Summary

Cable current ratings are not the headline number on the cable. The BS 7671 reference tables (Appendix 4) give the current a cable can carry in ideal conditions — open air, single cable, 30°C ambient, no thermal insulation contact. Real installations almost never match those conditions: cables run in groups through a hot loft, packed against thermal insulation, in conduit with other circuits, or above a hot ceiling. Each deviation derates the cable's capacity, and the factors are multiplicative.

The trade reality is that most electricians size cables from memory ("2.5mm² for 32A ring final, 4mm² for 32A cooker, 6mm² for 40A shower") and rely on the rule-of-thumb being conservative enough to cover most installations. For straightforward circuits in normal conditions this works. For modern installs — deep loft insulation, grouped solar PV strings, EV charger circuits routed alongside other cables, conduit runs containing multiple circuits — the rule of thumb undersizes the cable and the installation can run hot for years before showing damage.

This article covers the four main derating factors, how they combine, the practical situations where they bite, and the cable-up decision rules that protect against future EICR findings. It also covers the Zs implications — derating may force a larger cable that also has lower impedance, which is then favourable for fault loop disconnection.

Key Facts

Quick Reference Table

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Cable Size Method C Ref. Current (A) After 0.7 Cg (3 grouped) After 0.5 Ci (insulation surround) After both (Cg × Ci)
1.5mm² T&E 20 14 10 7
2.5mm² T&E 27 19 13.5 9.5
4mm² T&E 37 26 18.5 13
6mm² T&E 47 33 23.5 16.5
10mm² T&E 65 45.5 32.5 23
16mm² T&E 87 61 43.5 30.5
25mm² T&E 114 80 57 40
Installation Condition Typical Factor
Loft pulled over insulation (touching, on warm side) Ci ≈ 0.81
Loft buried in insulation (50–100mm depth) Ci ≈ 0.65
Loft fully encased in insulation Ci ≈ 0.5
In conduit with 1 other circuit Cg ≈ 0.8
In conduit with 2 other circuits Cg ≈ 0.7
In conduit with 5 other circuits Cg ≈ 0.55
Clipped on warm wall (boiler room) 35°C Ca ≈ 0.94
Loft 45°C (UK summer worst case) Ca ≈ 0.79
Outside in cold (5°C) Ca ≈ 1.12 (uprate)
Common Mistakes Effect
Using "2.5mm² for 32A ring" without checking Ci Cable may carry ~13A safely in insulated loft, overloaded at 32A
Bundling EV charger cable with existing house wiring EV cable + others = derated; may not meet design current
Routing solar PV through loft alongside others High operating current + heat + grouping = significant derating
Single-cable rule for shower circuits A bundled shower + lighting circuit could derate to below shower draw

Detailed Guidance

How Derating Works

Each derating factor reduces the cable's safe current-carrying capacity. The factors multiply (not add):

Iz = It × Ca × Cg × Ci × Cf

Where:

For the cable to be acceptable, the design current (Ib) must be less than Iz, and the protective device rating (In) must also be less than Iz. The order:

Ib ≤ In ≤ Iz

Method Selection — Where Most Derating Starts

The installation method sets the starting reference current. Common UK methods:

Pick the method that matches the dominant installation condition. A cable that runs partly on a clipped surface and partly buried in insulation is treated as the worst case for the length where derating applies.

Ambient Temperature

The reference temperature for BS 7671 Appendix 4 is 30°C ambient. UK summer loft temperatures regularly reach 40–50°C; equipment cupboards near boilers can be 35–40°C; outside in cold weather, ambient drops below 30°C and the cable could theoretically be uprated (rarely done — design for worst case).

Ca values (Table 4B1, PVC insulation):

A 2.5mm² T&E running on Method C (clipped) in a 45°C loft: 27 × 0.79 = 21.3A safe capacity (down from 27A).

Grouping Factor — The Big One in Commercial Work

Where multiple circuits run in close proximity, they heat each other. Cg factors from Table 4C1 (cables clipped together, Method C):

For cables in conduit or trunking (Method B):

The factor applies to the length of the run where the grouping occurs. A 10m run with 3m of grouped section uses the grouping factor for that 3m (and the cable needs to be sized for the worst section).

Thermal Insulation — The Loft Problem

Section 523.9 covers cables in thermal insulation. The factor Ci depends on:

Approximate values for cables fully surrounded by insulation:

For loft conversions where insulation is laid on top of cables running across joists, the cables are partially insulated (insulation on top, joist bay below). Treat as approximately Ci = 0.7 unless detailed assessment.

For cables passing through plasterboard wall cavities full of insulation: Ci = 0.5 (effectively surrounded).

Worked Examples

Example 1: 32A ring final, 2.5mm² T&E, clipped on joist tops in cold loft, no insulation contact, no grouping. Ambient 30°C.

Example 2: Same cable run, now insulation 100mm added over the joists.

Example 3: EV charger 7.4kW (32A), 4mm² T&E from CU through loft with two other lighting circuits.

This is why most EV charger installations specify 10mm² T&E or 6mm² SWA from the CU even though the headline rule of thumb "32A = 4mm²" would suggest 4mm².

Cable Capping (Routed in Plaster)

When cables are buried in plaster under cap-and-cover (a thin plastic channel for protection), BS 7671 treats them as installed in Method A or A1 — derating applies due to the thermal mass of plaster surrounding the cable. The effect is similar to thermal insulation but less severe — typically Ci ≈ 0.85.

Combined Effects — Whole-Run Worst Case

A cable run that goes through multiple environments takes the worst-case derating for the section that hits worst. For example:

Iz is calculated for each section; the overall cable must be sized for the worst section. In this case the conduit section dominates (0.7 × Ca for that section). Calculate three Iz values and use the minimum.

Zs Implications

A cable that fails derating must be uprated to the next size. The bonus: the larger cable has lower R1+R2, which reduces the earth fault loop impedance Zs. This may move a borderline Zs to comfortably compliant.

For TT supplies in particular, where Zs is often the design constraint rather than current-carrying capacity, the uprated cable for thermal reasons can be the same cable specified for Zs reasons — design once, install once.

Frequently Asked Questions

Can I just use a bigger cable to be safe?

Yes, but at a cost: material expense, weight, conduit fill ratio (a 10mm² cable doesn't fit easily in a 20mm conduit alongside others), and termination size at switchgear. Use the correct derating calculation rather than blanket oversizing — but err on the side of caution at the marginal case.

My loft has 300mm of insulation. Should I run cables on top of it (cold side) or under it (warm side)?

On top of insulation (cold side) is preferable — the cable sees ambient air temperature. Under insulation (warm side) means the cable is below the thermal envelope and runs hotter. If cables must run through the insulation layer, route them as straight as possible and minimise the length within the insulation.

Do I need to derate cables in conduit if they're all on the same circuit?

No — the grouping factor applies to multiple CIRCUITS, not multiple cores of the same circuit. A T&E cable carrying L+N+CPC is one circuit, not three; no derating for the cores against each other.

What's the simplest way to calculate Iz on site?

Use a sizing chart (e.g. ECA Cable Size Calculator, or the IET On-Site Guide Appendix 1) with the installation method, ambient temperature, grouping count, and insulation status entered. The chart returns Iz directly. Faster than the longhand calculation; result is the same. Many electricians use an app on phone (e.g. AMTECH Designer) for this.

Why does the IET On-Site Guide allow "rules of thumb" that ignore derating?

The On-Site Guide assumes typical, modest installation conditions (no extreme thermal insulation, no heavy grouping, standard ambient). The rule of thumb applies to those typical conditions. Anywhere outside that — loft insulation, multiple circuits clipped together, hot ambient — requires the full calculation. The On-Site Guide explicitly says: "if conditions are outside those described, refer to BS 7671 Appendix 4."

Regulations & Standards