Correction factors — MEP Tools
How the Correction factors panel derates the tabulated BS 7671 current capacity for your installation conditions.
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.
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 — replacing the tabulated voltage-drop coefficient with manufacturer R/X or a direct mV/A/m value.
- Cable & installation selection — cable families and installation methods.
- Reading the results — how Iz and the correction-factor audit trail appear in the result panel.