Transformer Sizing Calculator

NEC 450.3(B)

Size transformers and calculate overcurrent protection per NEC 450.3(B). Includes primary and secondary breaker sizing.

Load

Input Mode

WattskVA

Load

Power Factor

kVA = Watts / (Power Factor x 1000). Typical power factor ranges from 0.8 to 1.0.

Transformer

Primary Voltage

Secondary Voltage

Phase

Single-PhaseThree-Phase

Results

Calculation Breakdown (NEC 450.3(B))

kVA = W / (PF x 1000) | I = kVA x 1000 / V

Load:10,000W

Required kVA:11.11 kVA

Selected Transformer:15 kVA

Primary Current (480V):31.25A

Secondary Current (240V):62.50A

Primary OCP (125% rule):40A breaker

Secondary OCP (125% rule):80A breaker

15 kVA → 40A primary / 80A secondary

Required kVA

11.11kVA

Selected Transformer

15kVA

Primary Current

31.3A

Secondary Current

62.5A

Primary OCP

40A

Secondary OCP

80A

Load

Input Mode

WattskVA

Load

Power Factor

kVA = Watts / (Power Factor x 1000). Typical power factor ranges from 0.8 to 1.0.

Transformer

Primary Voltage

Secondary Voltage

Phase

Single-PhaseThree-Phase

Results

Calculation Breakdown (NEC 450.3(B))

kVA = W / (PF x 1000) | I = kVA x 1000 / V

Load:10,000W

Required kVA:11.11 kVA

Selected Transformer:15 kVA

Primary Current (480V):31.25A

Secondary Current (240V):62.50A

Primary OCP (125% rule):40A breaker

Secondary OCP (125% rule):80A breaker

15 kVA → 40A primary / 80A secondary

Required kVA

11.11kVA

Selected Transformer

15kVA

Primary Current

31.3A

Secondary Current

62.5A

Primary OCP

40A

Secondary OCP

80A