# Correction factors

Source: https://meptools.com/tools/voltage-drop-cable-checker/docs/correction-factors/

The **Correction factors** panel (stage 02, Cable selection) adjusts the tabulated
current-carrying capacity of the selected cable for your actual installation conditions. The
corrected capacity used in the current-capacity check is:

> Iz = It × Ctotal

where It is the BS 7671 Appendix 4 tabulated current capacity for the selected cable family,
installation method, and size, and:

> Ctotal = Ca × Cs × Cd × Cg × Ci × Cf × Cc × Ccores

Each term is a separate correction factor, each traceable to a specific BS 7671 table or
clause.

## The factors

| Symbol | Corrects for                                          | BS 7671 source                                                                   |
| ------ | ----------------------------------------------------- | -------------------------------------------------------------------------------- |
| Ca     | Ambient (in air) or ground (buried) temperature       | Table 4B1 (in air) or Table 4B2 (buried)                                         |
| Cs     | Soil thermal resistivity (buried only)                | Table 4B3                                                                        |
| Cd     | Burial depth (buried only)                            | Table 4B4                                                                        |
| Cg     | Grouping with other circuits or multicore cables      | Grouping table for the selected arrangement (shown as "table · label" in the UI) |
| Ci     | Thermal insulation in contact with the cable          | Clause 523.9                                                                     |
| Cf     | Protective-device type (semi-enclosed BS 3036 fuses)  | —                                                                                |
| Cc     | Overload protection for buried reference methods      | —                                                                                |
| Ccores | Number of loaded cores, for 1.5-4 mm² conductors only | Table 4B5                                                                        |

### Ca — ambient-temperature factor

UI field: **"Ambient temperature"** (°C) for in-air methods, or **"Ground temperature"** (°C)
for buried methods. Looked up from Table 4B1 (in air) or Table 4B2 (buried), against the
conductor's operating-temperature column (60/70/90 °C, from the selected cable's insulation).

### Cs — soil-resistivity factor

UI field: **"Soil resistivity"** (K·m/W), shown only for buried installation methods. Looked
up from Table 4B3.

### Cd — burial-depth factor

UI field: **"Burial depth"** (m), shown only for buried installation methods. Looked up from
Table 4B4.

### Cg — grouping factor

UI fields: **"Grouping arrangement"** — a dropdown of options valid for the selected cable,
each shown as "table · label" (the source table plus the arrangement description) — and
**"Grouped circuits or multicore cables"**, a count of 1 or more.

### Ci — thermal-insulation factor

UI field: **"Thermal insulation"** — a single select with six options, each showing the
resulting Ci directly:

| Option             | Ci   |
| ------------------ | ---- |
| Not in insulation  | 1.00 |
| 50 mm enclosed     | 0.89 |
| 100 mm enclosed    | 0.81 |
| 200 mm enclosed    | 0.68 |
| 400 mm enclosed    | 0.55 |
| Surrounded ≥ 0.5 m | 0.50 |

There is no separate length field — the enclosure length is baked into the option you pick.
Per clause 523.9 / Table 52.2, the last row is open-ended: any surrounded length of 0.5 m or
more lands on Ci = 0.50, and at that point the base capacity becomes 0.5 × Reference Method C
rather than a factor on the selected method.

This field is only shown for in-air installation methods — buried methods hide it (see
"Field visibility" below). If a stored selection is "Surrounded" but the current installation
method doesn't support the insulation correction, the panel shows a note that Ci stays at 1.00
regardless of the stored choice.

### Cf — protective-device factor

Not a field in this panel — it is set by the protective device type chosen on the Circuit &
load page (stage 01). Cf = 0.725 for BS 3036 semi-enclosed fuses, and 1.00 for every other
device type.

### Cc — buried-overload factor

Not a field in this panel — it follows from the selected installation method. Cc = 0.90 for
buried reference methods D1 and D2, and 1.00 otherwise.

### Ccores — loaded-core factor

UI field: **"Loaded cores"**, shown only for the multicore cable families that use it (4D2,
4E2, 4F2). Looked up from Table 4B5, and only active for conductor sizes in the 1.5-4 mm²
range.

## Field visibility

The panel shows only the fields relevant to the selected installation method:

- **In-air methods** — the soil-resistivity and burial-depth fields are hidden; the
  thermal-insulation fields are shown.
- **Buried methods** — the thermal-insulation field is hidden; the soil-resistivity and
  burial-depth fields are shown.

![Correction factors panel for a buried installation, showing soil resistivity and burial depth fields](./_assets/correction-factors-buried.png)

## Requested vs. effective values

BS 7671 correction-factor tables are only defined at discrete steps (specific temperatures,
depths, resistivities, and so on). Whatever value you type is snapped to the nearest
supported table value before the factor is looked up. Below the fields, the panel shows
**"Values actually used in the calculation"**, listing both what you entered and the
effective (snapped) value actually used, together with any warnings raised by the
normalization.

## Next

- [Manufacturer override](/tools/voltage-drop-cable-checker/docs/manufacturer-override) —
  replacing the tabulated voltage-drop coefficient with manufacturer R/X or a direct mV/A/m
  value.
- [Cable & installation selection](/tools/voltage-drop-cable-checker/docs/cable-and-installation) —
  cable families and installation methods.
- [Reading the results](/tools/voltage-drop-cable-checker/docs/results) — how Iz and the
  correction-factor audit trail appear in the result panel.