Cable Derating Calculator
Calculate the effective current-carrying capacity (Iz) of a cable after applying correction factors for ambient temperature, grouping with other circuits, and thermal insulation — to BS 7671 and AS/NZS. Skipping derating is one of the most common cable sizing errors.
Derated capacity Iz
38.4A
Combined factor
0.609
10 mm² derates from 63 A to 38.4 A
Ca (temperature) = 0.87 · Cg (grouping) = 0.7 · Ci (insulation) = 1. Iz = It × Ca × Cg × Ci.
Based on 70 °C thermoplastic copper, Method C reference ampacities. Buried-cable soil factors (Cs) are available in the Voltix app.
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How derating works
Cable ampacity tables in BS 7671 (Appendix 4) assume a reference ambient of 30 °C and a single circuit with no adjacent cables. Real installations are often warmer and more crowded. Correction factors adjust the usable capacity:
Iz = It × Ca × Cg × Ci
- It — tabulated current-carrying capacity for the cable cross-section and installation method (Method A, B, C, etc.)
- Ca — ambient temperature factor (Table 4C1 for PVC, Table 4C2 for thermosetting insulation)
- Cg — grouping factor when cables run together (Table 4C3)
- Ci — thermal insulation factor (0.5 if fully enclosed in insulation, 0.75 touching insulation on one side)
The corrected Iz must be greater than or equal to the design current Ib. The protective device rating In must also not exceed Iz.
Temperature correction factors (Ca) for 70 °C PVC cable
Approximate Ca values at common ambient temperatures (BS 7671 Table 4C1):
- 25 °C: Ca = 1.03
- 30 °C: Ca = 1.00 (reference)
- 35 °C: Ca = 0.94
- 40 °C: Ca = 0.87
- 45 °C: Ca = 0.79
- 50 °C: Ca = 0.71
- 55 °C: Ca = 0.61
- 60 °C: Ca = 0.50
Grouping factors (Cg) for cables touching on a tray or surface
Approximate Cg values from BS 7671 Table 4C3 (single layer, touching):
- 2 circuits: Cg = 0.80
- 3 circuits: Cg = 0.70
- 4 circuits: Cg = 0.65
- 6 circuits: Cg = 0.57
- 8 circuits: Cg = 0.52
- 10 circuits: Cg = 0.50
- 16 circuits: Cg = 0.41
- 20 circuits: Cg = 0.38
Worked examples
Example 1 — hot plant room:10 mm² copper PVC cable (It = 57 A, Method C), installed in a plant room at 45 °C alongside 2 other circuits.
- Ca at 45 °C = 0.79
- Cg for 3 circuits = 0.70
- Iz = 57 × 0.79 × 0.70 = 31.5 A
This cable is limited to 31.5 A — not the 57 A on the table.
Example 2 — fully insulated roof space:2.5 mm² twin-and-earth (It = 27 A, Method A) buried in loft insulation, ambient 40 °C.
- Ca at 40 °C = 0.87
- Ci fully in insulation = 0.50
- Iz = 27 × 0.87 × 0.50 = 11.7 A
A cable nominally rated at 27 A can only safely carry 11.7 A in this installation. This is why routing cables in insulation without derating is a serious safety issue.
Who is this calculator for?
Used when designing cable installations in high-temperature environments (roof spaces, boiler rooms, plant rooms) or where multiple circuits run together in conduit, tray, or trunking. Essential for BS 7671 compliance in domestic rewires, commercial fit-outs, and industrial installations. Also applies to AS/NZS 3008.1 cable sizing for Australian and New Zealand electricians — the process and formula are identical, with local correction factor tables.
Frequently Asked Questions
- What is cable derating?
- Derating reduces a cable's tabulated ampacity to account for conditions that make it harder to shed heat — high ambient temperature, bundling with other cables, or being surrounded by thermal insulation. The published rating assumes 30 °C ambient and a single circuit.
- Which correction factors does BS 7671 use?
- The main factors are Ca (ambient temperature, Table 4C1), Cg (grouping, Table 4C3), Ci or Cc (thermal insulation, Reg 523.9). They multiply together: Iz = It × Ca × Cg × Ci. All factors less than 1.0 reduce the ampacity.
- How much does thermal insulation reduce cable capacity?
- A cable touching insulation on one side over 500 mm or more derate to about 0.75; one totally surrounded by insulation derates to 0.5 — halving its capacity. This is a common issue in domestic roof space wiring after insulation is installed.
- Do I derate before or after choosing the cable size?
- Derating defines the minimum tabulated capacity you must start from: It ≥ Ib ÷ (Ca × Cg × Ci). Use this calculator to confirm that a chosen cable's derated Iz still exceeds the design current and satisfies the In ≤ Iz requirement.
- What ambient temperature do I use for a roof space in summer?
- Unventilated roof spaces can reach 50–60 °C on hot summer days in southern climates. For UK roof spaces, 45–50 °C is a reasonable design temperature. At 50 °C, the Ca factor for 70 °C PVC cable is 0.71 — a significant reduction.
- Can correction factors increase the ampacity above the tabulated value?
- Yes — Ca exceeds 1.0 when the ambient temperature is below the reference (30 °C for air installation). In a cold environment such as an unheated plant room in winter, you could use a slightly smaller cable. In practice, most designers ignore the uprating and size for worst-case conditions.
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