Overview
This lecture explains how to calculate the required kVA or amperage capacity for single and three phase transformers using voltage and current values, with examples and rounding guidance.
kVA Calculation Basics
- To size a transformer, use the winding voltage (V) and amperage (I).
- For single phase: kVA = (V × I) / 1000
- For three phase: kVA = (V × I × 1.732) / 1000, where 1.732 ≈ √3.
- Always round up to the next standard transformer kVA rating.
Example Calculations
- Single Phase Example: V = 240V, I = 175A → kVA = (240 × 175) / 1000 = 42 kVA; round up to standard size 50 kVA.
- Three Phase Example: V = 208V, I = 175A → kVA = (208 × 175 × 1.732) / 1000 ≈ 63.05 kVA; round up to standard size 75 kVA.
Amperage from kVA
- For single phase: Amperage = VA / Voltage (e.g., 50,000 VA / 240V = 208A).
- For three phase: Amperage = VA / 1.732 / Voltage (e.g., 75,000 VA / 1.732 / 208V ≈ 208.2A).
Using Charts for Sizing
- Standard kVA sizes and voltages can be located on transformer sizing charts.
- Find the voltage column, then select the first amperage entry that covers the required load, resulting in the corresponding kVA value.
Important Notes
- Calculations do not account for special load requirements, such as motors or sensitive equipment.
- Always refer to manufacturer charts for confirmation of standard sizes.
Key Terms & Definitions
- kVA (kilovolt-ampere) — Unit of apparent power for transformers.
- VA (volt-ampere) — Basic unit of apparent power; 1 kVA = 1,000 VA.
- Single Phase Transformer — Transformer with one alternating current phase.
- Three Phase Transformer — Transformer with three alternating current phases.
- Amperage (A) — The current flowing through the transformer.
- Voltage (V) — The electrical potential difference.
Action Items / Next Steps
- Practice sizing transformers using provided formulas and rounding rules.
- Refer to manufacturer’s transformer catalogs and documents for standard sizing tables.
- Review pages 3 & 4 of document 7400HO9501 for additional conversion tables.