Worked Example: Maximum Demand for a 5-Storey Commercial Building per AS/NZS 3000:2018

A new 5-storey commercial office building is under design. The electrical engineer must calculate the maximum demand to size the main switchboard (MSB), main incomer protective device, and building transformer. The building has a standard commercial office layout with fluorescent/LED lighting, general power outlets, centralised HVAC, passenger lifts, and a UPS system for the IT infrastructure. This example demonstrates the area-based load estimation method with demand factors from AS/NZS 3000:2018, Appendix C.

First, determine the total connected load for each category across the entire building:

Lighting:

P_lighting = lighting density x total area              -- (Eq. 1)
P_lighting = 12 W/m^2 x 3,000 m^2
P_lighting = 36,000 W = 36 kW

General power outlets (GPO):

P_GPO = GPO density x total area                        -- (Eq. 2)
P_GPO = 25 VA/m^2 x 3,000 m^2
P_GPO = 75,000 VA = 75 kVA

At PF 1.0 (resistive GPO loads assumed): P_GPO = 75 kW

Air conditioning:

P_AC = units per floor x kW per unit x number of floors  -- (Eq. 3)
P_AC = 4 x 10 kW x 5
P_AC = 200 kW

Lifts:

P_lifts = 2 x 22 kW = 44 kW                             -- (Eq. 4)

UPS:

P_UPS = 40 kVA at PF 0.90 = 36 kW                       -- (Eq. 5)

Total connected load:

P_total = 36 + 75 + 200 + 44 + 36 = 391 kW              -- (Eq. 6)

Apply the demand factors from AS/NZS 3000:2018, Appendix C, Tables C2–C5 to each load category. These factors account for the statistical likelihood that all loads will not operate at full capacity simultaneously.

Lighting (Table C2 — commercial office):

Demand factor = 0.90
MD_lighting = 36 kW x 0.90 = 32.4 kW                    -- (Eq. 7)

The 0.90 factor reflects that approximately 10% of lighting is typically off at any given time (unoccupied offices, corridors on sensors, meeting rooms not in use).

General power / GPO (Table C3 — commercial office):

Demand factor = 0.40
MD_GPO = 75 kW x 0.40 = 30.0 kW                         -- (Eq. 8)

The 0.40 factor is critical: while each floor has dozens of socket outlets, most are unused or lightly loaded (phone chargers, monitors, desk lamps). Only about 40% of the connected GPO capacity is drawn at any time.

Air conditioning (Table C4 — cycling loads):

Demand factor = 0.80
MD_AC = 200 kW x 0.80 = 160.0 kW                        -- (Eq. 9)

Split systems cycle on and off based on thermostat settings. The 0.80 factor accounts for this cycling and the fact that not all zones call for cooling simultaneously, even on hot days.

Lifts (Table C5 — 2 lifts):

Per Table C5, for 2 lifts: largest at 100%, second at 80%:

MD_lifts = 22 x 1.00 + 22 x 0.80                        -- (Eq. 10)
MD_lifts = 22.0 + 17.6
MD_lifts = 39.6 kW

The diversity factor for lifts reflects that in a 2-lift building, it is unlikely both lifts are accelerating simultaneously under full load. One lift typically serves while the other waits or travels lightly loaded.

UPS (continuous IT load):

Demand factor = 1.00
MD_UPS = 36 kW x 1.00 = 36.0 kW                         -- (Eq. 11)

UPS loads are continuous and do not cycle. The demand factor is 1.00 because the IT equipment draws a consistent load 24/7.

Tabulate the individual maximum demands:

Sum of individual maximum demands = 298.0 kW              -- (Eq. 12)

The ratio of maximum demand to connected load is 298.0 / 391 = 0.762 (76.2%), which is a reasonable result for a commercial building of this size and mix of loads.

For a building of this size, a building-level diversity factor can be applied per AS/NZS 3000:2018, Table C1. For a 5-storey commercial building with total MD below 500 kW:

Building diversity factor = 0.85                           -- (Eq. 13)

This factor accounts for the non-coincidence of peak loads across different categories. For example, maximum air conditioning load (hot afternoon) typically coincides with reduced lighting load (daylight), and peak lift usage (morning/evening) occurs when GPO loads are lower.

After-Diversity Maximum Demand (ADMD):
ADMD_kW = 298.0 x 0.85 = 253.3 kW                        -- (Eq. 14)

Convert to kVA at the average building power factor of 0.90:

ADMD_kVA = ADMD_kW / PF = 253.3 / 0.90                   -- (Eq. 15)
ADMD_kVA = 281.4 kVA

Rounding to a practical design figure: ADMD = 287 kVA (allowing a small margin for load estimation uncertainty).

Transformer selection:

Standard transformer ratings: 100, 200, 315, 500, 750, 1000, 1500 kVA.

The ADMD of 287 kVA requires a minimum transformer rating of 315 kVA. However, to allow for future load growth (EV charging, increased IT loads, tenant fit-outs), a 500 kVA transformer is recommended:

Loading factor (315 kVA) = 287 / 315 = 91.1%   -- too high
Loading factor (500 kVA) = 287 / 500 = 57.4%   -- healthy margin  -- (Eq. 16)

The 500 kVA transformer at 57.4% initial loading provides 42.6% spare capacity for growth over the building’s 25+ year life.

Main switchboard incomer sizing:

I_main = S_transformer / (sqrt(3) x V)                    -- (Eq. 17)

For 500 kVA transformer:
I_main = 500,000 / (1.732 x 415)
I_main = 500,000 / 718.8
I_main = 695.7 A

However, the MSB incomer should be sized for the actual maximum demand with growth margin, not the full transformer rating (unless the transformer is dedicated to this building):

I_ADMD = ADMD_kVA x 1000 / (sqrt(3) x V)                  -- (Eq. 18)
I_ADMD = 287,000 / (1.732 x 415)
I_ADMD = 287,000 / 718.8
I_ADMD = 399.3 A

Select a 400 A main incomer MCCB (or ACB for higher fault ratings). The MSB bus rating should be 630 A to allow for future capacity increase without replacing the switchboard.

Each floor’s distribution board (DB) should be sized for the per-floor maximum demand:

Per-floor connected load:
  Lighting: 36/5 = 7.2 kW
  GPO: 75/5 = 15.0 kW
  AC: 4 x 10 = 40.0 kW
  Per-floor total = 62.2 kW                                 -- (Eq. 19)

Per-floor MD (with demand factors):
  Lighting: 7.2 x 0.90 = 6.48 kW
  GPO: 15.0 x 0.40 = 6.00 kW
  AC: 40.0 x 0.80 = 32.00 kW
  Per-floor MD = 44.48 kW                                   -- (Eq. 20)

Per-floor current at 415 V 3-phase:
  I_floor = 44,480 / (1.732 x 415 x 0.90) = 68.8 A        -- (Eq. 21)

Each floor DB should have a 100 A incomer (next standard size above 68.8 A with margin for tenant loads). The sub-main cable from the MSB to each floor DB is sized based on this current plus derating for installation conditions.

Note: Lifts and UPS are fed directly from the MSB, not through floor DBs.

The building ADMD of 287 kVA drives a 500 kVA transformer selection (with growth margin) and a 400 A main incomer at 415 V. The dominant load category is air conditioning at 160 kW (53.7% of total MD), which is typical for commercial buildings in warm climates.

• AS/NZS 3000:2018, Appendix C — Demand factors for maximum demand estimation
• AS/NZS 3000:2018, Table C1 — Building diversity factors
• AS/NZS 3000:2018, Table C2 — Lighting demand factors
• AS/NZS 3000:2018, Table C3 — General power (GPO) demand factors
• AS/NZS 3000:2018, Table C4 — Air conditioning demand factors
• AS/NZS 3000:2018, Table C5 — Lift/motor diversity factors

Use the ECalPro Maximum Demand Calculator to calculate the maximum demand for your building. Enter the floor area, load densities, and equipment schedule to get the ADMD, transformer recommendation, and main incomer sizing — with full standard clause references for every demand factor applied.