8.2 Maximum Demand Principles

Purpose of Maximum Demand

Maximum demand (MD) is the estimated or determined design current that a part of an installation is expected to draw under design conditions. You use it when selecting and verifying:

• consumer mains (and related service equipment interfaces),
• submains to distribution boards,
• final subcircuits where load aggregation or fixed loads drive the rating,
• protective devices and cables that must carry that current without overload under normal design assumptions.

If MD is underestimated, cables and devices may overheat, nuisance-trip, or fail under real use. If MD is wildly overestimated without reason, conductors and switchgear become oversized, costly, and harder to coordinate—though the safety failure mode examiners care about first is undersizing relative to real simultaneous load.

What maximum demand is not

On the open-book exam, stems rarely require a full multi-page spreadsheet. They test whether you know why MD exists, what diversity means, and how MD locks to protective device and cable selection.

Diversity & After-Diversity Demand

Connected load is the total of equipment that could draw current if everything ran at once. Maximum demand after diversity is the realistic concurrent load the installation (or circuit) is designed for.

Why diversity is legitimate

In a dwelling, the oven, every heater, every socket, and every light almost never run at full load simultaneously for long periods. Wiring Rules calculation tables and factors (in the edition you may open) encode after-diversity assumptions for standard domestic arrangements. Using those factors is not “cutting corners”—it is the accepted method when the installation matches the assumptions.

When diversity shrinks or disappears

Diversity must be reduced or removed when use patterns force simultaneous full load:

• commercial kitchens at service time,
• workshops with multiple welders or large motors,
• plant rooms with known concurrent duty,
• dedicated circuits for a single fixed load (the load is the demand).

Exam language: “after-diversity maximum demand,” “diversity factor,” “not all load is simultaneous.” If a stem describes a workshop where three 15 A machines run together, do not apply a soft domestic diversity story that assumes they rarely coincide.

Methods Overview: Calculation, Assessment, Measurement, Limitation

AS/NZS 3000 guidance-style material (exact clause titles vary by edition) presents more than one way to determine maximum demand. Candidates should know the method families and when each idea applies—not invent a single universal formula.

1. Calculation

Calculation uses standard tables, load units, or formulas in the Wiring Rules (or referenced methods) for the type of installation—commonly domestic and similar. You enter numbers of points, fixed loads, and apply diversity factors from the permitted text. This is the default teaching path for a “new three-bedroom house” stem.

2. Assessment

Assessment is a competent engineering judgment of expected demand based on known equipment, duty cycles, and operating patterns—used when standard tables do not fit (special occupancy, unusual plant, mixed commercial). Assessment must still be defensible and documented in real practice; on the exam, choose assessment when the stem’s loads are non-standard.

3. Measurement

Measurement uses actual recorded demand (maximum demand meters, logging) on an existing installation—valuable for upgrades, board replacements, or proving that a supply capacity is adequate. You cannot “measure” a building that does not yet exist; new work leans on calculation or assessment.

4. Limitation

Limitation sets demand by restricting what can be drawn—e.g. load-limiting devices, diversity control, or supply arrangements that cap concurrent load. The design current then follows the limit. Exam angle: limitation is a valid conceptual method when the installation is deliberately constrained, not a free pass to ignore overcurrent protection on individual circuits.

Always confirm the procedural steps and any mandatory hierarchy in your permitted AS/NZS 3000 edition. This chapter teaches the concepts the exam expects; clause numbers and table references must match the book you open on the day.

Link to Protective Device Rating & Cable Selection

Maximum demand is not an academic number. It drives:

1. Protective device rating (In) for mains, submains, and circuits — the device must protect the cable and suit the load without chronic nuisance tripping under design load.
2. Cable current-carrying capacity (CCC / Iz) — after installation method, grouping, ambient temperature, and insulation type, CCC must be adequate for the design current (coordination rules: device, cable, and load relationship).
3. Voltage drop (Section 8.3) — design current × circuit impedance over length determines drop; higher MD or longer runs push larger conductors.
4. Fault and disconnection performance — larger cables can lower impedance (helpful for EFLI) but protective devices and discrimination still need design thought (Chapter 6 coordination themes).

Coordination snapshot

Exam trap: selecting a cable only for CCC while ignoring that MD-based current over a long run fails voltage drop—or selecting a breaker larger than the cable can safely support.

Worked Qualitative Scenarios

Scenario A — New three-bedroom house

Loads: lighting throughout, general power sockets, electric HWC, freestanding cooker, heat pump, garage door, outdoor sockets.

Reasoning path:

1. Identify final circuits (lights, sockets by area, dedicated HWC, dedicated cooker, heat pump as dedicated or assessed).
2. Determine after-diversity maximum demand for consumer mains using the domestic calculation method in your edition (or assessment if the stem adds atypical plant).
3. Size mains cable and main protective device from that MD; check voltage drop from point of supply to main board and onward.
4. Size each final circuit from its own load and rules—not from whole-house MD alone.

Expected insight: domestic diversity applies; HWC and cooker still get dedicated circuits; mains MD is less than the arithmetic sum of every nameplate.

Scenario B — Small trade workshop

Loads: fluorescent/LED high-bay lighting, many 10 A sockets, two 15 A outlets used for compressors, a welder, a dust extractor, and a small office annex.

Reasoning path:

1. List simultaneous workshop loads—compressors and welder may coincide with lighting and extraction.
2. Prefer assessment (or calculation only where tables truly fit); do not apply soft house diversity to three machines on one shift.
3. Consider submain to a workshop DB with MD based on concurrent use; final circuits dedicated where fixed machines require it.
4. Check protective devices for motor starting and welder duty as the stem implies; still apply CCC and VD.

Expected insight: lower diversity, higher mains/submain current, more dedicated circuits, possible measurement later if expanding an existing shop.

Scenario C — Board upgrade on an existing café

Historical bills and a logger show peak demand well below original oversizing folklore.

Reasoning path: measurement can support MD for the upgrade, but still verify that new equipment (extra ovens) is included by assessment of future concurrent load. Limitation devices might appear if the supply authority constrains capacity—still protect every final circuit correctly.

Study drill

For any practice stem, write one line: method (calculate / assess / measure / limit), diversity high or low, what is being sized (mains / submain / final). If you cannot fill those three blanks, open AS/NZS 3000 demand guidance in your flagged copy before guessing cable sizes.