The Grouping Factor Trap: Why 6 Cables in a Tray Don't Always Derate by 0.57

Open any cable sizing spreadsheet in any engineering office, and you'll find the same conservative pattern: count the cables in the tray, look up the grouping factor, apply it. Six cables on a tray? Factor of 0.57. Twelve cables? Factor of 0.45. Calculation done.

This approach is technically correct — but only under assumptions that almost never hold in practice. The standard grouping tables assume every cable in the group is simultaneously loaded to 100% of its rated current. In 18 years of electrical engineering in mining and industrial plants, I have never seen this condition exist in reality.

Understanding when and how grouping factors can be legitimately reduced is one of the most valuable cable sizing skills an engineer can develop. On a typical industrial project, proper grouping analysis saves 1–2 cable sizes on every grouped circuit — translating to tens of thousands of dollars in material costs.

What Grouping Factors Actually Represent

When multiple current-carrying cables are installed close together, each cable contributes heat to its neighbours. This mutual heating reduces the amount of current each cable can safely carry without exceeding its maximum operating temperature.

The grouping correction factor (k₂) accounts for this by reducing the effective current carrying capacity:

The standard grouping tables:

BS 7671, Table C.9 — Correction factors for groups of more than one circuit

These values assume the worst case: every cable in the group carries its full rated current simultaneously.

The Hidden Assumption Nobody Questions

Here's the critical note that most engineers overlook:

AS/NZS 3008.1.1, Clause 3.5.4 — Cables not fully loaded

AS/NZS 3008 goes further in Clause 3.5.4, providing a specific methodology for calculating reduced grouping factors when cables are not loaded to their full rated capacity.

The key principle: a cable that carries 30% of its rated current generates only 9% of its rated heat (because heat is proportional to I², and 0.3² = 0.09). This cable's thermal contribution to its neighbours is negligible.

The Adjusted Grouping Method

The adjusted method works as follows:

1. Identify how many cables in the group are loaded to more than 30% of their rated current
2. Use that number — not the total cable count — to look up the grouping factor
3. For cables loaded between 30% and 70%, a further refinement is possible using the formula in IEC 60364-5-52 Annex D

Simplified approach (widely accepted in industry):

• Cables loaded <30% of rating: count as zero
• Cables loaded 30–70% of rating: count as 0.5
• Cables loaded >70% of rating: count as 1

Add up the effective count and use the standard grouping table with that number.

A Real-World Example

Scenario: A cable tray in an industrial MCC room carries 18 multicore cables. Based on the standard table, the grouping factor would be approximately 0.39 — meaning every cable must be derated to 39% of its tabulated rating. This typically forces every cable up by 2–3 sizes.

But when we analyse the actual loading:

Using 7 as the effective group number instead of 18, the grouping factor becomes 0.54 instead of 0.39. For the 4 fully loaded cables, this is the difference between a 95 mm² cable and a 70 mm² cable — one full size smaller, on each of four circuits.

When Does This Apply?

The adjusted grouping method is valid and commonly applied in:

• MCC rooms — where motor feeders have widely varying load levels and duty cycles
• Distribution boards — where not all circuits operate simultaneously
• Cable trays with mixed circuits — lighting, power, and control cables on the same tray
• Mining plants — where process equipment operates intermittently and load diversity is high

When You Should NOT Reduce the Grouping Factor

There are situations where the full grouping factor should be applied without reduction:

1. All cables are sized to carry full load simultaneously — in emergency scenarios or when the process requires all equipment running
2. Future expansion — if empty cable positions will be filled with fully loaded cables later
3. The loading is uncertain — if you don't have reliable load data, the conservative approach is justified
4. Critical safety circuits — fire services, emergency lighting, life safety systems
5. The installation authority requires it — some jurisdictions and inspectors do not accept the adjusted method

The Standards Reference

BS 7671, Appendix C, Note 2 — Cables not fully loaded

BS 7671 acknowledges the adjusted method in Note 2 of Appendix C but doesn't provide a detailed methodology — it's left to engineering judgement.

IEC 60364-5-52, Annex D, D.52.1 — Cables in groups not loaded simultaneously

IEC 60364-5-52 provides the most detailed treatment in Annex D, including formulas for partially loaded cable groups.

AS/NZS 3008.1.1, Clause 3.5.4 — Grouped cables not all carrying rated current

AS/NZS 3008.1.1 provides specific guidance in Clause 3.5.4, acknowledging that cables carrying less than 35% of their rating need not be counted.

Another Common Grouping Mistake: Spacing

Engineers often apply "touching" grouping factors when cables on a perforated tray are actually spaced. The distinction matters:

• Touching: cables in direct contact with each other (conduit bundles, clipped cables)
• Spaced: cables separated by at least one cable diameter on an open or perforated tray

For 6 cables, the factor is 0.57 touching vs 0.72 spaced — a 26% difference. On a well-managed cable tray where cables are laid with intentional spacing, the "spaced" factor applies. But many engineers default to "touching" because it's conservative.

Check your installation drawings. If the cable tray specification calls for cables to be spaced (as most quality installations do), use the spaced factors. It's not being optimistic — it's being accurate.

The Bottom Line

Grouping factors are the most over-applied and under-analysed derating factor in cable sizing. The standard tables represent a worst case that rarely exists in practice. Before applying a blanket grouping factor to every cable on a tray:

1. Count the actually loaded cables, not just the total cable count
2. Check whether cables are touching or spaced
3. Document your load assumptions for future reference
4. Use the full grouping factor for safety-critical circuits regardless of loading

The potential savings — both in material cost and in not installing unnecessarily large cables in already-congested cable trays — make this analysis well worth the effort.

Related Resources

• Grenfell Tower: Cable Tray Fire Spread — Overloaded cable trays and the thermal cascade that follows
• Cable Derating: 12 Cables in a Tray at 40°C — How grouping factors differ across all four standards
• The Complete Cable Sizing Comparison — All derating factors compared
• View all worked examples →