Referring to NEC Table 310.16 (2002 Edition) , a #12 AWG copper conductor

would be selected for the primary. A #14 AWG copper conductor would not be

selected even though it appears to have adequate current -carrying capacity

because the footnote to NEC Table 310.16 requires that overcurrent protection

be limited to 15 amperes for a #14 AWG conductor.

The NEC has an additional requirement relating to the transformer primary

conductor. NEC Article 2 15.2(A)(1) (2002 Edition) requires that feeder

conductors be sized for the noncontinuous load plus 125 percent of the

continuous load. In this case, the primary conductor would be sized for 125

percent of 18 amperes, or 22.5 amperes. Referring again to NEC Table 310.16,

a #12 AWG copper conductor is still acceptable for use because it has an

ampacity of 25 amperes. Note that the footnote to NEC Table 310.16 requires

that overcurrent protection be limited to 20 amperes for a #12 AWG conductor;

however, this load limit still exceeds the 18 ampere actual load requirement and

is therefore acceptable.

Secondary Ampacity

The required secondary amperage is:

15 *kVA * 1000

= 41.6 *amperes*

3 208 *V*

NEC Article 215.2(A)(1) requires that feeders be sized for the noncontinuous

load plus 125 percent of the continuous load. In this case, the secondary

conductor would be sized for 125 p ercent of 41.6 amperes, or 52 amperes.

Referring to NEC Table 310.16, a #6 AWG copper conductor would be selected.

B-6

B-6.1

Significant energy savings can be realized by installing conductors one size

larger than required by the NEC. The following examples illustrate the evaluation

process as well as the potential savings that can be realized.

EXAMPLE: A three-phase circuit feeds a 125 horsepower (93,250 watts), 460

volt motor, operating at 75 percent load, 76.2 meters (250 feet) from the load

center. Assume that the motor operates only 50 percent of the time (4,380 hours

per year). The motor full load current is 156 amperes and 75 percent of this load

is 117 amperes.

A #3/0 AWG conductor satisfies the e lectrical requirements. As shown below, a

larger #4/0 AWG conductor pays for itself within 5 years. Thereafter, the

installation continues to save energy costs of almost per year compared to

the smaller #3/0 AWG conductor.

B-7

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