A cable rated at 32 A does not always carry 32 A safely. Once you account for ambient temperature and how many other cables are running alongside it, that rating can fall to 20 A or less. Applying derating factors correctly is one of the most important — and most commonly skipped — steps in cable sizing.
What is cable derating?
The published current-carrying capacity (ampacity) of a cable is measured under reference conditions: typically 30 °C ambient air temperature (or 20 °C for buried cables) with the cable running in open air, not touching any other cables. Real installations almost never match those conditions. Derating applies correction factors to reduce the rated current to the value the cable can actually carry safely in its installed environment.
If you skip this step and the cable runs hotter than its rated temperature, the insulation degrades prematurely — and in a fault condition, it may fail before the protective device trips.
The three main derating factors
1. Ambient temperature (Ca)
Cable insulation is rated to a maximum conductor temperature — 70 °C for standard PVC (thermoplastic), 90 °C for XLPE and thermosetting insulation. If the surrounding air is warmer than the reference temperature, the cable cannot shed heat as easily, so you must reduce the current it carries.
Under BS 7671, the temperature correction factor Ca for a 70 °C PVC cable at various ambient temperatures is approximately:
- 25 °C ambient: Ca = 1.03 (slight uprating)
- 30 °C ambient: Ca = 1.00 (reference — no correction needed)
- 35 °C ambient: Ca = 0.94
- 40 °C ambient: Ca = 0.87
- 45 °C ambient: Ca = 0.79
- 50 °C ambient: Ca = 0.71
- 55 °C ambient: Ca = 0.61
In a roof void on a hot summer day, or in a plant room next to process equipment, temperatures of 40–50 °C are common. Ignoring this is a serious error.
2. Grouping factor (Cg)
When multiple cables run together — clipped side by side on a tray, or bunched in a conduit — they cannot shed heat into each other. The combined thermal effect means each cable runs warmer than it would alone. The grouping factor Cg reduces the rated current to compensate.
Approximate grouping factors for cables touching, installed in a single layer on a wall or tray (BS 7671 Appendix 4, Table 4C1):
- 2 circuits: Cg = 0.80
- 3 circuits: Cg = 0.70
- 4 circuits: Cg = 0.65
- 5 circuits: Cg = 0.60
- 6 circuits: Cg = 0.57
- 7 circuits: Cg = 0.54
- 8 circuits: Cg = 0.52
- 9–10 circuits: Cg = 0.50
- 12 circuits: Cg = 0.45
- 16 circuits: Cg = 0.41
- 20 circuits: Cg = 0.38
These figures assume all cables are loaded simultaneously. Where you can demonstrate that some circuits are unlikely to be on at the same time, you may be able to justify a less onerous factor — but you will need to document that assessment for the installation record.
3. Thermal insulation factor (Ci)
A cable buried in or enclosed by thermal insulation — such as one run through a loft that is subsequently insulated over — loses virtually all ability to shed heat. BS 7671 requires you to either avoid this installation method or apply a significant derating. A cable fully surrounded by insulation must be derated by a factor of 0.5 — meaning it can only carry half its open-air rated current.
The safest approach is to run cables in conduit or trunking before the insulation is installed, or to use a cable rated for the expected temperature. This is a particularly common issue in domestic roof space rewires.
How to apply multiple correction factors
When more than one factor applies, you multiply them together. The corrected ampacity Iz is:
Iz = It × Ca × Cg × Ci
Where It is the tabulated current-carrying capacity for the cable in its installation method before correction.
The corrected current Iz must be greater than or equal to the design current Ib of the circuit. And the nominal current of the protective device In must also satisfy In ≤ Iz (the cable must be protected by its overcurrent device).
Worked example (BS 7671)
You need to run a 20 A circuit using 2.5 mm² twin-and-earth cable. The cable will be clipped to a surface alongside 5 other circuits, in a roof space where the ambient temperature reaches 45 °C in summer.
- Tabulated current (installation method C, 2.5 mm² Cu PVC): It = 27 A
- Temperature factor at 45 °C: Ca = 0.79
- Grouping factor for 6 circuits: Cg = 0.57
- Corrected ampacity: Iz = 27 × 0.79 × 0.57 = 12.2 A
Result:12.2 A is well below the 20 A design current. This cable cannot be used. You would need to either use 6 mm² cable (tabulated at 41 A, giving Iz = 41 × 0.79 × 0.57 = 18.5 A — still marginal) or reduce the grouping by routing some circuits separately. A 10 mm² cable (tabulated at 57 A) would give Iz = 57 × 0.79 × 0.57 = 25.7 A — acceptable.
Derating under NEC
The NEC handles derating through two mechanisms. Temperature correction factors are found in the notes below Tables 310.15(B)(1) through (16) and apply similarly to BS 7671 — ambient temperature above the 30 °C reference requires a downward correction. Bundling derating is covered by NEC 310.15(C): when more than three current-carrying conductors are bundled together or installed in a conduit, raceway, or cable tray, the allowable ampacity is reduced.
NEC bundling factors for conductors in a raceway or cable (310.15(C)(1)):
- 4–6 conductors: 80% of table ampacity
- 7–9 conductors: 70%
- 10–20 conductors: 50%
- 21–30 conductors: 45%
- 31–40 conductors: 40%
- 41 or more: 35%
Note that NEC counts current-carrying conductors, not circuits. A standard branch circuit has two current-carrying conductors (line and neutral); a three-phase circuit has three.
Derating under AS/NZS 3008.1
AS/NZS 3008.1 uses the same principle: a reference ampacity is adjusted by temperature factors (Table 23) and grouping factors (Table 22) that vary by installation method. The process is identical to BS 7671 — calculate the corrected current and ensure it exceeds the design current with the protective device also satisfying the relationship In ≤ Iz.
Common mistakes
The most frequent errors are: forgetting to derate when cables are bundled in a draw — especially when a cable tray is progressively filled over years as new circuits are added; failing to account for roof-space temperatures; and applying only one factor when two or three apply simultaneously. When all three factors compound, a cable's real capacity can be as little as a third of its nominal rating.
Calculate it instantly
Rather than working through these factors manually on every job, use the free Voltix cable derating calculator, which applies all relevant BS 7671 and AS/NZS factors automatically. For the full cable sizing workflow — including derating — read our BS 7671 cable sizing guide.