Cable Support Spacing
Proposed changes to the Wiring Regulations, also known as BS7671, were published on 1st July 2017. While the revised document does touch on the subject of cable supports, it does not detail what the distance between cable supports should be. This blog sets out to shed some light on the issue of cable support distances….
Fire Resistant Cable Supports
One of the most significant of the proposed changes to the wiring regulations is for fire resistant cable supports to be used throughout an installation. The 3rd amendment of the 17th edition made it a requirement only for cables in escape routes to use fire resistant cable supports. However, under the proposed revision the requirement for fire resistant cable supports has been expanded to include cables in all areas, regardless of whether or not the cables are installed in an escape route.
In the proposed revision, section 521.10.202 states: Wiring systems shall be supported such that they will not be liable to premature collapse in the event of fire.
Note1: Wiring systems hanging across access or egress routes may hinder evacuation and firefighting access
Note 2: This regulation precludes, for example, the use of non-metallic cable clips or cable ties as the sole means of support where cables are clipped direct to exposed surfaces or suspended under cable tray, and the use of non-metallic cable as the sole means of support of the cables therein.
Note 3: Suitably spaced steel or copper clips. Saddles or ties are examples that will meet the requirements of this regulation
Clauses 522-08-04 and 522-08-05 also refer to cable supports. These clauses remain unchanged from the current (17th) edition of the Wiring Regulations.
Clause 522-08-04 states: Where conductors or cables are not supported continuously due to the method of installation, they shall be supported by suitable means at appropriate intervals in such a manner that the conductors or cables do not suffer damage by their own weight.
Clause 522-08-05 states: Every cable or conductor shall be supported in such a way that it is not exposed to undue mechanical strain and so that there is no appreciable strain on the terminations of conductors, account being taken of mechanical strain imposed by the supported weight of the cable conductor itself.
What all of these clauses are effectively proposing is that all cables must be supported effectively throughout their length.
For cables clipped direct to a surface, the IET Selection and Erection Guidance Notes number 1 provides cable clipping distances based on their diameter and cable type. Prysmian Group was instrumental in providing this information and an extract is available to download here.
Vertical Run Cable
For vertical runs (exceeding 100m) then additional measures should be applied. The cable should not be allowed to have a straight vertical run without the addition of tension-reliving section. This normally involves the cable having a short horizontal section of at least 1 metre) included in the vertical run. Note: at the point of change from vertical to horizontal and horizontal to vertical the internal radius bend should not be less than the minimum bending radius for the cable. Cable supports should be applied either side of the bend.
For flexible systems, where the cable is not directly fixed to the support system, for example a J-hanger installation, calculations need to be undertaken to determine the required distance between the cable support positions to achieve a set deflection/off-set. It is not realistic to install cables in a flexible system without any initial off-set (typically 2%). If no offset is introduced, in the form of a deflection from the cable plane (either in the vertical or horizontal plane) then expansion will build up at discrete points as the cable heats up and the conductor expands, which could result in the cable becoming displaced from its hanger supports.
Heinhold Calculation Method
Prysmian UK use the Heinhold method for undertaking such calculations for major projects. This method considers the stiffness of the cable by summating the axial movement of inertia (Jo) of each element with the E-modulus for each material layer (eg insulation, armour and sheath). From this figure the length between support positions can be calculated for the define deflection (sag) percentage. The length between support positions will change depending on the cable design, size, materials and weight. For example, an MDPE sheathed cable will be stiffer and therefore require a greater distance between supports than the same LSOH cable to achieve the same 2% deflection (sag).
For examples and information on cable supports including fire resistant cable cleats see cable fixings.