Single-core in RMC, free air
Multi-core in RMC, free air
Buried in conduit
Direct buried
Single-core in cable tray
Multi-core in cable tray
Single-core on surface
Multi-core on surface
Single Phase (230V standard for calculations)
Three Phase (400V standard for calculations)
Understanding Electrical Cable Sizing
Properly sizing electrical cables is crucial for safety, efficiency, and longevity of electrical installations. An undersized cable can overheat, leading to insulation damage, fire hazards, and power loss. An oversized cable is inefficiently costly and can also cause issues like reduced fault current for protective device coordination.
Key Factors in Cable Sizing
Several factors must be considered to determine the correct cable size (cross-sectional area):
Current Carrying Capacity (Ampacity): The maximum amount of current a cable can safely carry continuously without exceeding its temperature rating. This is influenced by conductor material, insulation type, ambient temperature, and installation method (e.g., in conduit, free air, buried).
Voltage Drop: As current flows through a cable, a voltage is lost due to the cable's resistance. Excessive voltage drop can impair the performance of connected equipment. The allowable voltage drop is typically specified by electrical codes or project requirements.
Short Circuit Capacity: The cable must be able to withstand the thermal and mechanical stresses of a short circuit for the time it takes for protective devices (fuses, circuit breakers) to operate.
Environmental Factors: Ambient temperature, grouping of cables (which reduces heat dissipation), and physical protection all affect the cable's ability to carry current.
The Calculation Process
Cable sizing typically involves a multi-step process:
Determine the Design Current (Ib): This is usually the nominal current of the load or circuit, sometimes with a diversity factor applied. In this calculator, it's the "Maximum Continuous Current".
Select a Cable Type and Installation Method: Based on the project requirements and local electrical codes.
Calculate Required Ampacity: Based on the design current (Ib) and relevant correction factors for ambient temperature, grouping, etc. The selected cable's tabulated ampacity must be greater than or equal to the corrected design current.
Calculate Voltage Drop: Using the selected cable size, conductor material, length, and current, calculate the voltage drop and compare it to the allowable limit.
Check Short Circuit Current: Ensure the cable's short circuit withstand capability is sufficient.
Simplified Calculator Logic
This calculator provides an estimate based on common formulas, primarily focusing on current carrying capacity and voltage drop. For precise calculations and compliance with local electrical regulations, always consult relevant standards (e.g., IEC, NEC, BS 7671) and a qualified electrical engineer.
Formulas Used (Simplified):
Ampacity Correction Factor (Ca): This is a complex factor accounting for ambient temperature and installation method. Typical values are derived from standard tables and are simplified here.
Required Ampacity (Iz):Iz = Ib / (Ca * Cg * ...) where Ib is the design current, and Ca, Cg are correction factors. This calculator implies a required ampacity that must be met by selecting a suitable cable size.
Voltage Drop (VD):
VD (Volts) = (I * L * K) / CSA
For Single Phase: K = 2*rho (where rho is resistivity)
For Three Phase: K = sqrt(3)*rho
Where:
I = Current (Amps)
L = Length (Meters)
K = Material resistivity factor (different for Copper/Aluminum, Single/Three Phase)
CSA = Conductor Cross-Sectional Area (mm²)
The calculator aims to find the minimum CSA that satisfies both ampacity and voltage drop requirements.
Resistivity Factors (K) Approximation for Voltage Drop (for reference, calculator uses internal logic):
Copper, Single Phase: ~34.3 mΩ·m/mm² (using 2*17.15)
Copper, Three Phase: ~59.4 mΩ·m/mm² (using sqrt(3)*17.15)
Aluminum, Single Phase: ~55.7 mΩ·m/mm² (using 2*27.85)
Aluminum, Three Phase: ~96.5 mΩ·m/mm² (using sqrt(3)*27.85)
Example Usage
Scenario: You need to supply a continuous load of 25 Amps over a distance of 60 meters. You want to limit voltage drop to 3% and are using copper conductors installed in a multi-core cable in a tray. The ambient temperature is 35°C. The circuit is single phase.
Enter 25 for Maximum Continuous Current.
Enter 3 for Allowable Voltage Drop (%).
Enter 60 for Cable Length (Meters).
Select Copper for Conductor Material.
Select Multi-core in cable tray for Installation Method.
Enter 35 for Ambient Temperature (°C).
Select Single Phase for Circuit Type.
Click "Calculate Cable Size".
The result will suggest a minimum cable size (CSA in mm²) that meets these criteria, typically displayed with a warning if standard sizes are rounded up.
Disclaimer: This calculator is for educational and estimation purposes only. It uses simplified models and average values. Always refer to manufacturer data, electrical codes, and consult with a qualified professional for actual installations.
function calculateCableSize() {
var currentRating = parseFloat(document.getElementById("currentRating").value);
var voltageDropPercentage = parseFloat(document.getElementById("voltageDropPercentage").value);
var cableLength = parseFloat(document.getElementById("cableLength").value);
var conductorMaterial = document.getElementById("conductorMaterial").value;
var installationMethod = document.getElementById("installationMethod").value;
var ambientTemperature = parseFloat(document.getElementById("ambientTemperature").value);
var circuitType = document.getElementById("circuitType").value;
var resultDiv = document.getElementById("result");
resultDiv.innerHTML = "; // Clear previous results
// — Input Validation —
if (isNaN(currentRating) || currentRating <= 0) {
resultDiv.innerHTML = 'Please enter a valid Maximum Continuous Current (Amps).';
return;
}
if (isNaN(voltageDropPercentage) || voltageDropPercentage 10) { // Max 10% arbitrary limit
resultDiv.innerHTML = 'Please enter a valid Allowable Voltage Drop percentage (e.g., 1-5%).';
return;
}
if (isNaN(cableLength) || cableLength <= 0) {
resultDiv.innerHTML = 'Please enter a valid Cable Length (Meters).';
return;
}
if (isNaN(ambientTemperature) || ambientTemperature 60) { // Reasonable range
resultDiv.innerHTML = 'Please enter a valid Ambient Temperature (°C).';
return;
}
// — Constants and Material Properties —
var standardVoltage = (circuitType === "single_phase") ? 230 : 400;
var resistivity = (conductorMaterial === "copper") ? 1.72e-8 : 2.82e-8; // Ohm-meters at 20°C
var tempCoefficient = (conductorMaterial === "copper") ? 0.00385 : 0.00404; // Per °C
// Adjust resistivity for ambient temperature
var tempResistivity = resistivity * (1 + tempCoefficient * (ambientTemperature – 20));
var kFactor; // Factor for voltage drop calculation (depends on phase and material)
if (circuitType === "single_phase") {
// K for single phase = 2 * (resistivity_at_temp / Area)
// For VD = I * L * K / Area, the 'K' here is often defined without Area, so we use resistivity directly in its place implicitly
// VD (Volts) = I * L * (2 * rho_temp) / CSA (in Ohm*m*m / mm^2) – need consistent units
// Let's use a simplified approach where K implicitly contains resistivity for the voltage drop formula:
// VD = (I * L * K) / CSA. Where K is effectively (2 * rho_temp) for single phase and (sqrt(3) * rho_temp) for 3 phase
kFactor = 2 * tempResistivity; // Effectively rho_temp in Ohm*m
} else { // three_phase
kFactor = Math.sqrt(3) * tempResistivity; // Effectively rho_temp in Ohm*m
}
// — Ampacity Correction Factors (Simplified example values – REAL CALCS NEED TABLES) —
// These are highly dependent on specific cable types and standards (e.g., IEC 60364, NEC)
// This section is a placeholder for real-world lookup tables.
var ampacityCorrectionFactor = 1.0; // Default if no specific factors are applied
// Example: Temperature Correction (very simplified)
// A common reference is 30°C. Above that, ampacity decreases.
var tempCorrectionFactor = 1.0;
if (ambientTemperature > 30) {
tempCorrectionFactor = 1.0 – ((ambientTemperature – 30) * 0.004); // Arbitrary reduction factor per degree C
} else if (ambientTemperature < 30) {
tempCorrectionFactor = 1.0 + ((30 – ambientTemperature) * 0.002); // Arbitrary increase factor
}
// Installation method factors are complex and would require lookup tables based on specific cable types and grouping.
// For this simplified calculator, we will primarily rely on voltage drop and a basic temperature correction.
// Real-world tools would use IEC 60364-5-52 or similar tables extensively.
ampacityCorrectionFactor = tempCorrectionFactor; // Apply temp correction as the main factor for this example
// — Calculate Required Ampacity (Iz) —
// Iz = Ib / (Ca * Cg * …)
// For this simplified calc, we'll use Ib / Ca (where Ca is our combined correction factor)
var requiredAmpacity = currentRating / ampacityCorrectionFactor;
// — Determine Minimum CSA based on Ampacity —
// This requires a lookup table for specific cable types (e.g., PVC insulated, XLPE insulated) and installation methods.
// We will use a VERY simplified approximation here.
// Typical values:
// Copper: 1.5mm² (~16A), 2.5mm² (~21A), 4mm² (~27A), 6mm² (~34A), 10mm² (~50A) – These are rough, vary wildly.
// Aluminum: Roughly 1.6x Copper values.
// This is a lookup simulation. In reality, you'd use tables based on installation method, temperature, etc.
var minCsaAmpacity = 0;
if (conductorMaterial === "copper") {
if (requiredAmpacity <= 16 * ampacityCorrectionFactor) minCsaAmpacity = 1.5;
else if (requiredAmpacity <= 21 * ampacityCorrectionFactor) minCsaAmpacity = 2.5;
else if (requiredAmpacity <= 27 * ampacityCorrectionFactor) minCsaAmpacity = 4.0;
else if (requiredAmpacity <= 34 * ampacityCorrectionFactor) minCsaAmpacity = 6.0;
else if (requiredAmpacity <= 50 * ampacityCorrectionFactor) minCsaAmpacity = 10.0;
else if (requiredAmpacity <= 64 * ampacityCorrectionFactor) minCsaAmpacity = 16.0;
else if (requiredAmpacity <= 80 * ampacityCorrectionFactor) minCsaAmpacity = 25.0;
else if (requiredAmpacity <= 100 * ampacityCorrectionFactor) minCsaAmpacity = 35.0;
else minCsaAmpacity = 50.0; // Placeholder for larger sizes
} else { // Aluminum
if (requiredAmpacity <= 13 * ampacityCorrectionFactor) minCsaAmpacity = 1.5 * 1.6; // Rough scaling
else if (requiredAmpacity <= 17 * ampacityCorrectionFactor) minCsaAmpacity = 2.5 * 1.6;
else if (requiredAmpacity <= 22 * ampacityCorrectionFactor) minCsaAmpacity = 4.0 * 1.6;
else if (requiredAmpacity <= 27 * ampacityCorrectionFactor) minCsaAmpacity = 6.0 * 1.6;
else if (requiredAmpacity <= 40 * ampacityCorrectionFactor) minCsaAmpacity = 10.0 * 1.6;
else if (requiredAmpacity <= 51 * ampacityCorrectionFactor) minCsaAmpacity = 16.0 * 1.6;
else if (requiredAmpacity <= 64 * ampacityCorrectionFactor) minCsaAmpacity = 25.0 * 1.6;
else if (requiredAmpacity CSA = (I * L * K) / VD
// Need to use the actual current rating (Ib) for voltage drop calculation as it's a fixed value for the load.
var minCsaVoltageDrop = (currentRating * cableLength * kFactor) / allowableVoltageDropVolts;
// — Determine Final Required CSA —
// The required CSA must satisfy BOTH ampacity and voltage drop.
// We need to round UP to the nearest standard cable size (e.g., 1.5, 2.5, 4, 6, 10, 16, 25, 35, 50 mm² etc.)
var requiredCsa = Math.max(minCsaAmpacity, minCsaVoltageDrop);
// — Round up to nearest standard size —
var standardSizes = [1.5, 2.5, 4, 6, 10, 16, 25, 35, 50, 70, 95, 120, 150, 185, 240, 300, 400, 500, 630, 800]; // Common mm² sizes
var selectedCsa = 0;
for (var i = 0; i = requiredCsa) {
selectedCsa = standardSizes[i];
break;
}
}
// If requiredCsa is larger than the largest standard size listed, use the largest standard size and warn.
if (selectedCsa === 0) {
selectedCsa = standardSizes[standardSizes.length – 1];
resultDiv.innerHTML = 'Warning: Required size exceeds standard table. Use largest available standard size: ' + selectedCsa + ' mm².';
}
// — Display Result —
var vdActualVolts = (currentRating * cableLength * kFactor) / (selectedCsa * 1e6); // Convert mm^2 to m^2 for Ohm*m calculation
// Need to recalculate K for Ohms per meter directly for VD.
// Resistivity of Cu at 20C = 1.72e-8 Ohm-m
// Resistivity of Al at 20C = 2.82e-8 Ohm-m
// Resistance per meter (R/m) = rho / CSA (in m^2)
var resistancePerMeter = tempResistivity / (selectedCsa * 1e-6); // CSA in m^2
var actualVoltageDropVolts = 0;
if (circuitType === "single_phase") {
actualVoltageDropVolts = 2 * currentRating * cableLength * resistancePerMeter; // Factor of 2 for single phase
} else { // three_phase
actualVoltageDropVolts = Math.sqrt(3) * currentRating * cableLength * resistancePerMeter; // Factor of sqrt(3) for three phase
}
var actualVoltageDropPercentage = (actualVoltageDropVolts / standardVoltage) * 100;
resultDiv.innerHTML = '
Recommended Cable Size:
' +
'Minimum required CSA (approx): ' + requiredCsa.toFixed(2) + ' mm²' +
'Selected Standard CSA: ' + selectedCsa + ' mm²' +
'Actual Voltage Drop: ' + actualVoltageDropVolts.toFixed(2) + ' V (' + actualVoltageDropPercentage.toFixed(2) + '%)' +
'Note: Ampacity checks are simplified. Always verify against manufacturer data and local codes.';
}