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Enter either secondary voltage OR turns ratio
Primary Core Secondary
Np turns ┌──────────┐ Ns turns
│ ││ │
─┬─ ◎◎◎◎◎◎──│────││────│──◎◎◎◎◎◎ ─┬─
│ │ ││ │ │
Vp │ ││ │ Vs
│ │ ││ │ │
─┴─ ◎◎◎◎◎◎──│────││────│──◎◎◎◎◎◎ ─┴─
│ ││ │
└──────────┘
Vp/Vs = Np/Ns = Is/Ip| Rating | Primary | Secondary | Application |
|---|---|---|---|
| 25 kVA | 7200V | 240/120V | Residential pole mount |
| 50 kVA | 7200V | 240/120V | Commercial |
| 75 kVA | 13800V | 480/277V | Commercial/Industrial |
| 112.5 kVA | 480V | 208/120V | Commercial building |
| 150 kVA | 480V | 208/120V | Medium industrial |
| 500 kVA | 13800V | 480V | Large industrial |
| 1000 kVA | 34500V | 480V | Industrial substation |
A transformer is a static electrical device that transfers alternating current (AC) energy between two or more circuits through electromagnetic induction. It consists of a primary winding, a secondary winding, and a magnetic core — typically made from laminated silicon steel. Transformers are fundamental to modern power distribution, allowing electricity generated at power plants to be stepped up to high voltages (115 kV–765 kV) for efficient long-distance transmission, then stepped down to usable levels (120V/240V) at the point of consumption. According to IEEE Standard C57, power transformers are classified by their kVA rating, voltage class, and cooling method. They operate on Faraday's law of electromagnetic induction: a changing magnetic flux in the core induces a voltage in the secondary winding proportional to the turns ratio. Ideal transformers conserve power (Vp × Ip = Vs × Is), while real-world units achieve 95–99% efficiency depending on design and loading conditions.
Divide the primary voltage by the secondary voltage: Turns Ratio = Vp ÷ Vs. For example, 7200V primary and 240V secondary gives a 30:1 ratio. This ratio also equals Np ÷ Ns (primary turns divided by secondary turns).
Use the power conservation principle: Is = (Vp × Ip) ÷ Vs. Alternatively, Is = kVA × 1000 ÷ Vs for single-phase, or Is = kVA × 1000 ÷ (Vs × √3) for three-phase transformers.
Calculate required kVA from your total load: kVA = (Total Watts) ÷ (Power Factor × 1000). Add a 20–25% safety margin per NEC recommendations. For three-phase loads, kVA = (V × I × √3) ÷ 1000.
Real transformers have copper losses (I²R), core losses (hysteresis and eddy currents), and stray losses. Multiply ideal output power by the efficiency factor (typically 0.95–0.99) to get actual output. Larger transformers generally have higher efficiency.
An undersized transformer overheats, degrades insulation, and can cause fires. NEC Article 450 requires proper sizing and overcurrent protection based on the transformer's impedance and kVA rating.
Selecting the right transformer size minimizes losses. A 1000 kVA transformer running at 50% load is less efficient than a 500 kVA unit at full load. DOE efficiency standards (10 CFR 431) mandate minimum performance levels.
Correct turns ratio and impedance calculations ensure proper voltage regulation, fault current levels, and protection coordination. These values directly affect breaker sizing and cable selection throughout the installation.
| kVA Rating | 1φ @ 120V | 1φ @ 240V | 3φ @ 208V | 3φ @ 480V |
|---|---|---|---|---|
| 15 kVA | 125 A | 62.5 A | 41.6 A | 18.0 A |
| 25 kVA | 208 A | 104 A | 69.4 A | 30.1 A |
| 50 kVA | 417 A | 208 A | 138.8 A | 60.1 A |
| 75 kVA | 625 A | 313 A | 208.2 A | 90.2 A |
| 112.5 kVA | 938 A | 469 A | 312.3 A | 135.3 A |
| 225 kVA | 1875 A | 938 A | 624.6 A | 270.6 A |
| 500 kVA | 4167 A | 2083 A | 1388 A | 601 A |
| 1000 kVA | 8333 A | 4167 A | 2776 A | 1203 A |
* Single-phase: I = kVA × 1000 ÷ V. Three-phase: I = kVA × 1000 ÷ (V × √3). Values rounded to nearest whole ampere.
A transformer works by electromagnetic induction. An alternating current flowing through the primary winding creates a changing magnetic field in the iron core. This changing flux links with the secondary winding and induces a voltage proportional to the turns ratio (Vs = Vp × Ns ÷ Np). The device only operates on AC because a steady DC current would produce a constant magnetic field with no change in flux, resulting in zero induced voltage.
Calculate your total connected load in watts, divide by the power factor to get VA, then convert to kVA by dividing by 1000. Apply a demand factor if not all loads operate simultaneously, and add a 20–25% growth margin. For example, a building with 80 kW of load at 0.85 power factor needs at least 94.1 kVA, so you would select a standard 112.5 kVA transformer to allow for future expansion.
Single-phase transformers have one primary and one secondary winding and are used for residential and light commercial loads (up to about 167 kVA). Three-phase transformers use three sets of windings connected in delta or wye configurations and are standard for commercial and industrial applications above 15 kVA. Three-phase power delivers more watts per kilogram of conductor and provides smoother power delivery for motors.
Transformer losses (copper and core losses) depend on voltage and current, not on the power factor of the connected load. Since the manufacturer cannot predict what loads will be connected, the transformer is rated in kVA (apparent power), which represents the maximum voltage × current product it can handle regardless of power factor.
Impedance percentage (typically 2–10%) indicates the fraction of rated voltage needed to circulate full-load current through the short-circuited secondary. A 5% impedance transformer means 5% of rated primary voltage drives full-load current. Lower impedance gives better voltage regulation but allows higher fault currents, which affects breaker and bus bar sizing per NEC Article 450.
Convert volt-amps to kilovolt-amps for transformer sizing and apparent power calculations.
Calculate current from voltage using Ohm's Law for transformer secondary circuit design.
Convert real power to apparent power to properly size transformers for your loads.