Commonly used Installation/Reference Method Codes
Installation/Reference Methods for cables
These codes are entered into ‘Circuit Details’ section of Test sheets and used to identify cable types for determining current carrying capacities.
Note: Routing of cables
Where cables are routed through insulation, this can make a considerable difference to the size of the conductor required.
A Enclosed in conduit in thermally insulated wall
B Surface conduit or trunking on a wall
C Clipped or fixed direct ( not enclosed)
D Buried direct in the ground (with or without mechanical protection)
E Perforated Cable Tray/ Ladder/ Free Air (Multi core)
F Perforated Cable Tray/ Ladder/ Free Air (Single core)
G Bare/ non sheathed/ Sheathed with heat dissipation not impeded Free Air
Specifically for Flat Twin & Earth cables
100 Clipped direct to wooden joist above plasterboard ceiling.
Thermal insulation not exceeding 100mm
101 Clipped direct to wooden joist above plasterboard ceiling.
Thermal insulation exceeding 100mm
102 In a stud wall with insulation.
Cable touching the inner wall surface.
103 In a stud wall with insulation.
Cable not touching the inner wall surface.
Note
100, 101 and 102 must not be totally enclosed in thermal insulation. One side of the cable must be in contact with a thermally conductive surface.
Installation methods 103 relates to cables which are not in contact with such a surface and are subject to a de-rating factor of 0.5.
Examples of how to economically route cables
You can carry out your own calculations or use an online cable size calculator to prove the following example and see for yourself how this theory works.
Example
An immersion heater rated at 3 kW is to be installed using twin and earth cable. The circuit will be fed from a 16A 60898 mcb and routed for 14m through a roof space which is insulated with glass fibre.The roof space temperature is expected to rise to 50°C in summer.
The following options demonstrate how correction factors for grouping and ambient temperature can affect cable size.
A. Where it leaves the consumer unit and passes through a 50 mm insulation-filled cavity,the cable will be bunched with seven others.
Required Cable Size= 10mm
This would be an expensive option not to mention very difficult to terminate such a large cable in the accessories. A more sensible option would be to look for a method of reducing the required cable size.
B. Eight cables leave the consumer unit in two bunches of four
Required Cable Size= 6mm
D. To attempt to make the cable size smaller, if the eight cables left the consumer unit in four bunches of two:
Required Cable Size= 4mm
C. A single cable run out on its own then no grouping factor would apply.
Required Cable Size= 2.5mm
E. Finally, if the cable left the consumer unit on its own and the cable was not in contact with the glass fibre and clipped direct to the rafters in the loft space.
Required Cable Size= 1.5mm
See On-Site Guide Section 7 for a detailed list of descriptions
also see BS7671:2008 Appendix 4 & Table 4A2 for more extensive details
ElectriciansBlog.co.uk
{ 2 comments… read them below or add one }
Hi Bill
You could use the Cable Calculator in the ERA http://www.electriciansblog.co.uk/2009/01/resources/
Hope this helps.
What B type breaker size is the maximum I can use for lighting using 1.5mm cable where the cable is in a flat roof with thermal insulation, I am not sure of the thickness of the insulation but the cable run is not longer than 30 metres? Could you give me the size for both reference 100 & 101