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.
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.
Need more power?
The Voltix app gives you all 12 calculators, offline access, and built-in reference tables. Free to download - Pro unlocks everything.
How derating works
Cable ampacity tables assume a reference ambient of 30 °C and a single circuit. Real installations are often hotter and more crowded, so correction factors reduce the usable capacity:
Iz = It × Ca × Cg × Ci
- It - tabulated capacity for the installation method.
- Ca - ambient temperature factor.
- Cg - grouping factor.
- Ci - thermal insulation factor.
Worked example
10 mm² copper (57 A base, Method C), 40 °C ambient, grouped with two other circuits, not in insulation:
- Ca = 0.87, Cg = 0.70, Ci = 1.0
- Iz = 57 × 0.87 × 0.70 = 34.7 A
The same cable that carries 57 A in ideal conditions is limited to about 35 A here.
Frequently Asked Questions
- What is cable derating?
- Derating reduces a cable's tabulated ampacity to account for installation conditions that make it harder to shed heat - high ambient temperature, bundling with other cables, or being surrounded by thermal insulation.
- 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), and Cs/Cd for buried cables. They multiply together: Iz = It × Ca × Cg × Ci.
- How much does thermal insulation reduce capacity?
- A cable in contact with insulation on one side for 50 mm derates to about 0.89; one totally surrounded by insulation derates to roughly 0.5 - halving its capacity.
- Do I derate before or after choosing the cable?
- Derating defines the minimum tabulated capacity you must start from: It ≥ Ib ÷ (Ca × Cg × Ci). Use this calculator to confirm a chosen cable's derated Iz still exceeds the design current.
Related calculators
Take Voltix to the field.
Download the Android app and get 12 professional calculators, offline, always available, across NEC, BS 7671 and AS/NZS.