Methods for Installing Power and Control Cables as per NEC

 

1.         PURPOSE

 

1.1       This engineering standard defines the methods for installing power and control cables in accordance with the National Electrical Code, and defines and supplements those areas of the code in which options are available, or Air Products has chosen to exceed the minimum requirements of the code.

 

 

2.         scope

 

2.1       This standard applies to low-voltage power and control cables and medium voltage power cables. While directed towards Air Products-owned and -operated facilities, it shall be considered the minimum requirements for any facility design. For sale of equipment, the electrical designer shall verify these requirements with the customer’s representatives before proceeding with facility design.

 

 

3.         related documents

 

3.1       Air Products Engineering Documents

 

4EL64127A            Terminal Boxes

4EL64151A            Medium Voltage Power Cable (Over 600 Volts)

4EL64176A            Cable Trays

4EL64201A            Underground Conduit

4EL64203A            Direct Buried Cable Design

4AEL-620300         Electrical Work

4AEL-620303         Installation and Testing of Medium Voltage Cable

 

3.2       National Fire Protection Association (NFPA)

 

70                          National Electrical Code (NEC)

 

 

  1. CABLE PROTECTION

 

4.1       Outdoors

 

4.1.1   Cable installed in conduits and cable trays or cables installed underground in conduit must be suitable for wet-location conditions (for example, resisting moisture, acids, and alkalis).

 

4.1.2   When aboveground wiring is installed in conduit, the conduit shall be rigid galvanized steel (RMC) or rigid aluminum (RMC). Contract drawings shall identify the type of conduit to be utilized. Below-grade wiring shall be installed in rigid (schedule 40) polyvinyl chloride conduit (RNC). Refer to 4EL64201A for concrete-encasement requirements of underground PVC conduit. While these are the Air Products preferred wiring methods, other raceway-system materials might be utilized for special applications.

 

4.1.2.1   Rigid aluminum conduit (RMC) shall be used in cooling tower and water-treatment areas, in which the wiring system is continually exposed to moisture and possibly corrosive chemicals. All hardware for installing and mounting rigid aluminum conduit shall be aluminum or stainless steel.

 

4.1.2.2   In extremely corrosive atmospheres, the use of PVC-coated, rigid, galvanized-steel conduit might be required. However, because of its high cost and special requirements for installation, the use of PVC-coated, rigid, galvanized-steel conduit shall be reviewed with Project Engineering and determined to be the only acceptable wiring method for that particular application.

 

4.1.3      Liquid-tight, flexible, metal conduit shall be used to provide flexibility and vibration reduction when connecting to utilization equipment. Depending upon application, lengths shall be restricted to short runs up to 900 mm (36 in) maximum.

 

4.1.4      Because of the reduced cost over multiple-conduit systems and flexibility of a cable-tray system for future wiring installations, cable tray shall be used whenever possible for major wiring installations. Refer to 4EL64176A for additional cable tray details.

 

4.2          Indoors

 

4.2.1      Within buildings, cable shall be installed in NEC-approved raceways or cable trays. Electric metallic tubing (EMT) may be used in non-process areas if the complete conduit run remains within the confines of the building. The use of EMT to RMC conduit adapters is prohibited.

 

4.2.2      Whenever possible, conduit installed in areas having finished walls and ceilings shall be concealed by the finishing material.

 

4.2.3      In areas where the conduit system is exposed to high operating temperatures (for example, the uppermost level, or penthouses of a reformer structure), high temperature wiring shall be used. Wire shall be rated for the temperature requirement but rated at a minimum of 150°C (302°F).

 

 

  1. TERMINATIONS

 

5.1          Whenever possible, terminal lugs shall be used on all wire terminations. Refer to 4AEL-620300 for specific requirements for wire terminations.

 

5.2          Refer to 4EL64151A for additional information regarding terminations of medium voltage power cable (above 600 volts).

 

 

  1. SPLICES

 

6.1          Splices in wire shall be held to a minimum. Splices are not permitted in low voltage (600 volts and below) wires or cables, or in instrumentation and thermocouple wires. Splices are not permitted in any wires in either motor control center or switchgear pits, pull boxes, or manholes that are physically located beneath final grade of the area. Splices are not permitted in condulet fittings unless as shown on installation standards.

 

6.2          Conversion of single-conductor, control-system wiring to multi-conductor cable and individual instrumentation pairs and triads to multi-pair and multi-triad cables shall be performed in terminal boxes specifically designed for that function. Refer to 4EL64127A for terminal box design information.

 

6.3          Splices in lighting or 120-volt convenience-receptacle circuit wiring shall be made per methods described in 4AEL-620300.

 

6.4          Splices in medium voltage power cable (above 600 volts):  Under normal circumstances Air Products does not permit splicing of medium voltage power cable; however, in the event that splicing of medium voltage power cable would be required, splicing must be performed by the cable manufacturer’s authorized splicing technician only. All requests for splicing medium voltage power cable must be reviewed and approved by the Air Products GSS (Global Support Services) organization Electrical Engineering Department before performing any work. See specification 4AEL-620303.

 

6.5          Refer to 4EL64203A for information regarding splices in direct-buried cables.

 

 

  1. CABLE PULLING IN CONDUIT

 

7.1          Cable-pulling compound shall be used to facilitate pulling of all cables or wires. 4AEL-620300 identifies cable-pulling compounds acceptable to Air Products.

 

7.2          Cable-pulling tension calculations shall be performed whenever there are more than two 90‑degree bends, or equivalent, or long pulls in an underground conduit run without the benefit of an intermediate pull point to ensure that the cables can be installed without exceeding the maximum allowable pulling tension. Pulling-tension calculations shall be performed from both ends of the conduit run to determine which direction of installation would provide the least pulling tension. This information shall be given to the contractor responsible for installing the cable.

 

7.2.1      To accurately perform the calculations, the maximum allowable tension for the particular cable that is to be installed must be calculated. This value is dependent upon the method of attaching the pulling grip to the cable, the allowable sidewall bearing pressure, and the basic makeup of the cable. Second, knowing the weight of the cable and the details of the conduit run, the estimated pulling tension that can occur during installation is calculated and compared with the maximum allowable tension.

 

7.2.1.1   For medium-voltage cable, refer to the cable manufacturer’s technical data, the maximum allowable pulling tensions, and installation recommendations.

 

7.2.1.2   Pulling cable by the conductor is the preferred method because it generally provides the highest allowable pulling tension. For medium-voltage cable where the manufacturer is unknown, the maximum-allowable pulling tension for pulling cable by the conductor is calculated as follows:

 

Tm = .008 x n x CM

 

Where Tm  =          maximum allowable tension in pounds

.008     =          formula constant

N          =          number of conductors being pulled (assumes equal tension in each conductor)

CM        =          circular mil area of each conductor

 

Using the above formula, the maximum-allowable pulling tension Tm for pulling (3) 1/c 500 kcmil copper conductors with pulling grip attached directly to conductor would be 12,000 pounds [.008 x 3 x 500,000 = 12,000].

 

7.2.1.3   The maximum-allowable sidewall bearing pressure around a conduit bend for (3) 1/c paralleled medium-voltage cables is calculated as follows:  Refer to the manufacturer’s technical data for detailed values. For medium-voltage cable where the manufacturer is unknown, the following formula may be used for preliminary conduit design.

 

Tm = 675 x D x R

 

Where Tm = maximum allowable tension for all three cables in pounds

675     = formula constant

D     = outside diameter of one individual cable in inches

R      = radius of bend in feet

 

Using the above formula, the maximum-allowable sidewall pulling tension Tm for pulling (3) 1/c 500 kcmil 15 kv paralleled copper conductors around a 4-foot radius elbow would be 4320 pounds [675 x 1.6 x 4 = 4320].

 

7.2.1.4   For cable installation conditions other than listed in paragraphs 7.2.1.2 and 7.2.1.3, refer to manufacturer’s Installation Data portion of manufacturer’s catalog or website.

 

7.3          To calculate pulling tensions, use a cable-pulling program. If a cable-pulling program is not available, pulling tensions can be calculated manually by using calculations found in the technical portion of many cable manufacturers’ catalogs (for example, The Kerite Co. Power Cable, Cable Data Catalog). Many medium voltage cable suppliers will also offer engineering assistance.

 

7.4          The radius of a conduit bend, not the internal diameter, directly affects the sidewall pulling tension; therefore, electrical designers must give consideration to conduit bends when developing underground conduit routings. The use of long-radius elbows will reduce the pulling tension on cables around an elbow.

 

7.4.1      In all installations of underground, medium-voltage cable, the electrical designer shall identify, on the contract drawings, that conduit elbows shall be as a minimum, the 900 mm (3 ft) long-radius type.

 

7.4.2      The electrical designer shall also review conduit bends for underground, low-voltage cable applications. As a minimum, the radius of conduit elbows shall not be smaller than the minimum bending radius of the cable being installed as permitted by the NEC.

 

7.5          For installations in which cable is not in duct or conduit, certain mechanical and physical restrictions must be considered. Formulas for calculating the maximum weights under various installation conditions can be applied to provide adequate cable installations. These formulas can be found in the technical portion of many cable manufacturers’ catalogs.

 

 

  1. CABLE PULLING IN CABLE TRAY

 

8.1          The electrical contractor is responsible for the setup and rigging of the cable pulls in cable tray per 4AEL-620300. The electrical designer should account for the ability to rig and pull cable when designing the cable tray system. If necessary the electrical designer may need to consult the cable manufacturer for assistance in determining if the cable can handle the tension and sidewall pressures required to install the cable based on the initial design. The electrical designer may also need to consult the civil and structural designers to provide additional structural supports necessary to allow for proper rigging of pulling equipment. If necessary modifications to the cable tray and/or structural support design may be required to obtain additional pull points.

 

 

  1. SEPARATION OF SPECIAL POWER SYSTEM CABLES

 

9.1          All wiring carrying power derived from regulating transformers and uninterruptible power supplies (UPS) shall be separated from normal utility power wiring to reduce the potential for induced voltages from the unregulated utility power wiring.

 

9.1.1      Wires carrying regulated power and UPS power may be installed in the same conduit.

 

9.1.2      Regulated power and UPS power conductors may be installed in the same cable tray system with normal utility power wiring, operating at 600 volts or less, if a metallic barrier is installed in the cable tray system to provide a minimum of 50 mm (2 in) of separation between the wiring systems. Regulated power and UPS power cables may occupy the same separated portion of the cable tray.

 

9.1.3      Where it is not cost affective to install separate conduit or tray systems, such as long field runs, shielded tray cable may be used for regulated power and UPS power conductors. This type of design shall be limited in use where possible.

 

 

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