The following notes are intended to be a guide to the installation of Marshall-Tufflex PVC-U conduit and fittings. For further advice
contact our Technical Hotline 0870 121 9990.

CHOICE CONDUIT/CHANNEL

The choice is dependent on the type of work being undertaken and the specification. Heavy gauge round conduit is normally used in surface work and for casting in-situ. Light gauge round conduit is suitable for concealed work and in screeds. Oval conduit is normally chosen for use in plastered walls and can be used as switch drops in surface work. The channel sections are frequently used as an inexpensive method of installing cables in domestic installations beneath plaster. See Material Data page 200 for Thermal Properties.

SURFACE INSTALLATION

All horizontal runs of conduit should be secured at a maximum distance of 0.9m and vertical runs should be secured at a maximum of 1.2m. For high ambient temperatures or where rapid changes in temperature are likely to be encountered this distance should be reduced. At fittings or where directional changes takes place the conduit should be fastened approximately 150mm either side to maintain support. The fastenings should not be over tightened to permit thermal movement of the conduit.

JOINTS AND COUPLERS

To accommodate for thermal movement due to temperature change (Materials Data) on surface installations, it is recommended that expansion couplings be used at a maximum distance of 6m intervals. Where high ambient temperatures or frequent variations in temperature are likely to occur this distance should be reduced. Expansion couplers are installed with the short side coated with solvent cement (MSC2) and the coupler pushed firmly over the conduit down to the shoulder. The slip side coated inside with lubricant sealant (MSC1) receives the conduit to a midpoint to the nib. This will then permit for expansion or contraction providing the conduit is free to move in the saddles. Conduit fittings are installed in the system using solvent cement (MSC2)for permanent installations and lubricant sealant (MSC1) where the installation is subject to frequent changes.

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BENDS

Note: Care should be taken not to make too tight a bend and attention is drawn to BS7671 1992 (Wiring Regulations) 522-08-03. The radius of every bend in a wiring system shall be such that conductors and cables shall not suffer damage

COLD BENDING 20-25MM CONDUIT

This may be carried out on all conduit sizes up to 25mm in diameter using the correct size and gauge of bending spring. It should be noted that the heavy gauge spring is colour banded green and the light gauge spring colour banded white near the tip of the spring. These springs are not interchangeable under any circumstances. Make sure they are not damaged in any way as this can cause the conduit to kink and fracture making removal of the spring difficult. (In cold weather the Conduit should be warmed by rubbing with a rag or some other suitable means before bending.) To bend the conduit insert the spring to the desired position, grip the conduit on either side of the bend and bring slowly together to form the bend. The bend should be made more acute than
necessary because of the tendency of the PVC-U to ‘recover’ after bending. To remove the spring twist in an anti-clockwise direction which will reduce its diameter. At the same time turn the conduit in a clockwise direction gently pulling the spring and conduit apart. If the spring fails to release during this operation do not pull too hard otherwise damage to the spring may occur. Repeat the removal procedure turning the spring again in an anti-clockwise direction and rotating the conduit clockwise slowly pulling them apart. The conduit should then be fastened into position to prevent further ‘recovering’ of the bend.

HOT BENDING

This should be carried out on all conduit above 25mm diameter using the correct size and gauge ofbending spring. Insert the bending spring into the conduit as previously described, gently heating the conduit with a hot air torch, hot water or by other suitable means, with care being taken to avoid the direct application of a flame to the conduit. When the conduit is in a pliable state, slowly bend around a suitable former, holding in position for about one minute until set when the bending spring may then be removed by twisting in an anticlockwise direction and gently withdrawing from the conduit. If the conduit is bent too fast or, particularly in the case of light gauge across the knee, there is a risk of damage to both the conduit and spring. Similarly once the bend has been made it should not be forced backwards but allowed to recover naturally.

EARTHING

The properties of PVC-U make it an all insulated system and the use of a separate earth cable is essential which greatly reduces the risk of poor earth continuity that can result from any breakdown of joints which may occur in a steel system.

JOINT SEALANTS

Lubricant Sealant MSC1. For use with expansion couplers or installations subject to frequent change. Water resistant. Solvent Cement MSC2. A slow acting solvent cement especially formulated for watertight conduit fittings. Solvent Adhesive MSC3. A watertight fast acting solvent adhesive mainly for trunking systems with good take-up properties. Health and Safety Data Sheets are available from our Technical Department.

SPACER BAR SNAP SADDLE ASSEMBLY

Slide Saddle into groove until Saddle locks into the Spacer Bar.

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DISMANTLE
Insert 4mm blade screwdriver into slot on side. Twist screwdriver which releases the Saddle in the Spacer Bar and slide Saddle out of the Spacer Bar groove.

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CONDUIT AND CHANNEL CABLE CAPACITIES

INTRODUCTION
This appendix describes a method which can be used to determine the size of conduit necessary to accommodate cables of the same size, or different sizes, and provides a means of compliance with Regulations, which states ‘The number of cables drawn into conduit of a wiring system shall be such that no damage is caused to the cables or to the conduit during their installation.’

The method employs a ‘unit system’ each cable size being allocated a factor. The sum of all factors for the cables intended to be run in the same conduit is compared against the factors given for conduit in order to determine the size of conduit necessary to accommodate those cables.

It has been found necessary, for conduit, to distinguish between –

1. Straight runs not exceeding 3 metres in length, and
2. Straight runs exceeding 3 metres, or runs of any length incorporating bends or sets.

The term ‘bend’ signifies a British Standard 90º bend and one double set is equivalent to one bend. For the case 1, each conduit size is represented by only one factor. For the case 2, each conduit size has a variable factor which is dependent on the length of run and the number of bends or sets. For a particular size of cable the factor allocated to it for case 1 is not the same as for case 2.

Because of certain aspects, such as the assessment of reasonable care of pulling-in, acceptable utilisation of the space available and the dimensional tolerance of cables and conduit, any method of standardising the cable capacities of such enclosures can only give guidance on the number of cables which can be accommodated.

Thus the sizes of conduit determined by the method given in this appendix are those which can be reasonably expected to accommodate the desired number of cables in a particular run using an acceptable pulling force and with the minimum probability of damage to cable insulation.

Only mechanical considerations have been taken into account in determining the factors given in the following tables.

As the number of circuits in a conduit increases, the current-carrying capacities of the cables must be reduced according to the appropriate grouping factors. It may therefore be more attractive economically to divide the circuits concerned between two or more enclosures.

Single-core PVC-insulated cables in straight runs of conduit not exceeding 3 metres in length. For each cable it is intended to use, obtain the appropriate factor from Table 1A. Add all the cable factors so obtained and compare with the conduit factors given in Table 1B. The conduit size which will satisfactorily accommodate the cables is that size having a factor equal to or exceeding the sum of the cable factors.

TABLE 1A

Cable factors for short straight runs

TABLE 1B

Conduit factors for short straight runs

Single-core PVC-insulated cables in straight runs of conduit exceeding 3 metres in length, or in runs of any length incorporating bends or sets. For each cable it is intended to use, obtain the appropriate factor from Table 1C. Add all the cable factors so obtained and compare with the conduit factors given in Table 1D, taking into account the length of the run it is intended to use and the number of bends and sets in that run. The conduit size which will satisfactorily accommodate the cables is that size having a factor equal to or exceeding the sum of the cable factors.

TABLE 1C
Cable factors for long straight runs or runs incorporating bends

For MT Supertube increase cable factor by 15%.

TABLE 1D
Conduit factors for runs incorporating bends