Feeder Cable Connections

While the electric generator is being installed and serviced, a part of your crew can connect it to the load. Essentially, this consists of running wire or cable from the generator to the load. At the load end, the cable is connected to the substation or to the distribution center. At the generator end, the cable is connected either to the output terminals of a main circuit breaker or a load terminal board. Before the wires are run and connections are made, it will be up to you to do the following:

1. Determine the correct size of wire or cable to use.

2. Decide whether the wire or cable will be buried or carried overhead on poles.

3. Check the generator lead connections of the plant to see that they are arranged for the proper voltage output.

The information contained in the following paragraphs will help you in these tasks.

CABLE SELECTION.\If the wrong size conductor is used in the load cable, various troubles may occur. If the conductor is too small to carry the current demanded by the load, it will heat up and possibly cause a fire or a break in the circuit. Even though the conductor is large enough to carry the load current safely, its length might result in a lumped resistance that produces an excessive voltage drop. An excessive voltage drop results in a reduced voltage at the load end. This reduced voltage is incapable of operating the equipment safely. In this respect, it might be well to point out the voltage drop should not exceed 3 percent for power loads and 3 percent for lighting loads or 3 percent for combined power and lighting loads.

Select a feeder conductor capable of carrying 150 percent of rated generator amperes to eliminate overloading and voltage drop problems. Refer to tables 5-3 and 5-4 to select the proper cable to carry the computed capacity according to insulation class.

CABLE INSTALLATION.\The load cable may be installed overhead or underground. In an emergency installation, time is the important factor. It may be necessary to use trees, pilings, four-by-fours or other temporary line supports to complete the installation. Such measures are temporary; eventually, you will have to erect poles and string the wire on crossarms or bury it underground. If the installation is near an airfield, it may be necessary to place the wires underground at the beginning. Wire placed underground should be direct-burial, rubber-jacketed cable; otherwise, it will not last long.

Direct burying of cable for permanent installation calls for a few simple precautions to ensure uninterrupted service. They are as follows:

1. Dig the trench deep enough so that the cable can be buried at least 18 inches (24 inches in traffic areas and under roadways) below the surface of the ground to prevent disturbance of the cable by frost or subsequent surface digging.

Table 5-3.\Ampacities of Insulated Conductors

Table 5-4.\Ampacities of Insulated Conductors Free Air Page 5-8

Notes to Tables 5-3 and 5-4

1. Explanation of Tables. For explanation of Type Letters, and for recognized size of conductors for the various conductor insulations, see Section 310-13. for installation requirements, see Sections 310-1 through 310-10, and the various articles of this Code. For flexible cords, see Tables 400-4, 400-5(A), and 400-5(B).

3. 120/240 Volts, 3-Wire, Single-Phase Dwelling Services. In dwelling units, conductors, as listed below, shall be permitted to be utilized as 120/240-volt, 3-wire, single-phase serviceentrance conductors and feeder conductors in raceway or cable with or without an equipment grounding conductor. The grounded conductor shall be permitted to be not more than two AWG sizes smaller than the ungrounded conductors for application of this note, provided the requirements of Sections 215-2, 220-22, and 230-42 are met.

5. Bare Conductors. Where bare conductors are used with insulated conductors, their allowable ampacities shall be limited to that permitted for the adjacent insulated conductors.

6. Mineral-Insulated, Metal-Sheathed Cable. The temperature limitation on which the ampacities of mineral-insulated, metal-sheathed cable are based is determined by the insulating materials used in the end seal. Termination fittings incorporating unimpregnated, organic, insulating materials are limited to 90C (194F) operation.

7. Type MTW Machine Tool Wire.

(FPN): For the ampacities of Type MTW wire, see Table 13-5(a) in the Electrical Standard for Industrial Machinery, NFPA 79-1987.

8. Ampacity Adjustment Factors.

(a) More than Three Conductors in a Raceway or Cable. Where the number of conductors in a raceway or cable exceeds three, the ampacities shall be reduced as shown in the following table:

*These factors include the effects of a load diversity of 50 percent. **No diversity.

(FPN): Column A is based on the following formula:

A1 = Table ampacity multiplied by factor from Note 8(a)

N = Total number of conductors used to obtain factor from Note 8(a)

E = Desired number of energized conductors

A2 = Ampacity limit for energized conductors

Where single conductors or multiconductor cables are stacked or bundled longer than 24 inches (610 mm) without maintaining spacing and are not installed in raceways, the ampacity of each conductor shall be reduced as shown in the above table.

Exception No. 1: When conductors of different systems, as provided in Section 300-3, are installed in a common raceway or cable the derating factors shown above shall apply to the number of power and lighting (Articles 210, 215, 220, and 230) conductors only.

Exception No. 2: For conductors installed in cable trays, the provisions of Section 318-11 shall apply. Exception No. 3: Derating factors ahall not apply to conductors in nipples having a length not exceeding 24 inches (610 mm).

Exception No. 4: Derating factors shall not apply to underground conductors entering or leaving an outdoor trench if those conductors have physical protection in the form of rigid metal conduit, intermediate metal conduit or rigid nonmetallic conduit having a length not exceeding 10 feet (3.05 m) above grade and the number of conductors does not exceed 4.

(b) More than One Conduit, Tube, or Raceway. Spacing between conduits, tubing, or raceways shall be maintained.

9. Overcurrent Protection. Where the standard ratings and settings of overcurrent devices do not correspond with the ratings and settings allowed for conductors, the next higher standard rating and setting shall be permitted.

Exception: As limited in Section 240-3.

10. Neutral Conductor.

(a) A neutral conductor which carries only the unbalanced current from other conductors, as in the case of normally balanced circuits of three or more conductors, shall not be counted when applying the provisions of Note 8.

(b) In a 3-wire circuit consisting of 2-phase wires and the neutral of a 4-wire, 3-phase wye-connected system, a common conductor carries approximately the same current as the other conductors and shall be counted when applying the provisions of Note 8.

(c) On a 4-wire, 3-phase wye circuit where the major portion of the load consists of electric-discharge lighting, data processing, or similar equipment, there are harmonic currents present in the neutral conductor and the neutral shall be considered to be a current-carrying conductor.

11. Grounding or Bonding Conductor. A grounding or bonding conductor shall not be counted when applying the provisions of Note 8.

Unless otherwise specifically permitted elsewhere in this Code, the overcurrent protection for conductor types marked with an obelisk () shall not exceed 15 amperes for 14 AWG. 20 amperes for 12 AWG, and 30 amperes for 10 AWG copper; or 15 amperes for 12 AWG and 25 amperes for 10 AWG aluminum and copper-clad aluminum after any correction factors for ambient temperature and number of conductors have been applied.

Reprinted with permission from NPFA 70-1990, the National Electrical Code^R, Copyright^c1989, National Fire Protection Association, Quincy, MA 02269. This reprinted material is not the complete and official position of the National Fire Protection Association, on the referenced subject which is represented only by the standard in its entirety.

2. Lay the cable over a sand cushion (fig. 5-4). If this is impractical, loosen the trench base so it is cleared of rocks and stones.

3. Space the cables on 6-inch centers for further mechanical and electrical protection.

4. After laying the cable and before backfilling, cover it with earth free from stones, rocks, and so forth. This will prevent the cable from being damaged in the event the surrounding earth is disturbed by flooding or frost heaving.