A common misapplication that is encountered is the improper sizing of the six motor leads between the still very popular wye-delta reduced voltage motor starter and the motor. This is best demonstrated by an example.

Assume that you have a 500-ton electrical centrifugal chiller operating at 460 V, three-phase, 60 Hz, with a nameplate rating of 588 full load amps (FLA).You would normally apply the correct factor of 125 percent required by NEC Article 440, to arrive at the required conductor ampacity: 588 × 1.25 = 735 ampacity for each of the three conductors.

Because there will be six conductors between the load side of the starter and the compressor motor terminals, the 735 ampacity is divided by two; you would select six conductors, each having an ampacity of not less than 368 A. Referring to NEC Article 310, Table 310-16 for insulated copper conductors at 75°C would result in the selection of 500-kcmil conductors.

This wire size is incorrect when used between the wye-delta starter and motor terminals. The problem is caused by a common failure to recognize that the motor may consist of a series of single-phase windings.

To permit the transition from wye-start to delta-run configuration, the motor is wound without internal connections. Each end of the three internal motor windings is brought out to a terminal, as shown in Figure 8.5.

FIGURE 8.5 Wye-to-delta internal motor windings brought out to terminals.

The motor windings are configured as required for either starting or running at the starter as shown in Figure 8.6, panels a and b, respectively. In the running-delta configuration, the field wiring from the load side of the starter to the compressor motor terminals consists of six conductors, electrically balancing the phases to each of the internal motor windings as described below in Figure 8.7.

FIGURE 8.6 Wye-start, delta-run motor winding configuration.

FIGURE 8.7 Field wiring between starter and motor in wye-start, delta-run configuration.

Note, for example, that motor winding T1 − T4 is connected to the line voltage across phase L1 − L2. It should be apparent that the windings within the motor are single phase- connected to the load side of the starter. Thus, the interconnecting field wiring between the starter and motor must be sized as though the motor were single-phase.

Electrical terminology simply describes this motor as being phase-connected, and the current carried by the interconnecting conductors as phase amps.

To correctly size the conductors between the motor starter and the motor, therefore, it is necessary to calculate the ampacity with the 125 percent feeder-sizing factor required by the NEC on a single-phase basis as follows:

Ampacity per terminal conductor = three-phase FLA × 1.25/1.73

For the example given:
Ampacity per terminal conductor = 588 × 1.25/1.73 = 424

Thus, it is clear that the current in the conductors between the starter and the motor on a single-phase basis is 58 percent of the three-phase value, not 50 percent as originally assumed, because the current in one phase of a three-phase system in the delta-connected winding is one divided by the square root of three due to the vector relationship.

In the original example, the conductors were sized for a minimum ampacity of 368 A.From the NEC, 500-kcmil copper conductors at 75°C have a maximum allowable ampacity of 380.The preceding calculation discloses that the conductors should be selected for not less than 424 ampacity.

Referring to the NEC again, 600-kcmil conductors have a maximum allowable ampacity of 420. In many cases, depending upon the interpretation of the local electrical inspector, 600 kcmil would be acceptable (usually within 3 percent is acceptable).

Five-hundred– kilocircular mil wire would not be. Almost needless to say, the conductors supplying the line side of the wye-delta starter are sized as conventional three-phase motor conductors.

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