4.3 Grouping & Thermal Derating Factors

Derating is how real installations meet the tables halfway

AS/NZS 3008 base tables assume defined spacing, a defined number of circuits, and reference temperatures. Real switchboards, roof spaces and cable trays violate those assumptions daily. Derating factors (also called rating factors or correction factors) scale the tabulated current so the conductor still stays within its temperature limit when heat dissipation is worse than the reference case.

Two high-yield families for Queensland licence work are:

1. Grouping / bunching factors — mutual heating from neighbouring loaded cables.
2. Thermal insulation factors (or dedicated insulated-installation capacities) — building batts and similar.

Ambient and soil factors from Section 4.2 sit in the same mathematics. Treat all applicable independent factors as multipliers unless a specific AS/NZS 3008 note gives a combined method for that arrangement.

Grouping — why neighbours make you hotter

Each loaded cable is a heat source. When cables are:

• bunched in a loom;
• sharing conduit or trunking;
• touching on a tray;
• stacked in layers;

…the air (or soil) around each cable is preheated by the others. The Standard's grouping tables relate the number of circuits / cables and the installation configuration to a factor Cg typically between about 0.5 and 1.0 for common cases (exact values from your edition).

Counting circuits correctly

Grouping tables care about loaded cables/circuits as defined in AS/NZS 3008 notes. Multicore cables are counted per the Standard's rules (a twin-and-earth serving one circuit is not "three circuits" simply because three cores exist). Capstone traps include:

• Counting earth conductors as separate loaded circuits.
• Forgetting a second circuit added later in the same conduit.
• Applying a tray grouping factor to a single spaced free-air cable incorrectly.

Thermal insulation derating

When cables pass through ceiling insulation, are enclosed behind insulated plasterboard with no free air, or are otherwise thermally blanketed, use the AS/NZS 3008 factors or table columns for cables in contact with / surrounded by thermal insulation. Partial penetration rules may allow different treatment for short lengths — read the note rather than inventing a 50% guess.

Queensland housing stock with thick ceiling batts makes this a default question, not an edge case. Air-conditioner final subcircuits and oven circuits often climb through insulated roofs.

Multiplying factors — the non-negotiable method

If a cable needs:

• ambient factor Ca = 0.94;
• grouping factor Cg = 0.80;
• thermal insulation factor Ci = 0.75;

then the combined factor is:

Ctotal = Ca × Cg × Ci = 0.94 × 0.80 × 0.75 = 0.564

If the tabulated CCC for the size/method is 40 A:

Iz = 40 × 0.564 ≈ 22.6 A

Worked example A — bunched conduit

Given: Tabulated enclosed-method CCC for 4 mm² Cu V-90 = 32 A (illustrative). Three loaded circuits in one conduit → Cg = 0.70 (illustrative). Ambient at reference → Ca = 1.0. No thermal insulation.

Iz = 32 × 1.0 × 0.70 = 22.4 A

If In = 25 A, Iz < In → increase conductor size or split circuits into separate enclosures to improve Cg, then re-check.

Worked example B — roof insulation + grouping + hot ambient

Given: Tabulated CCC for 6 mm² Cu clipped = 46 A (illustrative). Factors: Ca = 0.91 (hot roof), Cg = 0.80 (two circuits bunched for part of run — assume factor applies), Ci = 0.70 (surrounded by insulation).

Ctotal = 0.91 × 0.80 × 0.70 = 0.5096

Iz ≈ 46 × 0.5096 ≈ 23.4 A

A 32 A circuit-breaker on a continuous 28 A load cannot be justified on this 6 mm² run under those factors — despite 6 mm² "looking big" to apprentices who only memorise clipped tables.

Worked example C — why "use the worst factor only" fails

Same 46 A base with factors 0.91, 0.80 and 0.70. If a candidate wrongly applies only the worst factor 0.70:

Wrong Iz = 46 × 0.70 = 32.2 A (looks acceptable for In = 32 A).

Correct Iz ≈ 23.4 A (fails).

That single mistake is enough to select an undersized cable. Assessors who ask for working catch it immediately.

Order of work (licence-friendly algorithm)

1. Choose cable type and base table CCC for the governing installation method.
2. List every applicable independent factor: ambient/soil, grouping, insulation, depth/resistivity as required.
3. Multiply factors → Ctotal.
4. Iz = CCC_table × Ctotal.
5. Compare with Ib and In; iterate size upwards until Iz is adequate.
6. Re-check voltage drop and short-circuit (Section 4.4) — larger size from derating often helps those checks too.

Interaction with protective device selection

Sometimes derating forces a cable increase that then allows a different protective device coordination outcome. Do not reduce In below what the load and AS/NZS 3000 require just to "make Iz look fine" on an undersized cable. Fix the cable or the installation method (separate conduits, spacing, avoid insulation contact) rather than defeating overload protection.

Tray fill, spacing and "free air" optimism

Designers sometimes claim free-air CCC while cables are touching for tens of metres on a tray. If the Standard's free-air tables require spacing, touching cables need grouping factors or a different method column. Photograph evidence in capstone practicals of jammed looms is a gift to markers looking for derating discussion.

Exam traps for Section 4.3

• Applying only one factor when ambient, grouping and insulation all apply.
• Adding factors (0.9 + 0.8) instead of multiplying.
• Using grouping factors for unloaded spare cores incorrectly counted as circuits.
• Ignoring insulation because "the cable is only in batts for a metre" without checking the Standard's short-length provisions.
• Derating the protective device rating instead of the cable when explaining Iz.

Capstone sentence that scores

"Base CCC from AS/NZS 3008 method X is … A; factors Ca=…, Cg=…, Ci=…; product …; Iz=… A; In=… A; therefore size … mm² is required / adequate."

If you can say that cleanly with numbers from the book, you have Section 4.3 under control. Section 4.4 then asks whether that same conductor can survive the fault energy until the protective device clears.