Cable tray fittings : Names and functions

 Types of  electrical conduits and pipes

• PVC pipes
• Rigid PVC Conduit
• EMT Conduits
• RMC Conduits
• IMC conduits
• RGS conduits
• Flexible Metallic Conduits (FMC)
• Liquidtight Flexible Metal Conduits (LFMC)
• Liquidtight Flexible Non metallic Conduits (LFNC)
• Electrical Nonmetallic tubing (ENT)

PVC pipes

• PVC stands for (Poly Vinyl Chloride)
• It contains plastic materials so it is highly flexible and easy to adapt
• It is considered the lightest in weight and cheapest type of conduits compared to other types of electrical conduits
• PVC tubes are only used for embedded use in concrete or walls
• The plastic material makes the pipe resistant to any moisture
• It can be bent simply by using special heating tools in field

Rigid PVC Conduit

• It is like a pvc plumbing pipe but thinner than it .
• It can be bent to change direction by using a portable heater box .
• Rigid pvc conduits are installed using plastic fittings that are glued together so that the raceway can be watertight making rigid pvc conduits suitable for outdoor and direct burial in ground applications
• It can be installed embedded in concrete and in outdoor and underground applications .
• It is also ideal for use in dry and corrosive locations .
• It is available in various sizes ,the small sizes are installed in concrete inside ceilings and walls or exposed on ceilings or walls but the large sizes can be installed underground between buildings and structures to allow installation of power and communication cables .

EMT Conduits

EMT Conduits 

• EMT stands for (Electrical Metallic Tubing)
• This type of pipes is commonly made of galvanized steel or steel-coated aluminum
• it is a light weight and thin wall tube and less costly compared with GRC tubes
• It is used for the installation of electrical conductors in both exposed and concealed work
• It is commonly used in commercial and industrial buildings than in residential applications
• It is commonly used for exposed indoor wiring runs in commercial and industrial buildings
• This type of tube can be used in wet locations or in concrete or even direct contact with the earth on condition all tubes and their fittings are coated with corrosion resistant material
• It is a rigid pipe but can be bent by a simple tool called conduit bender in case of need to change its direction
• It is a famous to be used to protect fire alarm system cables in case of exposed installations over false ceilings
• EMT pipe is internally coated with organic insulation which makes it smooth so it can be easy to pull the wires inside .
• EMT is available in many sizes commonly from 1/2 through 4 inch
• EMT must be supported at no more than 10 ft (3.0 m) intervals, and within 3 ft (900 mm) of each "outlet box, junction, device box, cabinet ,conduit body and other tubing terminations) and a strap must be used within 3 ft (900 mm) of each coupling.

• The maximum no of bends in EMT tube from a one wire pull point to another should not exceed four 90 degree bends or equivalent 360 degree
• EMT can be used as an equipment grounding conductor.

RMC Conduits 

• RMC stands for (Rigid Metallic Conduit)
• They are thick pipes made of galvanized steel or steel coated aluminium
• It is one of the heaviest and thickest metal pipes and can be coated with a zinc casing inside and outside and covered externally with a non-carrial material such as pvc and has a broiled tooth
• These pipes can be used indoor, outdoor , underground and also concealed or exposed
• RMC conduits are used in dry , wet and corrosive locations

IMC conduits

• IMC stands for (Intermediate Metallic Conduit)
• This type of pipe is lighter than RMC, heavier than EMT and also less thick than RMC by one third and thicker than EMT .
• It can be used in the same applications as RMC
• The outer layer of these pipes is coated with a layer of zinc and the inner layer is coated with an organic layer that is shock resistant
• RMC Conduits are threaded on both ends so that can be connected with other conduits
• These pipes can not be bent so a ready elbows of them are available
• IMC conduit Can be considered (exposed seen) which is installed visibly not buried in concrete or walls but can be seen in offices and workplaces .

RGS conduits

• RGS stands for (Rigid Galvanized Steel)
• It is the thickest wall and heaviest weight steel conduit that is used in indoor, outdoor, underground, concealed and exposed applications .
• RGS conduit is resistant to rust and chemical reactions so used frequently in places where gases are abundant and places near water
• It is considered the most expensive types of conduits and mainly used over suspended ceilings and in exposed electrical installations in Electrical and mechanical rooms, some laboratories, bathrooms and Wet Area

Flexible conduits (FC)

There are three types of flexible conduits as follows:

Flexible Metal Conduits (FMC)

• It is a very flexible conduit due to its spiral construction
• It has a smooth inner layer which makes it easy to pull wires inside .
• FMC conduits can be bent very easily without using any special bending tools
• It is very easy to install in field without any need to use elbows or couplings or any other fittings
• It is mainly used in dry indoor locations where it is difficult to install rigid metal conduits or where excess number of fittings would be needed to use rigid connections
• FMC conduits are widely used above suspended ceilings for carrying branch circuit conductors from the junction box to the hot wiring compartment in lighting fixtures.
• When it is used above suspended ceilings, its length should not exceed 6 ft (1.8 m)
• It is also used to carry wires for short run from a wall box to a motor or fixed appliance
• FMC conduits should not be used underground (directly buried or in duct which is buried) or embedded in poured concrete or aggregate ,or in direct contact with earth .
• FMC conduits should not be used where subjected to corrosive conditions ,also these conduits shall not be installed in direct contact with masonry in damp locations .
• FMC conduits can be used ideally with vibration equipment like air conditioning units.
• The NEC specifies that straps or any other means of supporting these conduits in place should be used every 4.5 ft (1.4 m) and within 12 in (300 mm) of each end .
• The NEC specifies that FMC conduits in 1.25 (35) trade size or smaller, installed with not more than 6 ft (1.8 m) total length and terminated in fittings investigated for grounding ,these conduits can be used for grounding when used for carrying circuits rated 20A or less .
• FMC conduits are available in sizes (3/8 , 1/2 , 3/4)

Liquidtight Flexible Metal Conduits (LFMC)

• LFMC conduit is similar in construction to  FMC conduit but it is covered with outer sheath of  thermoplastic material that makes it water and oil resistant and provides protection against most liquids, sunlight, acids ,vapour and impact/crush.
• LFMC conduit is made with galvanized steel inner core makes it stronger and less vulnerable tto kinking and also  provides mechanical protection for the conductors or wires installed inside. 
• LFMC conduits can be used exposed or concealed and also can be used for direct burial in earth .
• It is suitable for use in wet and damp locations .
• LFMC conduits are also used with outdoor equipment as an example to carry wires from the disconnect switch to the air conditioner external unit

Liquidtight Flexible Non metallic Conduits (LFNC)

• It is made with a spiral of rigid PVC reinforcement embedded within a flexible PVC wall
• LFNC conduit has a smooth inner layer that enables easy wire pulling making the installation easier and it also has a smooth outer layer that is sunlight and oil resistant making it suitable for outdoor use .
• It is lighter than LFMC conduits (approximately 50% lighter),  easier to cut  and lower installation cost 
• LFNC conduit has no sharp edges or burrs that protects conductors from any damage .
• LFNC conduits provide protection against corrosion, vapour, acids, oil, liquids and impact/crush .
• LFNC conduits can be used exposed or concealed and also can be used for direct burial in earth .
• It is suitable for use in applications that require high flexibility such in extremely tight areas .
•  LFNC conduits are used in many applications like motors, pumps, HVAC, marine equipments, outdoor lighting and etc

Electrical Nonmetallic tubing (ENT)

• It is composed of a material that is resistant to moisture and chemical atmospheres and is flame retardant.
• ENT can be bent by hand, when being installed, to establish direction and lengths of runs.
• Conduit fittings used for these conduits are rigid and don't have bending properties .
• ENT can be used concealed in walls, floors, and ceilings that provide a thermal barrier with at least a 15-min fire rating from listings of fire-rated assemblies .
• ENT can be used or installed above suspended ceilings with at least a 15-min fire rating .
• ENT can be used in indoor wet locations, or in slab on or below grade .
• ENT shall not be used in hazardous locations or direct burial.
• ENT shall not be used in locations where the ambient temperature exceeds that for which the ENT is rated for.
• ENT should be fastened every 3 ft (900 mm) and there is the usual 6 ft (1.8 m) allowance for luminaire and equipment whips above suspended ceilings .
• ENT is available in sizes (from 1/2 to 2 inch)

Note :

• It is necessary to avoid installing a single-phase cable or single-core cable into any type of metal conduits because the magnetic field arising from the cable will cut off the surface of the metal conduit, producing eddy currents in the conduit, causing heat of the pipe and may lead to overheating or burning of the cable ,so in single phase circuits both the pahse and neutral conductor feeding the same load should be installed inside the same conduit as the magnetic field resulting from the two conductors will cancel each other .
• For multi-core cables, the magnetic field generated by ththree phases of the cable will be equal to zero or cancel each other , resulting in no eddy currents causing heating of the conduit and cable .

Filling ratio for conduits

Filling ratio : is the ratio between the sum of the cross section areas of all cables to the cross section area of the conduit containing them .

Filling ratio NEC table

Calculation of the appropriate conduit size according to cross section areas of cables

Example: if we have three copper cables the C.S.A of each cable is (16*4 mm2) .So what is the appropriate conduit size for these cables ?

Solution

 since the number of cables is more than (2), according to NEC table , the Filling Ratio = 40%    so,

C.S.A of cables = 40% C.S.A of Conduit 

π * (D² / 4) = 0.4 * (d²/ 4)           

D²  = 0.4 * d²    

      d = 1.58 * D      

where d = diameter of Conduit        

          D = sum of diameters of Cables

So from the catalogue of cable manufacturer (as below) we can find the outer diameter of the cable (16*4 mm2)

From the catalogue, we find that the outer diameter of the cable (16*4 mm2) equals (19.6 mm) , so

  D = 3 * 19.6 = 58.8 mm        

   d = 1.58 * D                           

 = 1.58 * 58.8 = 92.9 mm

so we can choose a conduit with size of 110 mm or 4 inches.

Electrical conduits fittings and accessories

PVC Conduits fittings

PVC couplings / Connectors

Coupling is a short piece of conduit that allows two pieces of the main conduit to be joined together

PVC elbow

Elbow is a piece of conduit that is installed between two pieces of the main conduit to allow changing the direction of conduit or bypass obstructions

PVC reducer

Reducer is a short piece of conduit that allows two pieces of the main conduit of different diameter or size to be joined together

PVC crank

It is used to join two pieces of conduits with an obstruction between them or to cross the other conduits in the same level

PVC male/female adaptor

Male/female adaptor is used to connect conduit to electrical back box or junction box

PVC saddle

Saddles are used for fixing conduits to walls or ceiling and there are many types of saddles like : 

- Spacer bar saddle 

- open or strap saddle 

- spring or U clip saddle 

- oval clip saddle 

- oval strap saddle

PVC Ceiling boxes

These are boxes that are used to connect wires to ceiling lighting fixtures

PVC conduit cutter

The conduit is cut using special cutter to make the cutting place without sharp edges that may injure the wires when pulled inside the conduit. 

Metal conduits fittings

Metal Coupling / connector

Coupling is used to connect two pieces of metal conduit to be joined together and the two pieces of the conduit are connected to both ends of coupling through two screws .

Metal  conduit elbow

Elbow is used to allow changing the direction of conduit or bypass obstructions and the two pieces of the conduit are connected to both ends of elbow through two screws 

Sometimes ready elbows are not used but the elbow is made in field using Conduit Bender which is used to bend the conduiy according to the required angle in the field

Metal male/female adaptor

Adaptor is used to connect conduit to an electrical  outlet box and has two ends one end is connected to the conduit by using screw and the other end is connected to the electrical box by using a nut .

Metal conduit crank

Metal conduit cutter

Metal Extension Box

Embedded conduits in wall  (using UPVC pipes) can be  converted into exposed conduits (using EMT or RGS metal conduits) by using  extension Box as shown