Cable Derating Factors Explained: Grouping, Ambient & Installation

The current rating you read from a cable table is a best-case figure — one cable, in free air, at a comfortable ambient temperature. Real routes are hotter and more crowded than that, so you multiply the rating down by a set of correction factors. Getting these factors wrong, in either direction, is the single most common cable-sizing mistake.

Why Derating Exists: It's All About Heat

A cable's rating is simply the current at which its conductor reaches its maximum safe temperature (70 °C for PVC, 90 °C for XLPE). Anything that makes it harder for the cable to shed that heat lowers the current it can safely carry. Every derating factor is a different way of saying "this cable can't lose heat as easily as the table assumed."

The Main Factors

Ambient temperature (Cₐ). The tables assume a reference ambient (30 °C in air, 20 °C in ground). A hot plant room at 40 °C derates the cable; a cold one can uprate it.
Grouping (Cₛ). Cables bunched together warm each other, so each carries less. More circuits in the group, or closer spacing, means a bigger reduction.
Thermal insulation (Cₓ). A cable surrounded by thermal insulation can barely lose heat — a cable totally enclosed in insulation for more than half a metre can be derated to around 0.5.
Installation / reference method. Clipped direct, in conduit, on tray, buried — each starts you on a different rating column before any factor is applied.
Buried cables. Add soil thermal resistivity, burial depth and ground grouping as further factors.
Protection by a semi-enclosed (rewireable) fuse. A historic 0.725 factor still appears where BS 3036 fuses are involved.

Illustrative: four touching circuits are commonly derated to roughly two-thirds of the single-cable rating.

How They Combine: Multiply

The factors multiply together. The required tabulated rating is the design current divided by the product of the factors: Iₜ ≥ Iₛ / (Cₐ × Cₛ × Cₓ …). Say a 40 A circuit runs through a 40 °C plant room (Cₐ = 0.87) grouped with three others (Cₛ = 0.65): you need a cable whose tabulated rating is at least 40 / (0.87 × 0.65) ≈ 71 A — nearly double the naive figure. Skip the factors and you've undersized it.

The Traps

Phantom grouping. Applying a grouping factor to cables that spread out after leaving the board is over-conservative and costs copper.
Hidden grouping. Missing a bunched section — the reverse, and the dangerous one.
Forgotten insulation factor. A cable through an insulated loft or wall cavity is easy to overlook and heavily derated.
Optimistic ambient. Using 30 °C when the room genuinely runs at 40 °C.

What This Means for Your Design

Derating is exactly the kind of bookkeeping that software does reliably and spreadsheets get wrong when a route changes. Our ProDesign cable calculation packs apply the installation method, ambient, grouping and insulation factors per circuit and show them on the output — so the derating is visible, not buried. See also cable sizing to BS 7671: spreadsheets vs ProDesign.