Question

An installation consists of a 10-kVA, single-phase transformer with a 440-volt primary and a 110-volt, 2-wire secondary using insulation type THWN copper conductors. No secondary OCPD is installed. The secondary loads are considered to be noncontinuous loads. Calculate the minimum size secondary conductors required to handle the secondary full load current for this transformer.

Answer 1

Answer:

Use a 150-amp breaker and No. 1/0 copper conductors

Explanation:

An installation consists of a 10-kVA, single-phase transformer with a 440-volt primary and a 110-volt, 2-wire secondary using insulation type THWN copper conductors. No secondary OCPD is installed. The secondary loads are considered to be noncontinuous loads. Calculate the minimum size secondary conductors required to handle the secondary full load current for this transformer

Calculate primary full-load amps:

10 kVA ÷ (440 volts ÷ 1000) = 22.72 amps

Calculate required feeder breaker size and conductor ampacity:

22.72 amps x 1.5 = 34.09 amps

Use an 80-amp breaker and No. 3 AWG copper conductors 1

Calculate secondary full-load amps:

10 kVA ÷ (110 volts ÷ 1000) = 90.9 amps

Calculate required secondary breaker size and conductor ampacity:

90.9 amps x 1.25 = 113.6 amps

Use a 150-amp breaker and No. 1/0 THWN copper conductors.