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Correction factors (Ca, Cg and more)Why does a cable in a hot loft carry less current?
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A simple shower example has no correction factors. In real installations, the environment around a cable can reduce how much current it can safely carry, so we correct for it and usually end up with a bigger cable.
Only multiply in the factors that apply to the question, then pick a cable whose tabulated rating It is at least that figure. Its capacity as installed is Iz = It × Ca × Cg × …, which is then at least In.
| Factor | What it corrects for | Where to find it |
|---|---|---|
| Ca | Ambient temperature around the cable (not the room). Tables are based on 30 °C air, where the factor is 1. Buried cables use a 20 °C ground temperature instead. | OSG Table F1 · BS 7671 Table 4B1 |
| Cg | Grouping: several circuits run bunched together along the same route | OSG Table F3 · BS 7671 Table 4C1 |
| Ci | Thermal insulation around the cable | OSG · BS 7671 |
| Cc | Cable buried in the ground (in a duct or direct): factor 0.9 | BS 7671 only |
| Cf | Circuit protected by a BS 3036 rewireable fuse: factor 0.725 | BS 7671 only |
In exam questions you'll be given the temperature and the number of grouped circuits. You don't measure them. In the exam you mainly need Ca, Cg and Ci.
On-Site Guide · Appendix F · Table F1 ambient temperature factors (Ca) · p.184 On-Site Guide · Table F3 grouping factors (Cg) · p.186 On-Site Guide · It ≥ In ÷ factors formula and some factors · p.183
BS 7671 · Appendix 4 · list of symbols Ca, Cg, Ci, Cc, Cf (incl. Cf 0.725 and Cc 0.9) · p.452 Highlight Cf and Cc
BS 7671 · Table 4B1 ambient temperature factors · p.469 BS 7671 · Table 4C1 grouping factors · p.471
Choose the arrangement row. The shower cable isn't on a ladder, a tray, or a single layer on a wall or floor, so it's "bunched in air, on a surface, embedded or enclosed". Then read across to the number of circuits. "Grouped with two other circuits" means 3 circuits in total.
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