Correction Factors

Correction factors for ambient and ground temperature, grouping, thermal insulation, and semi-enclosed fuses.

Ambient Temperature (Ca) — Table 4B1

Ambient Temperature Correction Factor (Ca), cables in air

Temp (°C)	70 °C thermoplastic (PVC)	90 °C thermosetting (XLPE)	Mineral, 70 °C sheath	60 °C thermosetting (flex)
25	1.03	1.02	1.07	1.04
30	1.00	1.00	1.00	1.00
35	0.94	0.96	0.93	0.91
40	0.87	0.91	0.85	0.82
45	0.79	0.87	0.78	0.71
50	0.71	0.82	0.67	0.58
55	0.61	0.76	0.57	0.41
60	0.50	0.71	0.45	—
65	—	0.65	—	—
70	—	0.58	—	—
75	—	0.50	—	—
80	—	0.41	—	—

Reference ambient 30 °C. — : BS 7671 gives no factor; the cable may not be used at that temperature.

Mineral 70 °C: thermoplastic covered, or bare and exposed to touch (pairs with Table 4G1A).

BS 7671: Table 4B1 Open Calculator →

Ground Temperature (Ca) — Table 4B2

Ground Temperature Correction Factor (Ca), Reference Method D

Ground temp (°C)	70 °C thermoplastic (PVC)	90 °C thermosetting (XLPE)
10	1.10	1.07
15	1.05	1.04
20	1.00	1.00
25	0.95	0.96
30	0.89	0.93
35	0.84	0.89
40	0.77	0.85
45	0.71	0.80
50	0.63	0.76
55	0.55	0.71
60	0.45	0.65
65	—	0.60
70	—	0.53
75	—	0.46
80	—	0.38

Reference ground temperature 20 °C, not 30 °C. Method D ratings also assume soil thermal resistivity 2.5 K·m/W and 0.7 m depth.

BS 7671: Table 4B2

Grouping Factor (Cg) — Table 4C1

Grouping Correction Factor (Cg), cables touching — Table 4C1

No. of circuits	Row 1: Bunched / enclosed (A to F)	Row 2: Single layer on wall or floor (C)	Row 3: Multicore on perforated tray (E)	Row 4: Multicore on ladder or cleats (E)
1	1.00	1.00	1.00	1.00
2	0.80	0.85	0.88	0.87
3	0.70	0.79	0.82	0.82
4	0.65	0.75	0.77	0.80
5	0.60	0.73	0.75	0.80
6	0.57	0.72	0.73	0.79
7	0.54	0.72	0.73	0.79
8	0.52	0.71	0.72	0.78
9	0.50	0.70	0.72	0.78
12	0.45	0.70	0.72	0.78
16	0.41	0.70	0.72	0.78
20	0.38	0.70	0.72	0.78

Between tabulated numbers use the next higher number's (lower) factor. No factor is given beyond 20 circuits.

Grouping factors assume uniform groups of equally loaded cables (Table 4C1 NOTE 1). Where horizontal clearances between adjacent cables exceed twice their overall diameter, no factor need be applied (NOTE 2).

BS 7671: Table 4C1 Open Calculator →

Grouping in Ducts (Cg) — Table 4C3

Multicore Cables in Single-way Ducts, Reference Method D1 — Table 4C3

No. of ducts	Ducts touching	0.25 m	0.5 m	1.0 m
2	0.85	0.90	0.95	0.95
3	0.75	0.85	0.90	0.95
4	0.70	0.80	0.85	0.90
5	0.65	0.80	0.85	0.90
6	0.60	0.80	0.80	0.90

Duct-to-duct clearance (α) between adjacent ducts. One duct: no grouping factor (1.00). No factor is given beyond 6 ducts.

BS 7671: Table 4C3

Thermal Insulation (Ci) — Reg 523.9

Thermal Insulation Correction Factor (Ci)

Length of run in insulation	Factor (Ci)
Not in contact with insulation	1.00
50 mm run in insulation	0.88
100 mm run in insulation	0.78
200 mm run in insulation	0.63
400 mm run in insulation	0.51
500 mm or more in insulation (totally surrounded)	0.50

For cables up to 10mm² totally surrounded by thermal insulation over short lengths. At 0.5m or more, use 0.50 unless more precise information is available.

The factor multiplies the cable's Reference Method C (clipped direct) rating (Reg 523.9). A cable with one side in contact with a thermally conductive surface is covered by the tabulated reference methods, not by a factor.

BS 7671: Reg 523.9 / Appendix 4 Section 2.6

Semi-enclosed Fuse Factor (Cc)

When BS 3036 semi-enclosed (rewirable) fuses are used, apply a correction factor Cc = 0.725 to the tabulated cable current rating.

BS 7671: Reg 433.1.1

Cable Capacities

Voltage Drop

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