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Transformer Sizing Calculator

Calculates required transformer kVA capacity based on load current, voltage ratio, power factor, and duty cycle.

Results

Calculated kVA
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Recommended Size
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Primary Current
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Primary Breaker
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Details

Verify transformer specs with manufacturer data. Efficiency varies by design (96-99% typical). Consult NEC Article 450 for overcurrent protection requirements.

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How to Use This Calculator

1. Select single-phase or 3-phase. Most commercial and industrial transformers are 3-phase. Residential step-down transformers are typically single-phase.

2. Enter voltages and load current. The load current is the expected amps on the secondary side of the transformer.

3. Set the power factor and duty. Motors and compressors are typically 0.80-0.90 PF. Resistive loads are 1.0. Continuous duty uses full load; intermittent can reduce the sizing.

Transformer Sizing Formula

Single-Phase: Output kVA = (V x I) / 1000

3-Phase: Output kVA = (V x I x 1.732) / 1000

Design kVA = Output kVA x Duty Factor x 1.25 (safety margin)

Primary Current = kVA x 1000 / (V_primary x 1.732) [3-phase]

Primary Breaker = Primary Current x 1.25 (NEC 450)

Frequently Asked Questions

How do I calculate transformer kVA size?
For single-phase: kVA = (Voltage x Current) / 1000. For 3-phase: kVA = (Voltage x Current x 1.732) / 1000. Then multiply by a 1.25 safety margin and round up to the next standard size.
What is the difference between kVA and kW?
kVA is apparent power. kW is real power. They are related by the power factor: kW = kVA x PF. For resistive loads, PF is near 1.0. For motors, PF is typically 0.80-0.90.
What size breaker do I need for a transformer?
Per NEC Article 450, primary overcurrent protection is typically 125% of the primary full-load current. Calculate, multiply by 1.25, and round to the next standard breaker size.

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