rule that reflects how that type of load actually behaves in practice, and the diversified sub-totals get added together for the final maximum demand figure in amps and kVA. This calculator applies that grouping and diversity automatically once you enter the loads for the installation.

AS/NZS 3000 Appendix C splits maximum demand into two main tables depending on what you are supplying. Table C1 covers domestic installations — a single dwelling uses Column 2, while blocks of living units use the other columns to account for diversity across multiple homes on one supply. Table C2 covers non-domestic installations — commercial and industrial sites — with different load groupings suited to that kind of building: things like continuous processes, larger motor loads, and equipment that does not follow a household usage pattern.

This calculator implements Table C1 Column 2, the single domestic case. It is the right tool for a house, a granny flat, or a single-unit switchboard upgrade. For a block of units, a shop fit-out, or an industrial supply, you need the appropriate column of Table C1 or the relevant part of Table C2, and those often call for engineering judgement on top of the table figures rather than a straight lookup.

Take a single-phase 230V house with 25 lighting points, 20 general-purpose 10A socket outlets, a storage hot water system with a 15.7A element, an 8kW electric cooktop and oven, and a 7kW ducted reverse-cycle air conditioner. If you simply added up the connected load on the fixed appliances alone, ignoring diversity altogether, it would look like this:

No consumer mains would ever be sized off that raw figure, and no distributor would expect it to be. Once the AS/NZS 3000 Table C1 diversity factors are applied to each group — lighting, socket outlets, cooking, and heating and cooling all have their own rule — the diversified maximum demand comes out noticeably lower than the connected total, because the calculation accounts for the fact that the cooktop, the air conditioner, and every light in the house are not all running flat out at the same instant. Enter these same loads into the calculator above for the actual diversified figure in amps and kVA, itemised by load group.

AS/NZS 3000 recognises more than one way to arrive at a maximum demand figure. Calculation applies the Appendix C diversity factors to each load group — the most common approach for a new domestic or commercial fit-out, and what this calculator automates for Table C1 Column 2. Assessment relies on engineering judgement where load patterns are unusual, intermittent, or do not fit the standard groups, and needs the reasoning documented. Measurement uses a maximum demand indicator or recorder on an existing installation to log the actual peak demand sustained over time, which then overrides a calculated or assessed figure if it comes out higher.

A free calculator like this one covers the calculation method for a single house fast, on site, with no signup. It is not a substitute for full building services software when you are modelling a multi-storey development, staged construction, or a site with dozens of tenancies — that scale of job calls for purpose-built design software or a specialist assessment, not a single-page browser tool. Know which category your job falls into before you rely on any maximum demand figure for a distributor application.