Note:

1. These factors are applicable to uniform groups of cables, equally loaded.
2. Where horizontal clearances between adjacent cables exceeds twice their overall diameter, no rating factor need be applied.
3. The same factors are applied to:
   – groups of two or three single-core cables:
   – multicore cables.
4. If a system consists of both two- and three-core cables, the total number of cables is taken as the number of circuits, and the corresponding factor is applied to the tables for two loaded conductors for the two-core cables, and to the Tables for three loaded conductors for the three-core cables.
5. If a group consists of n single-core cables it may either be considered as n/2 circuits of two loaded conductors or n/3 circuits of three loaded conductors.
6. The rating factors given have been averaged over the range of conductor sizes and types of installation included in Tables B1.1 to B1.8 the overall accuracy of tabulated values is within 5%.
7. For some installations and for other methods not provided for in the above table, it may be appropriate to use factors calculated for specific cases, see for example Table C3.1 to C3.2.
8. When cables having differing conductor operating temperature are grouped together, the current rating is to be based upon the lowest operating temperature of any cable in the group.
9. If, due to known operating conditions, a cable is expected to carry not more than 30 % of its grouped rating, it may be ignored for the purpose of obtaining the rating factor for the rest of the group. For example, a group of N loaded cables would normally required a group rating factor of Cg applied to the tabulated It. However, if M cables in the group carry loads which are not greater than 0.3 CgIt amperes the other cables can be sized by using the group rating factor corresponding to (N-M) cables.

Note:

1. Values given apply to an installation depth of 0.7m and a soil thermal resisitivity of 2.5K.m/W. These are average values for the range of cable sizes and types quoted for Table B1.1 to B1.8.
   The process of averaging, together with rounding off, can result in some cases in errors of up to ± 10%. (Where more precise values are required they may be calculated by methods given in BS 7769 (BS IEC 60287)).
2. In case of a thermal resistivity lower than 2.5 K.m/W the rating factors can, in general, be increased and can be calculated by the methods given in BS 7769 (BS IEC 60287).

Table C2.3: Rating factors for more than one circuit, cables laid in ducts in the ground – Reference Method D in Tables B1.1 to B1.8

i. Multicore Cables in a Single-Way Ducts

Number of cables	Duct-to-duct Clearance (a)
	Nil (ducts touching)	0.25m	0.5m	1.0m
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

Note:

1. Values given apply to an installation depth of 0.7m and a soil thermal resisitivity of 2.5K.m/W. These are average values for the range of cable sizes and types quoted for Table B1.1 to B1.8.
   The process of averaging, together with rounding off, can result in some cases in errors of up to ± 10%. (Where more precise values are required they may be calculated by methods given in BS 7769 (BS IEC 60287).
2. In case of a thermal resistivity lower than 2.5 K.m/W the rating factors can, in general, be increased and can be calculated by the methods given in BS 7769 (BS IEC 60287).

ii. Single-Core Cables in Non-Ferrous Single-Way Ducts

Number of single-core circuits of 2 or 3 cables	Duct-to-duct Clearance (a)
	Nil (ducts touching)	0.25m	0.5m	1.0m
2	0.80	0.90	0.90	0.95
3	0.70	0.80	0.85	0.90
4	0.65	0.75	0.80	0.90
5	0.60	0.70	0.80	0.90
6	0.60	0.70	0.80	0.90

Note:

1. Values given apply to an installation depth of 0.7m and a soil thermal resistivity of 2.5K.m/W. These are average values for the range of cable sizes and types quoted for Table B1.1 to B1.8.
   The process of averaging, together with rounding off, can result in some cases in errors of up to ± 10%. (Where more precise values are required they may be calculated by methods given in BS 7769 (BS IEC 60287)).
2. In case of a thermal resistivity lower than 2.5 K.m/W the rating factors can, in general, be increased and can be calculated by the methods given in BS 7769 (BS IEC 60287).

Table C3.1

Rating factors for groups of more than one multicore cables, to be applied to reference current-carrying capacities for multicore cables in free air
– Reference Method E in Tables B 1.1 to B1.8

Installation Method in Table A1.1 to A1.5			Number of trays or ladders	Number of cables per tray or ladder
				1	2	3	4	6	9
Perforated cable tray system (Note3)	31		1	See item 4 of Table C2.1
			2	1.00	0.87	0.80	0.77	0.73	0.68
			3	1.00	0.86	0.79	0.76	0.71	0.66
			6	1.00	0.84	0.77	0.73	0.68	0.64
Perforated cable tray system (Note3)	31		1	1.00	1.00	0.98	0.95	0.91	–
			2	1.00	0.99	0.96	0.92	0.87	–
			3	1.00	0.98	0.95	0.91	0.85	–
Vertical perforated cable tray system (Note 4)	31		1	See item 4 of Table C2.1
			2	1.00	0.88	0.81	0.76	0.71	0.70
Vertical perforated cable tray system (Note 4)	31		1	1.00	0.91	0.89	0.88	0.87	–
			2	1.00	0.91	0.88	0.87	0.85	–
Unperforated cable tray systems	30		1	0.97	0.84	0.78	0.75	0.71	0.68
			2	0.97	0.83	0.76	0.72	0.68	0.63
			3	0.97	0.82	0.75	0.71	0.66	0.61
			6	0.97	0.81	0.73	0.69	0.63	0.58
Cable ladder systems, cleats, wire mesh tray, ect (Note 3)	32 33 34		1	See item 4 of Table C2.1
			2	1.00	0.86	0.80	0.78	0.76	0.73
			3	1.00	0.85	0.79	0.76	0.73	0.70
			6	1.00	0.84	0.77	0.73	0.68	0.64
Cable ladder systems, cleats, wire mesh tray, ect (Note 3)	32 33 34		1	1.00	1.00	1.00	1.00	1.00	–
			2	1.00	0.99	0.98	0.97	0.96	–
			3	1.00	0.98	0.97	0.96	0.93	–

Note:

1. Values given are averages for the cable types and range of conductor sizes considered in Tables B1.1 to B1.8. The spread of values is generally less than 5%.
2. Factors apply to single layer groups of cables as shown above and do not apply when cables are installed in more than one layer touching each other. Values for such installations may be significantly lower and must be determined by an appropriate method.
3. Values are given for the vertical spacing between cable tras of 300mm and at least 20mm between cable trays and wall. For closer spacing the factors should be reduced.
4. Values are given for horizontal spacing between cable trays of 225mm with cable trays mounted back to back. For closer spacing the factors should be reduced.

Table C3.2

Rating factors for groups of one or more circuits of single-core cables to be applied to reference current-carrying capacity for one circuit of single-core cables in free air
– Reference Method F in Tables B1.1 to B1.8

Installation Method in Table A1.1 to A1.5			Number of trays or ladders	Number of cables per tray or ladder			Use as a multiplier to rating for:
				1	2	3
Perforated cable tray system (Note 3)	31		1	0.98	0.91	0.87	Three cables in horizontal formation
			2	0.96	0.87	0.81
			3	0.95	0.85	0.78
Vertical perforated cable tray systems (Note 4)	31		1	0.96	0.86	–	Three cables in vertical formation
			2	0.95	0.84	–
Cable ladder systems, cleats, wire mesh tray, etc. (Note 3)	32 33 34		1	1.00	0.97	0.96	Three cables in horizontal formation
			2	0.98	0.93	0.89
			3	0.97	0.90	0.86
Perforated cable tray system (Note 3)	31		1	1.00	0.98	0.96	Three cables in trefoil formation
			2	0.97	0.93	0.89
			3	0.96	0.92	0.86
Vertical perforated cable tray systems (Note 4)	31		1	1.00	0.91	0.89	Three cables in trefoil formation
			2	1.00	0.90	0.86
Cable ladder systems, cleats, wire mesh tray, etc. (Note 3)	32 33 34		1	1.00	1.00	1.00	Three cables in trefoil formation
			2	0.97	0.95	0.93
			3	0.96	0.94	0.90

Note:

1. Values given are averages for the cable types and range of conductor sizes considered in Tables B1.1 to B1.8. The spread of values is generally less than 5%.
2. Factors apply to single layer groups of cables (or trefoil groups) as shown above and do not apply when cables are installed in more than one layer touching each other. Values for such installations may be significantly lower and must be determined by an appropriate method.
3. Values are given for vertical spacing between cable trays of 300mm and at least 20mm between cable trays and wall. For closer spacing the factors should be reduced. Values are given for horizontal spacing between cable trays of 225mm with cable trays mounted back to back. For closer spacing the factors should be reduced.
   For circuits having more than one cable in parallel per phase, each three-phase set of conductors is to be considered as a circuit for the purpose of this table.