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padding: 25px;
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font-family: -apple-system, BlinkMacSystemFont, "Segoe UI", Roboto, Helvetica, Arial, sans-serif;
max-width: 650px;
margin: 20px auto;
color: #333;
}
.transformer-calc-container h2 {
color: #2c3e50;
text-align: center;
margin-top: 0;
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.calc-field {
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}
.calc-field label {
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font-weight: bold;
margin-bottom: 5px;
color: #444;
}
.calc-field input, .calc-field select {
width: 100%;
padding: 10px;
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border-radius: 5px;
box-sizing: border-box;
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}
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border-radius: 5px;
width: 100%;
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cursor: pointer;
transition: background-color 0.3s;
}
.calc-btn:hover {
background-color: #34495e;
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.results-box {
margin-top: 20px;
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border-radius: 5px;
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.result-row:last-child {
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.result-label {
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.result-value {
color: #c0392b;
font-weight: bold;
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max-width: 800px;
margin: 40px auto;
line-height: 1.6;
color: #333;
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.transformer-article h2, .transformer-article h3 {
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.example-box {
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border-left: 5px solid #2980b9;
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}
Understanding Transformer Percentage Impedance (%Z)
Transformer impedance is a critical parameter in electrical engineering, representing the internal resistance and reactance of the transformer windings. It is expressed as a percentage of the rated voltage required to circulate full-load current through one winding when the other winding is short-circuited.
The Significance of Percentage Impedance
In power systems, %Z determines the voltage drop across the transformer under load and the magnitude of fault current during a short-circuit event. Higher impedance limits short-circuit current (protecting downstream equipment) but causes a greater voltage drop under normal operating conditions.
Mathematical Formulas Used
The calculator uses the following engineering formulas:
- Full Load Current (3-Phase): I = (kVA × 1000) / (V × √3)
- Full Load Current (1-Phase): I = (kVA × 1000) / V
- Actual Impedance (Zohms): ZΩ = (%Z / 100) × (V² / (kVA × 1000))
- Short Circuit Current (Isc): Isc = Ifull_load / (%Z / 100)
Practical Example Calculation
Suppose you have a 500 kVA, 3-phase transformer with a secondary voltage of 480V and a nameplate impedance of 5%.
- Full Load Amps: (500,000) / (480 × 1.732) = 601.4 Amps
- Short Circuit Current: 601.4 / 0.05 = 12,028 Amps
- Impedance in Ohms: (0.05 × 480²) / 500,000 = 0.023 Ohms
Why Use This Calculator?
Electrical engineers and contractors use this tool to size circuit breakers, determine Arc Flash categories, and perform coordination studies. By knowing the available short-circuit current, you can ensure that the Interrupting Capacity (AIC) of your switchgear is sufficient to handle a fault without catastrophic failure.
function calculateTransformer() {
var phase = parseFloat(document.getElementById('phaseType').value);
var kva = parseFloat(document.getElementById('kvaRating').value);
var volts = parseFloat(document.getElementById('secondaryVoltage').value);
var zPerc = parseFloat(document.getElementById('percentZ').value);
if (isNaN(kva) || isNaN(volts) || isNaN(zPerc) || kva <= 0 || volts <= 0 || zPerc <= 0) {
alert("Please enter valid positive numbers for all fields.");
return;
}
var fullLoadAmps = 0;
var zDec = zPerc / 100;
// Calculate Full Load Amps
if (phase === 3) {
fullLoadAmps = (kva * 1000) / (volts * Math.sqrt(3));
} else {
fullLoadAmps = (kva * 1000) / volts;
}
// Calculate Short Circuit Amps
var shortCircuitAmps = fullLoadAmps / zDec;
// Calculate Actual Impedance in Ohms
// Z_ohms = (Z% / 100) * (V^2 / S_va)
var zOhms = zDec * (Math.pow(volts, 2) / (kva * 1000));
// Display Results
document.getElementById('fullLoadAmps').innerHTML = fullLoadAmps.toFixed(2);
document.getElementById('actualOhms').innerHTML = zOhms.toFixed(5);
document.getElementById('shortCircuitAmps').innerHTML = shortCircuitAmps.toFixed(2);
document.getElementById('transformerResults').style.display = 'block';
}