6.3 Commercial Demand & Mains/Main-Switch Sizing

Commercial Demand & Mains/Main-Switch Sizing

Quick Answer: For commercial/industrial installations, determine maximum demand using the appropriate AS/NZS 3000 Appendix C guidance (or competent assessment / measurement / limitation where calculation tables are insufficient), then size consumer mains and the main switch so their ratings suit that diversified design current and the supply arrangement. Domestic Table C1 rows are not a universal template for every shop, factory or multi-tenancy board.

Domestic Fluency Is Not Automatic Commercial Competence

Section 6.2 drills single-dwelling Table C1 logic. Commercial work changes the problem shape:

• Load lists include lighting, power, mechanical services, lifts, kitchen plant, IT UPS, EV charging, and process equipment with awkward coincidence factors.
• Operating hours and duty cycles differ sharply between a café, a refrigerated warehouse and a daylight-only warehouse office.
• Multiple distribution boards and submains introduce cascaded diversity decisions — diversity applied at a final board is not always re-applied identically at the main switchboard without engineering judgement.
• Distributors may specify service fuse, CT metering or capacity limits that interact with your MD figure.

Appendix C still matters, but candidates must identify whether the scenario is domestic, multiple domestic, or non-domestic, and open the matching guidance in the live standard. Copying “first 10 A of GPOs then …” from a house calculation into a commercial mechanical switchboard is a classic fail mode.

Commercial / Industrial Approaches

Calculation from commercial tables and notes

Where Appendix C provides tables or methods for non-domestic installations, follow those load categories and diversity factors exactly as printed. Commercial calculation often groups:

• Lighting (sometimes with different diversity from domestic),
• Socket-outlet / general power,
• Heating, air conditioning and ventilation plant,
• Cooking / food-prep loads in hospitality,
• Motors and machinery (consider starting versus running demand where the method requires),
• Dedicated high-load equipment assessed with little diversity.

Show working category by category. If the table uses watts per square metre or other area methods in your edition’s informative material, apply them only when the scenario and notes support that approach.

Assessment by a competent person

Complex sites frequently rely on assessment: a competent licensed person judges MD from load schedules, diversity experience, and known process behaviour. For the capstone, “assessment” still needs transparent reasoning — list major loads, state which are continuous, which are intermittent, and how you arrived at a diversified peak. Assessment is not a synonym for “pick 63 A because the board is already 63 A.”

Measurement

For upgrades, tenancy changes or suspected undersizing, measured maximum demand over a representative period (logging ammeters / power analysers) can validate or revise a calculated figure. Measurement must cover busy seasons where relevant (summer A/C peaks, winter heating, production shifts). A quiet Sunday log is a poor basis for a warehouse MD.

Limitation

A main switch, service protection device, or agreed supply capacity may limit the demand the installation can draw. Designers sometimes use limitation deliberately (for example controlled load management) but must ensure final circuits remain protected and that intentional limitation is documented and safe. Limitation does not permit omitting overload protection on subcircuits.

From MD to Consumer Mains

Once MD is established in amperes:

1. Select consumer mains so current-carrying capacity Iz (after installation method and derating per AS/NZS 3008) is adequate for the MD design current — coordination thinking still respects Ib ≤ In ≤ Iz where protective devices are part of the story.
2. Check voltage drop from the point of supply to the main switchboard (and onward if required) against Wiring Rules limits commonly studied at 5% for the installation — MD (or the appropriate design current) feeds the mV/A·m calculation.
3. Consider fault level and cable withstand where short-circuit duties require it.
4. Confirm installation method (underground, enclosed, bunched) matches the CCC table you used.

Undersized consumer mains relative to true diversified peak risk overheating and nuisance issues; wildly oversized mains without a coherent MD story suggest the candidate never determined demand properly.

Main Switch Sizing

The main switch (or main switches in a multi-supply arrangement) must be suitable for:

• The maximum demand it is expected to carry in normal service,
• The fault and isolation duties required by AS/NZS 3000 switching and isolation rules,
• Coordination with upstream service protection and downstream distribution protective devices,
• Any number-of-poles and switching configuration the supply arrangement needs (single-phase, multiphase, multiple supplies).

Teaching points for capstone answers:

• Main-switch current rating is chosen with MD as a primary input — not by copying an arbitrary catalogue number.
• If MD is 95 A, a 63 A main switch is not “diversity by hardware wishful thinking”; it is undersized relative to the calculated demand unless the MD determination itself is revised by a valid limitation/assessment method and the installation’s loads are controlled accordingly.
• Conversely, selecting a 250 A main switch for a carefully calculated 40 A domestic MD without reason is poor practice even if CCC would allow larger cables.

Where multiple main switches or alternative supplies exist, state which MD applies to which switch. Do not blend two supply paths into one vague number.

Supply Arrangement Narrative (What Assessors Want)

A complete commercial/mains answer often reads like a short professional story:

1. Installation type and reference — “Non-domestic per AS/NZS 3000 Appendix C (live edition).”
2. MD method — calculation / assessment / measurement / limitation (and why).
3. MD result — amperes, phase, voltage basis.
4. Consumer mains — conductor size/method, Iz versus MD, voltage-drop check summary.
5. Main switch — rating and poles suited to MD and isolation requirements.
6. Notes — distributor constraints, load-control, or future spare capacity if the scenario mentions them.

That narrative is exactly how licensed work is defended on site and in Queensland regulatory culture: standards-referenced, numeric, and traceable.

Multi-Tenancy and Submains (Exam Awareness)

Commercial buildings often have:

• A main switchboard MD for the whole building or landlord services,
• Tenant submain MDs assessed separately,
• Rising mains or busways sized on diversified tenant coincidence.

Do not assume the arithmetic sum of every tenant’s MD equals the building MD — further diversity may apply at the incomer, subject to the method you are using and any Appendix C / engineering guidance applicable. Equally, do not diversify twice in a way that understates a continuous central plant load (chillers, fire pumps where relevant to the electrical design scenario, large UPS).

Worked Commercial Sizing Sketch (Method, Illustrative)

Suppose a small commercial tenancy calculation (illustrative) yields MD = 78 A three-phase after Appendix C / assessment working.

• Consumer mains: select a cable method and size whose Iz after derating is at least adequate for 78 A (and check voltage drop at 78 A over the run length). Exact mm² comes from AS/NZS 3008 tables for the installation method — MD tells you the current to design for; 3008 tells you which cable passes.
• Main switch: choose a switch rating ≥ MD and suitable for the supply (for example a 80 A or 100 A device depending on available standard ratings and coordination), ensuring isolation compliance.
• Protection: confirm upstream/downstream devices and cable CCC still satisfy coordination rules.

If measurement later shows peaks of 92 A in summer, revisit assessment — MD is a living design input when change of use or climate load appears.

Queensland Capstone Relevance

The capstone packs written calculation with practical installation judgement. Maximum demand questions in the commercial/mains framing test whether you can:

• Navigate Appendix C beyond domestic Table C1,
• Choose an appropriate determination method when data is messy,
• Convert MD into mains and main-switch decisions without skipping voltage-drop or CCC logic,
• Explain the result in Australian Wiring Rules language suitable for licensed work under the Electrical Safety Act 2002 (Qld) pathway.

Study sequence that pays off: domestic Table C1 fluency (Section 6.2) → commercial method selection (this section) → cable selection chapters (AS/NZS 3008) using MD as Ib. Keep the live standard open; keep working neat; keep diversity honest.