Conduit and Trunking Systems:Overview of Conduit Types

Overview of Conduit Types

A conduit is a tube, channel or pipe in which insulated conductors are contained. The conduit in effect replaces the PVC outer sheath of a cable, providing mechanical protection for the insulated conductors. A conduit installation can be rewired easily or altered at any time and this flexibility, coupled with mechanical protection, makes conduit installations popular for commercial and industrial applications.

Examples of types of conduit used in electrical installation work are as follows :

· PVC

· Steel

· Flexible

· Stainless Steel

Steel Conduit System

Types and Sizes of Steel Conduit

Steel conduits are manufactured to specifications defined by the British Standards Institute (BS).

Class 4 conduit has a hot dip galvanized finish. It is of heavy gauge with a welded seam. This conduit is suitable for use in external or damp installations.This is the conduit most widely used in this country.

Note: The galvanized finish on conduit is to prevent the steel from corroding in normal use. Also manufactured is a solid drawn seamless conduit which is more expensive and only used

for special gas tight, explosion proof or flame proof installations.

Conduit is normally supplied in 3 or 3.75 metre lengths and typical conduit external diameters are 16, 20, 25 and 32mm.

Eddy Currents in Steel Conduit

Metal conduits in which a.c. circuit wiring is installed MUST contain all the current carrying conductors of each circuit in the same conduit, to eliminate the possibility of induced eddy currents. Eddy currents could result in the metal conduit and cables becoming hot.

Some Advantages of Steel Conduit

· Affords cables good mechanical protection

· Permits easy rewiring

· Minimises fire risks.

· Can be utilized as the Circuit Protective Conductor. ( CPC )

Some Disadvantages of Steel Conduit

· Under certain conditions moisture is liable to form on the inside wall of the conduit.

· Expensive compared with some other wiring systems.

· Liable to corrosion where subject to acids, alkalis and corrosive fumes.

Cable Capacity of Steel Conduit ( Space Factor )

Having determined the correct number and cross-sectional area of cables for a given load it is necessary to select the size of conduit that will accomodate them.

If a greater number of cables are installed in the conduit, over-heating, insulation damage and fire may result. As a general rule the number of cables drawn into a conduit should not be such as to cause damage to either the cables or the conduit during the installation.

The maximum number of cables which may be drawn into a conduit can be calculated by using the factor system described in the ETCI Rules.

Cutting Steel Conduit

Conduit should be cut with a hacksaw, using a blade having 32 teeth / inch.

The cut should be made at right angles to avoid difficulty in threading the conduit. Figure 1. illustrates this process.

· Hold conduit to be cut securely in a pipe vice. Avoid damage to the galvanised coating.

· Stand square to the job and make sure your movement is unobstructed.

· Grip the hacksaw lightly and apply light pressure on the forward cutting stroke. Use the full length of the blade.

Threading Steel Conduit

It is necessary to cut threads on the conduit ends in order to screw them into conduit couplings and accessory boxes. The conduit ends are threaded using stocks and dies. After threading the ends, all internal burrs must be removed to ensure that cables are not damaged as they are being drawn into the conduits.

All threaded ends must be screwed up tightly into accessories to ensure earth continuity.

· Hold the conduit securely in a pipe vice ( ensure that pipe vice jaw bolts are tight ).

· While threading keep the die well lubricated preferably with a manufacturer’s paste or tallow.

· Thread to required length and remove any burrs with a file or reamer, this will prevent sharp edges causing damage to cables while being drawn into the conduit.

· To ensure a clean, unbroken thread the die should be rotated counter-clockwise frequently and finally run up and down the threads to remove any swarf ( metal fragments ).

Fitting and Fixing Steel Conduit

Conduit must be securely fixed and where it is liable to mechanical damage, it should be suitably protected.

Drainage outlets must be provided at points where condensation is likely to collect.

Spacing of Supports for Conduits

The table below shows a suggested spacing of supports for steel conduits up to 25mm in diameter. Supports should be positioned within 300mm of bends and fittings. The table assumes that the conduit is not subject to other external mechanical stresses.

A range of clips and saddles are available for conduit installations. The spacer bar saddle, is the device most commonly used for fixing conduit, see Figure 2. They provide a very secure method of fixing and generally should be spaced as per the table on the previous page.

Spacer bar saddles incorporate a spacing plate. These spacing plates are approximately 5mm thick. This spacing plate tends to align the conduit with the knockouts in switch and socket outlet boxes. On vertical runs ensure that saddles are fitted the correct way round. An important function of the spacer bar saddle is to keep the conduit out of contact with plaster and cement surfaces. Contact between conduit and these surfaces could result in corrosion of the conduit and discolourment of the surfaces.

When a conduit is fixed to concrete, much installation time is spent in plugging the concrete for fixings. Spacer bar saddles have the advantage of a central, single-hole fixing. The additional cost of spacer bar saddles over the plain saddle (see Figure 3.) can be offset against the saving in time and effort required in fixing.

Distance Saddles

Distance saddles are designed to space the conduit approximately 10mm from the wall or ceiling. These saddles are generally made of malleable cast iron. They are much more substantial than spacer bar saddles and the additional spacing provides greater protection against corrosion. The use of distance saddles eliminates the possibility of dust and dirt collecting behind the conduit. By adequate spacing it is possible to keep the conduits free of dust and it is for this reason distance saddles (Figure 4A) and hospital saddles (Figure 4B) are generally specified.

Multiple Saddles

Where two or more conduits follow the same route it is generally an advantage to use multiple saddles, see Figure 5. Proper spacing of the conduits ensures that there is no need to set the conduits where they enter conduit accessories

Termination of Steel Conduit to Enclosures

Two methods of terminating steel conduit are commonly used.

· The coupling and male bush method, see Figure 6A (Usually used and preferred)

· The locknut and female bush method, see Figure 6B (Used where space is tight)

Coupling and Male bush Method

The coupling and male bush method is slightly more expensive than the locknut and female bush method but it allows more wiring space and is a stronger form of termination. Using this method a coupling is screwed securely on to a standard threaded conduit end and then butted against the accessory box. A male bush is passed through the box entry and screwed tightly into the coupling using an open-ended spanner or a bushing spanner.

Locknut and Female Brass Bush Method

Using this method a locknut is screwed tightly onto a standard threaded conduit end and the thread passed through the hole in the accessory box. A second locknut is screwed onto the conduit effectively sandwiching the wall of the box. To prevent damage to cables, from the exposed end of the conduit, a female bush is then screwed onto the conduit thread. One disadvantage in using this method of terminating a conduit to a switch or socket box is that the conduit protrudes into the box and may inhibit the fixing of the switch or socket.

Galvanised Steel Conduit Accessories

Figure 7A illustrates a range of galvanised steel conduit boxes, reducers and adaptors.

Jointing Steel Conduit

Where two lengths of conduit are to be joined a plain coupling is used. To ensure good electrical continuity and maximum mechanical strength the tube ends must tighten inside the coupling (Max gap 2 mm), see Figure 7B. Care must be exercised to do this without leaving threads outside the coupling.

Where neither tube can be turned it is necessary to resort to the technique known as the “running coupling” , see Figure 7C. After tightening up the lockring the exposed thread must be painted to prevent corrosion.

Setting up Sequence of a Bending Machine

· Place the semi-circular former ‘B’ (to suit the size of conduit to be bent) in position and secure the centre-pin, see Figure 8.

Bending Steel Conduit

The most common method of bending steel conduit is to use a bending machine. However, for bending 32mm diameter or larger conduits it is recommended to use a portable ratchet bender. The following are the main steps to be taken when using a bending machine.

· Insert the conduit under the stop and into the groove in the former.

· Pull down the handle, allowing the roller to bend the conduit around the former.

· Use a template to compare the angle of the bend formed with the desired angle.

Bends should be formed to an internal radius of not less than 2.5 times the conduit diameter

Making a 90o Bend in Steel Conduit

The next step is to place the conduit in the former with the mark to the rear. Position the conduit so that a try-square, held against the mark touches and forms a tangent to the edge of the former as illustrated in Figure 13.

Making a Double Set or Offset in Steel Conduit

Normally offsets should be formed at either 30° or 45°. A 30° offset is preferred for two reasons:

· Ease of measurement.

· Ease of drawing-in cables.

Forming of a 30° Offset

Now go to the bending machine. Mark a point on the former as shown in Figure 17. Place the conduit in the former matching the first mark on the conduit with the mark on the former and bend to 30° as shown in Figure 18. Now remove the conduit and check the offset angle against a 30° template.

Place the conduit back in the machine pointing in the same direction as before but inverted and match the second mark on the conduit with the mark on the former, see Figure 19. The second offset is now formed until it is in parallel with the first offset.

Second Mark

Forming a 45° Offset

A 45° degree offset is formed in exactly the same manner as the 30° version except that the measurements between the first and second bends are calculated using the following formula 1:1:Ö2. Figure 20 illustrates the use of this formula, in this case Ö2 =141mm or 100 x Ö2 = 141mm.

Making a Saddle Set (Jump) in Steel Conduit

The following formula will help you calculate the position of the next two bends which must be formed in order to achieve the 30mm clearance required.

Reverse the conduit in the former and match mark 3 with the existing former mark. Bend down until the edges of the conduit are in line, see Figure 26.

Mark 3 on conduit Former Mark

Making a Double 90o Bend in Steel Conduit

Measurements for a double 90° bend in steel conduit can be taken using any of the three methods shown in Figure 27.

If you add the outside diameter of the conduit (20mm) to the centre to centre measurments (200mm) we can now mark the position of the second bend (220mm), see Figure 28. Bend down until the 900 angle is achieved.

Note: – The above methods may require adjustments due to manufacturer’s tolerances and wear in different bending machines

PVC Conduit

Composition of PVC Conduit

The basic material is poly-vinyl-chloride ( PVC ) which is produced in both flexible and rigid forms. It is impervious to acids, alkalis, oil, aggressive soils, fungi and bacteria and is unaffected by sea, and air. It withstands all pests and does not attract rodents.

PVC conduit may be buried in lime, concrete or plaster without harmful effects.

Choice of PVC Conduit

The choice is dependant on the type of work being undertaken and the job specification.

· Heavy gauge round conduit is normally used in surface work and for forming on-site.

· Light gauge round conduit is suitable for concealed work.

· Oval conduit is normally chosen for use in plaster walls and can be used for drops in surface work.

Typical electrical installations use a heavy guage standard impact tube manufactured to BS 4607. The conduit sizes and range of fittings are the same as those available for metal conduit.

Jointing PVC Conduit

PVC conduit is most often jointed by placing the end of the conduit into the appropriate fitting. Joints into accessories can be easily made dry or by the use of silicone grease or a permanent solvent, dependant on the installation conditions. The silicone grease should be used for expansion couplings and where an installation may be subject to future alteration.

Precautions When Using Adhesives

Care should be taken when using adhesives, which are pro-chemical based, and as such are quite flammable and volatile.

· Replace the lid on the tin immediately after use.

· Use in a ventilated area away from naked flame.

·Under no circumstance should you smoke while working with this adhesive.

· If adhesive should get into your eyes, seek first-aid immediately.

Expansion of PVC Conduit

Expansion couplings should be used for surface installations at a recommended maximum of 4 metre intervals.

Where frequent variations in ambient temperature are likely to occur this distance must be greatly reduced.

Note: PVC conduit is not suitable for installations subject to temperatures below -5oC or above

65oC

Fitting and Fixing of PVC Conduit

A range of fixings for PVC conduit is available as shown in Figure 29.

PVC conduit is fixed in the same way as metal conduit. All horizontal runs should be saddled at not more than 900 mm intervals unless high ambient temperatures or rapid changes in temperature are likely to be encountered, when the intervals should be reduced.

Vertical runs should be saddled at not more than 1200 mm intervals except where directional changes are made. Saddles should be installed approximately 150 mm either side of a bend. Conduit should be able to move freely in the saddle.

Advantages of PVC Conduit

· Lightweight and easy to handle

· Easy to cut and deburr

· Simple to form and bend

· Does not require painting

· Minimal condensation due to low thermal conductivity in wall of conduit.

· Speed of installation

· Excellent electrical and fire resistant properties

Disadvantages of PVC Conduit

· Care must be taken when glueing joints to avoid forming a barrier across the inside of the conduit.

· If insufficent adhesive is used the joints may not be waterproof.

· PVC expands about 5 times as much as steel and this expansion must be allowed for.

· PVC does not offer the same level of mechanical protection as steel.

· A separate Circuit Protective Conductor must be run inside the conduit.

Working with PVC Conduit

PVC conduit is easily cut using a junior hack saw. Any roughness of cut and burrs should be removed with the aid of knife blade or simply by wiping with a cloth. There are proprietary tools available for cutting PVC conduit.

Cold bending of PVC conduit may be carried out on conduit sizes up to 25mm diameter using a bending spring, see Figure 30. It is essential to use the correct size bending spring and it should be noted that springs for heavy guage conduit are colour coded green and those for light gauge conduit are colour coded white. They are not interchangable.

It is important that the spring is not damaged or kinked. A faulty spring can cause the conduit to

kink and fracture. The spring can also jam inside the bend.

The radius of the bends should be as near as possible to that of a similar bend formed in steel conduit of a similar size.

The conduit should be bent to a more acute angle than the angle finally desired because of the tendency of PVC to “recover” after bending. This tendency lessens as the temperature decreases.

In very cold weather it is recommended that the conduit is warmed slightly by rubbing with a cloth held in the hand. If the conduit is bent too quickly there is a risk of kinking and / or fracturing of the conduit and possible damage to the bending spring.

If the conduit is intended for surface work it should be saddled and fixed as soon as possible after bending. Once a bend has been formed it should not be forced backwards as this can damage both the conduit and the spring.

To remove the spring, twist it in an anti-clockwise direction. This will reduce the diameter of the spring and make it possible to pull the conduit and spring apart. If the spring jams during this operation do not pull hard on it, otherwise the spring will be damaged.

Hot Bending of PVC Conduit

Hot bending of conduit should be carried out on sizes over 25mm diameter. A bending spring of the correct size should be used. Apply a gentle heat preferrably with a hot air torch, electric heating element or hot water. Bend the conduit in accordance with above instructions and Fig. 31.

Care should be taken to avoid the direct application of flame to the conduit. When the conduit is in a pliable state it should be slowly bent around a suitable former and held in position for about one minute until set. After setting, the bending spring may be twisted anti-clockwise and removed.