Maximum Demand Calculator

Parameters

The form changes shape depending on the installation type you pick. Everything below is documented with the allowance the calculator actually applies to it, so you can see how each entry moves the total.

Shared parameters

Installation typeresidential or commercial

Selects which set of allowances runs and which fields appear. Residential uses per-circuit kilowatt allowances with reducing diversity. Commercial uses flat percentage diversity against total connected load. A mixed-use site, such as a shop with a flat above it, should be calculated as two separate runs and the results added.

Supply phases1 or 3

Sets the conversion from kilowatts to amps and, with it, the supply voltage. There is no separate voltage field: single phase is fixed at 230 volts and three phase at 400 volts. Single phase divides by 230; three phase divides by 400 times 1.732, the square root of 3.

Residential parameters

Lighting circuitscount, 0 or more

The number of final subcircuits dedicated to lighting, not the number of light points. The first circuit is allowed 1 kilowatt and every circuit after it 0.75 kilowatts. Six circuits therefore contribute 4.75 kilowatts, not 6.

Power point circuitscount, 0 or more

The number of general purpose socket outlet subcircuits, again counting circuits rather than outlets. The first is allowed 1 kilowatt and every circuit after it 0.5 kilowatts. Eight circuits contribute 4.5 kilowatts.

Permanent heatingkilowatts, 0 or more

Connected load of fixed space heating. Paired with permanent cooling below.

Permanent coolingkilowatts, 0 or more

Connected load of fixed air conditioning or cooling. Heating and cooling are assessed together: whichever is larger counts at 100 percent and the smaller one at 50 percent, on the basis that a building rarely heats and cools at once. Enter zero in one field if the installation only has one of them.

Cooking applianceslist of kilowatt ratings

Add one entry per cooking appliance. The list is sorted largest first inside the calculator, so the order you type them in does not matter. The largest counts at 100 percent. The second gets its first 1 kilowatt at 100 percent and everything above that at 80 percent. The third and every one after gets its first 1 kilowatt at 100 percent and the remainder at 60 percent.

Water heaterkilowatts, 0 or more

Connected load of electric water heating, counted at 100 percent as a continuous load. The calculator does not distinguish storage from instantaneous, and it applies no reduction for an off-peak or controlled-load tariff. If the water heater is on a separate controlled circuit that your distributor excludes from the maximum demand, leave this at zero and note it on the calculation.

Other fixed applianceslist of name and kilowatt rating

Any other permanently connected appliance: pool pump, spa heater, wall oven on its own circuit, workshop machine. Each entry counts at 100 percent with no diversity. The name is carried into the breakdown and the exported report, so use something you will recognise later.

Motor loadslist of kilowatt ratings

One entry per motor. The largest counts at 100 percent and every other motor at 80 percent. Enter mechanical nameplate ratings converted to electrical input where the two differ, since the calculator treats every figure as electrical kilowatts.

EV chargerkilowatts, 0 or more

Electric vehicle charging equipment, counted at 100 percent as a continuous load. It only appears in the results breakdown when it is greater than zero. A 32 amp single phase charger is about 7.4 kilowatts and a common three phase unit is 22 kilowatts. The calculator takes no credit for dynamic load management or a controlled charging schedule, so if the installation has either, the real assessed demand may be lower than shown.

Commercial parameters

Total lightingwatts, 0 or more

The total connected lighting load for the premises in watts, not kilowatts. The calculator converts to kilowatts and applies a flat 75 percent diversity.

Floor areasquare metres, greater than 0

Collected on the form and validated, but not used anywhere in the commercial calculation. It does not affect the result. It is there for future watts-per-square-metre lighting estimation and is worth filling in for the record only.

Total socket outlet circuitscount, 0 or more

The number of general purpose outlet circuits. Each is allowed 0.5 kilowatts and then 50 percent diversity is applied, so the net contribution is 0.25 kilowatts per circuit. Thirty circuits contribute 7.5 kilowatts.

Total fixed appliance loadkilowatts, 0 or more

The sum of all permanently connected appliances, counted at 100 percent with no diversity.

Total motor loadkilowatts, 0 or more

The sum of all motor loads, to which the calculator applies a single flat 85 percent factor. Because it takes only a total, it cannot apply the largest-motor-at-full-load rule that the standard uses, so a site with one dominant motor will be assessed less accurately than one with many similar motors.

Assumptions and limits

Maximum demand is the calculation most likely to be argued over, so it is worth being blunt about what this implementation actually does.

What the calculator assumes

• The diversity model is a simplified kilowatt-per-circuit method. It is not a transcription of AS/NZS 3000:2018 Appendix C. The residential path uses kilowatt allowances per circuit, whereas Table C1 assigns amperes per number of points and treats load groups such as cooking, water heating and space conditioning with their own rules. The commercial path uses three flat percentages, whereas Table C2 varies the allowance by premises class. The two approaches will not produce identical numbers.
• Everything is treated as unity power factor. Kilowatts convert straight to amps with no kilovolt-ampere step and no reactive allowance. A site heavy in motors or discharge lighting will draw more current than the figure shown here.
• Three phase is assumed balanced. The conversion divides by 400 times the square root of 3 with no phase imbalance. On a real domestic three phase board, one phase commonly carries far more than a third of the load, and it is that phase that decides the main switch and the consumer mains.
• No controlled load or tariff separation. Off-peak water heating, controlled EV charging and load-shed circuits are all counted at full connected load.
• Main switch selection is arithmetic only. The calculator picks the next standard rating from 40, 63, 80, 100, 125, 160, 200, 250, 315, 400 and 630 amps that is greater than or equal to the calculated demand. It does not consider fault rating, consumer mains capacity, metering limits, or the declared supply capacity of the distributor. Demand of 63.5 amps will select an 80 amp switch, because 63 amps is not enough.
• Floor area is inert. Entering it changes nothing in the commercial result.

What this should not be used for

• A connection application or supply upgrade request lodged with a distributor. Every distributor has its own assessment form and its own accepted method.
• The design of record for consumer mains or main switchboard sizing.
• Multi-dwelling blocks, where the per-living-unit diversity in Table C1 does most of the work and is not implemented here.
• Generator, uninterruptible power supply or battery system sizing, which need real load profiles and starting currents rather than a diversified average.
• Any site where the actual metered demand is available. Recorded interval data beats a diversity table every time.

The diversity allowances behind this calculator are indicative and have not been validated by a chartered professional engineer. Check each load group against the current edition of AS/NZS 3000 Appendix C and the applicable distributor service and installation rules, and have the responsible person for the installation sign off the final assessment.

Worked examples

Three assessments showing the allowances applied load group by load group, one single phase residential, one three phase residential, and one three phase commercial.

Maximum Demand for AS/NZS 3000:2018

Maximum demand is the greatest average power drawn over a defined period, used to size main switches and determine meter and supply capacity. In Australia, maximum demand is calculated using diversity factors from AS/NZS 3000:2018 Tables C1 (residential) and C2 (commercial). Diversity factors account for the fact that not all loads operate simultaneously at full capacity.

Residential maximum demand (Table C1)

• Lighting: 3 A for 1 to 20 points, plus 2 A for each additional 20 points or part thereof.
• Socket outlets not exceeding 10 A: 10 A for 1 to 20 points, plus 5 A for each additional 20 points or part thereof. Add 10 A if the installation includes any 15 A socket outlets, or 15 A if it includes any 20 A socket outlets.
• Fixed space heating or airconditioning: 75% of connected load. Where the installation has both a heating system and a cooling system, only the one with the greater load is counted.
• Ranges, cooking appliances, laundry equipment and socket outlets rated over 10 A: 50% of connected load for a single domestic installation or individual living unit.
• Water heating: storage water heaters at full load current, instantaneous water heaters at 33.3% of connected load.
• Fixed appliances: Each at 100%.
• Motors: assessed under Table C2 Column 2, being the full load of the highest rated motor plus 50% of the full load of the remainder.
• EV charger: Continuous load (100%).

Commercial maximum demand (Table C2)

• Lighting: 75% of connected load for residential institutions, hotels, boarding houses, hospitals, accommodation houses and motels. Full connected load for factories, shops, stores, offices, business premises, schools and churches.
• Socket outlets not exceeding 10 A: 1000 W for the first outlet, plus 400 W for each additional outlet in residential institutions, hotels and similar, or plus 750 W for each additional outlet in factories, shops, offices and similar. Where the building has permanently installed heating or cooling, the allowance is 1000 W for the first outlet plus 100 W for each additional outlet.
• Fixed appliances: 100%.
• Motors: full load of the highest rated motor plus 50% of the remainder for residential institutions, hotels and similar. For factories, shops, offices and similar, full load of the highest rated motor plus 75% of the second highest plus 50% of the remainder.

Main switch selection

Main switch rating must be ≥ the calculated maximum demand current. The calculator selects the next standard size (40A, 63A, 80A, 100A, 125A, 160A, 200A, 250A, 315A, 400A, 630A).