Prior to the installation of an above-grade conduit system, the Electrical Engineer should examine the various factors that will affect the installation. These factors include the type of equipment that is to be installed, the installation method that will be used, and the classification of the area in which the equipment will be installed. The requirements of SAES-P-104 (Wiring Methods and Materials) should be followed for Saudi Aramco above-grade conduit installations.
The conduit installation method that is used (rigid-steel conduit, EMT, or flexible liquid-tight conduit) will affect the routing requirements of the installation. Other determinations, such as the correct size of the conduit for the installation, must also be made with respect to routing the conduit and cabling. The hazardous classification of the installation location determine the sealing and termination requirements. This section of the Module describes the following aspects of determining conduit installation requirements:
o Conduit Types and Applications o Conduit Sizing and Routing o Conduit Bending o Conduit Threading
o Indoor and Outdoor Conduit Termination o Conduit Supports
Conduit Types and Applications
The types of conduit that are available for use by Saudi Aramco in above-grade conduit systems are rigid-steel conduit, EMT, and flexible liquid-tight conduit. IMC is prohibited for use in Saudi Aramco installations. The additional cost of rigid-steel conduit is considered to be worth the added protection that it offers over IMC.
Hot-dipped, galvanized, rigid-steel conduit is specified for all Saudi Aramco installations in which rigid-steel conduit is used. The application requirements of rigid-steel conduit systems have been described previously in this module and are listed briefly as follows:
o Used for Class I, Division 1 areas.
o Used when exceptional mechanical protection is required.
o Used when conduit is installed above ground in outdoor industrial facilities.
o Used when conduit is installed in severe corrosive environments
o If installed in severe corrosive environments, the rigid-steel conduit should be PVC-coated.
EMT does not offer the same degree of mechanical strength that is offered by rigid-steel conduit, and it should not be used where it is subjected to severe physical damage. EMT is only acceptable in nonhazardous, indoor locations, and it should not be used where corrosion can cause damage.
The applications of flexible liquid-tight conduit are limited to connections in which vibration, movement, or adjustments will occur. Flexible liquid-tight conduit is allowed for use in all areas except Class I, Division 1 hazardous locations.
Conduit Sizing and Routing
Two critical aspects of a conduit installation are conduit sizing and conduit routing. When the conduit size is chosen, the conduit inside diameter should be large enough to install all of the cables that were selected to be installed in that conduit without damage to any of the cables. The conduit should also be large enough to minimize any adverse heating effects on the conduit or on the cables that are contained within the conduit. When the conduit is installed, there are also routing and placement requirements that should be met. The routing requirements are important to minimize inter-conduit heating and conduit heating that results when a conduit is routed near process facility equipment that radiates heat.
Conduit Fill
Conduit fill is expressed as a percentage of the cross-sectional area of the conduit that the cables are allowed to occupy, and it depends on the number of conductors that are to be installed in the conduit. The allowable percentage of conduit fill is based on the combined heating effects of all of the cables that are installed in the conduit. Knowledge of the allowable percentage of conduit fill helps the Electrical Engineer to select the proper size of conduit for a particular installation.
The table in Figure 5 describes the various size requirements that are specified for rigid-steel conduit, EMT, and flexible liquid-tight conduit; the references for each requirement are also listed. The size requirements include the minimum size of the conduit, the maximum size of the conduit, and the allowable conduit fill.
Conduit Sizing Requirements
Rigid-Steel Conduit EMT Flexible Liquid Tight Conduit
Minimum Size ¾” except instrument panels, inside
buildings,
prefabriacted skids, or non-industrial areas
Refer to Figure 6 Allowable conduit fill
requirements are the same as those for rigid-steel conduit.
All conduit fill requirements are the same as those specified for rigid-steel conduit.
Figure 5: Conduit Sizing Requirements
To determine the allowable fill, the Electrical Engineer should first choose an applicable duct from the tables for selecting conduit size that are shown in Work Aid 2. Once the size of the conduit is selected, the total cross sectional area of all of the cables that will be contained in the conduit should be determined through use of the table of cable dimensions that is shown in Work Aid 2. Now that the cross-sectional area of the cables has been determined and the conduit has been chosen, the percentage fill of the conduit can be determined. Work Aid 2 provides the tables and the details on the procedure that is used to size conduit for Saudi Aramco, above-grade installations. The allowable percentage of conduit fill, based on the number of conductors that are to be installed in the
When conduit sealing fittings are used (type EYS or similar), the wire fill of the conduit sealing must not exceed 25% based on the conduit size (i.e., the ratio of the sum of the cross-sectional areas of wires and multi-conductor cables to the internal cross-sectional area of a conduit of the same trade size must not exceed 25%). If the percentage of fill of the conduit sealing exceeds 25%, oversized sealing fittings with reducers may be used in order to use the highest permissible conduit wire fill.
Percent of Cross Section of Conduit and Tubing for Conductors
Number of Conductors 1 2 Over 2
All conductor types 5
3
31 40
Note. A multi-conductor cable of two or more conductors shall be treated as a single conductor cable for calculating percentage conduit fill area. For cables that have elliptical cross section, the cross-sectional area calculation shall be based on using the major diameter of the ellipse as a circle diameter.
Figure 6: Allowable Percentage of Conduit Fill (from NEC, Chapter 9)
Jam Ratio
The natural weight of the cables that are contained in the conduit will cause them to settle to the lowest part of the conduit that the conduit space will allow. Depending on the size, configuration, and number of cables, the cables could get jammed in the conduit during installation. A useful unitless value that is used when cables are installed in conduit is called the "jam ratio." The jam ratio is used primarily during cable pulling tension calculations, and it will be explained in detail in that section of this Module; but it is also used in the conduit selection process, and, so, it will be described briefly here. The jam ratio is the ratio of the conduit's inside diameter to the diameter of the largest cable that will be installed in the conduit. The jam ratio provides a factor that describes the probability that the cable will jam during its installation in the conduit. The equation below is used to calculate the jam ratio:
D
=1.05 Ratio Jam
where:
"D" is the conduit inside diameter.
"d" is the diameter of the largest cable that is in the conduit.
"1.05" includes a correction factor of 5% that accounts for the oval cross-section of conduit bends.
Cable Clearance Within the Conduit
If the jam ratio is greater than 3.0, jamming is not likely to occur, and cable clearance can be ignored. If the jam ratio is between 2.5 and 2.8, jamming is probable; if the jam ratio is between 2.8 and 3.0, serious jamming is probable. If jamming is probable, the Electrical Engineer should evaluate the need to increase the size of the conduit.
Cable clearance is the distance between the uppermost cable in a conduit and the inside top of the conduit. A gap should be present between the uppermost cable in a conduit and the top of the conduit to prevent rubbing during pulls and to allow for expansion and contraction.
For a single cable installation, the cable clearance is calculated through use of the following equation:
Clearance = D - d where:
"D" is the conduit inside diameter.
"d" is the diameter of the largest cable that is in the conduit.
For a three cable installation (or three tripled conductors), the cable clearance is calculated using the following equation:
⋅
d - d 2 1
d) -+(D 1.366(d) 2
-= D Clearance
2
The cable clearance should be maintained within a band of 6 to 25 mm (1/4 to 1 inch). If the cable clearance is less than 6 mm (1/4 inch), the cable clearance is not satisfactory, and the Electrical Engineer should evaluate the need to increase the size of the conduit.
Magnetic Heating Effects
Metallic raceways are susceptible to magnetic heating effects, which include induced current heating and hysteresis heating. In order to avoid magnetic heating effects, all phase and neutral conductors of a three phase system must be in one conduit, or if there are parallel conductors, each conduit must have all phases and neutral.
Conduit Clearances
The conduit clearance is defined as the distance between the outside surface of a conduit and walls, other conduit, or other equipment. When routing conduit for above-grade installations, proper conduit clearances should be established. The conduit clearance is required to ensure that the conduit is not routed too close to process facility equipment that radiates heat.
Conduit runs should be symmetrical and should be routed vertically, horizontally, or parallel to structure lines. Conduit should not be installed near ladder rungs or at platform levels so that the conduit restricts passage or interferes with existing steps.
As a guideline, the minimum clearance for conduits that cross or run parallel to process lines should be 150 mm (6 inches) for uninsulated process lines and 100 mm (4 inches) for insulated process lines.
Fire Proofing
Fireproofing is required for critical power and control cables that are located above ground in a fire-hazardous zone, e.g., within 7.5 m (25 feet) horizontally of fire-hazardous equipment. Critical power and control cables are cables whose loss would render emergency shutdown, fire protection, or alarm systems inoperative. Fire-hazardous equipment is defined as equipment
Fireproofing must provide a minimum of 15 minutes of protection to the integrity of the circuit against temperatures of 1100°C (2000°F) in accordance with UL 1709.
Conduit Bending
Conduit bending requirements for a given installation are designed so that the conduit is not injured during the installation and so that the internal diameter of the bent conduit is not effectively reduced.
No more than four quarter conduit bends (360 degrees total) should be made in one run of conduit between pull points.
Minimum conduit bending radii requirements are based on the minimum cable training radii for the cable that is to be installed in the conduit and the physical size of the conduit.
Minimum Bending Radii
If a wire or cable is bent too much, the act of bending may cause damage to the wire or cable that results in subsequent cable failure. To prevent cable failure, a minimum bending radius (curvature of bend) limits cable and wire bending. With large power distribution cables, the construction of the cables (e.g., insulation and shielding) places additional restrictions on the minimum bending radius that a cable can withstand before damage to the cable will occur. As a cable passes, enters, or exits a conduit, the cable will usually have to be bent. The act of bending a wire or cable during the installation process is called
"training." The minimum bending radius of any cable should not be exceeded when the cable is trained in a conduit.
To prevent damage to the cables during the installation process, the minimum bending radii of the cables must also be considered during cable installation. The minimum bending radii of the inner surface of a given cable is determined through use of a calculation in which a specific multiplication factor is multiplied by the overall diameter of the cable.
Before the minimum bending radii of the conduit is specified, the minimum cable training radii of the cables should be determined.
To ensure that damage will not occur to the cables when they are trained, the minimum conduit bending radii should not be less than the minimum cable training radii. The procedure that is used to determine the cable minimum bending radii is provided in Work Aid 2.
The minimum conduit bending radii is selected from a table based on the size of the conduit that is used for the installation. A hand bender can be used to make field bends for conduit that is sized at 1-1/2 inches or less. A bending machine should be used for conduit that is larger than 1-1/2 inches. Bends that are accomplished with a hand bender are measured from the inner surface of the conduit; bends that are accomplished with a machine bender are measured from the center line of the conduit.
Work Aid 2 provides the procedure that is used to determine conduit bending requirements for Saudi Aramco above-grade installations.
Conduit Threading
Rigid-steel conduit is required to be threaded on both ends for Saudi Aramco conduit installations. Because EMT has a thin wall, individual sections of EMT are only permitted to be joined through use of threadless couplings.
Rigid-steel conduit is manufactured in standard lengths of 10 feet (3 m). During an installation, the conduit must be cut into proper lengths. The proper length to which the rigid-steel is cut is dependent on the location and conditions of the conduit installation. After the conduit has been cut to the proper length, it must be field-threaded and then chamfered (reamed) to remove the burrs and sharp edges that are formed during cutting. All field threads for rigid-steel conduit are required to be full and continuous. A minimum thread engagement of five full threads should be made at all fittings. Field threads should be cut with a standard conduit cutting die that has a 3/4 inch taper per foot. All conduit threads must be tapered; running threads are not permitted for any application. Raw threads should be protected from corrosion with CRC "Zinc-It" or an equivalent coating.
Rigid-metal conduit is available in sizes of 1/2 to 6 inches.
Conduit trade sizes are referred to as the approximate inside diameter of the raceway. All fittings and knockouts on boxes are identified by the trade size of the raceway for which the device is intended. Figure 7 describes the required dimensions of conduit threads for the various trade sizes of rigid-steel conduit. Critical measurements include the effective length of the threads (L2), the total length of the threads (L4), the pitch diameter at the end of the conduit (E0), and the required number of threads per inch of conduit.
L4 total length of threadsa
L2 effective length of threads
E0 pitch diameter at end of conduit Trade
size of conduit in inches
Number of threads per inch
inches mm inches mm inchesb mmb
3/8 18 0.60 15.2 0.41 10.4 0.612 15.5
½ 14 0.78 19.8 0.53 13.5 0.758 19.3
¾ 14 0.79 20.1 0.55 14.0 0.968 24.6
1 11 ½ 0.98 24.9 0.68 17.3 1.214 30.8
1 ¼ 11 ½ 1.01 25.7 0.71 18.0 1.557 39.5
1 ½ 11 ½ 1.03 26.2 0.72 18.3 1.796 45.6
2 11 ½ 1.06 26.9 0.76 19.3 2.269 57.6
2 ½ 8 1.57 39.9 1.14 29.0 2.720 69.1
3 8 1.63 41.4 1.20 30.5 3.341 84.9
3 ½ 8 1.68 42.7 1.25 31.8 3.838 97.5
4 8 1.73 43.9 1.30 33.0 4.334 110.1
5 8 1.84 46.7 1.41 35.8 5.391 136.9
6 8 1.95 49.5 1.51 38.4 6.446 163.7
a A minus tolerance of one thread applies to the total length of threads L4 b Plus and minus tolerances of one turn apply to the pitch diameter E0
Figure 7: Required Dimensions of Conduit Threads (from UL 6)
Indoor and Outdoor Conduit Terminations
Metal raceways should be mechanically joined to form a continuous electric conductor; raceways should also be connected to all boxes, fittings, and cabinets for effective electrical continuity.
Conduit terminations are used to complete the conduit system through connection of the metal raceway (i.e., rigid-steel conduit or EMT) to the boxes, fittings, and/or cabinets that are used in the conduit system.
Conduit systems can terminate at service entrance fittings, panels, pull boxes, or access fittings, and they can include the use of insulated bushings and conduit seals. For example, indoor conduit runs that terminate in the open should be equipped with an insulating bushing. Outdoor conduit that terminates in the open should be equipped with a service entrance fitting. Also, insulating grounding bushings should be installed on conduit that is inside of all boxes except where a threaded hub is provided as part of the conduit thread connection.
Fittings
A fitting is an accessory that is provided for a conduit system.
Fittings are used to perform mechanical connections to conduit and associated conduit support equipment. Items, such as lock nuts, bushings, conduit couplings, EMT connectors and couplings, and threadless connectors, are considered to be fittings. Conduit fittings should be made of cast or forged steel, cast iron, or malleable iron that is either hot-dip galvanized or zinc electroplated (as supplied by the manufacturer). Aluminum fittings are not allowed for use in Saudi Aramco conduit installations.
Only malleable iron sealing fittings are to be used for new installations, However, for repair purposes, gray, cast iron, split-type retro-fit sealing fittings are allowed.
For the connection of conduit, EMT, or other raceways (except cable trays), a box or fitting should be installed at each conductor splice connection point, outlet, switch, junction point, or pull point.
Conduit bodies are considered to be fittings and are allowed to contain splices if they have adequate volume.
All conduit fittings should be accessible from a platform, ladder, or stairwell. Cover openings should not be blocked by any structural steel or pipe that would prevent access to the interior of the fitting for maintenance.
For indoor and outdoor conduit terminations, there are certain requirements that should be met. Indoor conduit runs that terminate in the open should be equipped with an insulating bushing. Outdoor conduit that terminates in the open should be equipped with a service entrance fitting. Insulated grounding bushings should be installed on conduits inside of all boxes except where a threaded hub is provided as part of the conduit thread connection.
Seals (Explosion Proof)
Explosion proof seals in a conduit system should only be provided where required by the NEC. Non-required sealing is expensive and an operational problem since for any future circuit modifications the seal fitting must be cut out and, the conductors spliced or removed. In additon, each conduit entering a process unit control house should be sealed outside the point of entry for above grade runs and inside at the point of entry for below grade runs.
Seals, when required, should be located within 450 mm (18 in.) of an enclosure.
Vertical or horizontal conduit runs which require sealing should be sealed with combination vertical/horizontal seals, EYS or equal.
Explosion proof seals should be filled as follows:
(a) A dam of fiber (Chico “X” or equal) should be made around and between the wires to prevent the sealing compound from entering a conduit run.
(b) After fixing the dam, the sealing compound (Chico “A” or equal) equal to the diameter of the conduit (but not less than 5/8 inch) should be poured into the seal.
(c) All sealing compound should be mixed with clean fresh water.
(d) Do not pour sealing compound into draining chambers of