Service Entrance Wire Size Calculator
ANA›Life Services Authority›National Calculator Authority›Service Entrance Wire Size Calculator
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Service Entrance Wire Size Calculator
Determine the minimum AWG or kcmil copper or aluminum service entrance conductor size based on total load (amperes), service voltage, and run length using NEC voltage drop and ampacity guidelines.
Total Load (Amperes)
A
Service Voltage
120V (Single-phase) 240V (Single-phase) 208V (Three-phase) 480V (Three-phase)
Number of Phases
Single-phase (1Ø) Three-phase (3Ø)
One-Way Run Length (feet)
ft
Conductor Material
Copper Aluminum
Max Allowable Voltage Drop (%)
%
Calculate Wire Size
Results will appear here.
function serCalc() { var load = parseFloat(document.getElementById('ser_load').value); var voltage = parseFloat(document.getElementById('ser_voltage').value); var phases = parseInt(document.getElementById('ser_phases').value); var distance = parseFloat(document.getElementById('ser_distance').value); var material = document.getElementById('ser_material').value; var vdropPct = parseFloat(document.getElementById('ser_vdrop').value);
// --- Validation --- var errors = []; if (isNaN(load) || load 10) errors.push("Voltage drop must be between 0.5% and 10%."); if (phases === 3 && (voltage === 120 || voltage === 240)) errors.push("Three-phase service is typically 208V or 480V. Check your voltage selection.");
if (errors.length > 0) { document.getElementById('ser_result').innerHTML = 'Input Error:' + errors.join('') + ''; return; }
// ----------------------------------------------------------------------- // NEC Table 310.16 Ampacity (75°C column, most common for service entrance) // Format: [AWG/kcmil label, copper ampacity, aluminum ampacity, circular mils] // ----------------------------------------------------------------------- var wireTable = [ // label, Cu, Al, CM ["14 AWG", 15, 15, 4110], ["12 AWG", 20, 20, 6530], ["10 AWG", 30, 30, 10380], ["8 AWG", 50, 40, 16510], ["6 AWG", 65, 50, 26240], ["4 AWG", 85, 65, 41740], ["3 AWG", 100, 75, 52620], ["2 AWG", 115, 90, 66360], ["1 AWG", 130, 100, 83690], ["1/0 AWG", 150, 120, 105600], ["2/0 AWG", 175, 135, 133100], ["3/0 AWG", 200, 155, 167800], ["4/0 AWG", 230, 180, 211600], ["250 kcmil",255, 205, 250000], ["300 kcmil",285, 230, 300000], ["350 kcmil",310, 250, 350000], ["400 kcmil",335, 270, 400000], ["500 kcmil",380, 310, 500000], ["600 kcmil",420, 340, 600000], ["700 kcmil",460, 375, 700000], ["750 kcmil",475, 385, 750000], ["800 kcmil",490, 395, 800000], ["900 kcmil",520, 425, 900000], ["1000 kcmil",545,445, 1000000] ];
// ----------------------------------------------------------------------- // Step 1 – Ampacity-based minimum size (NEC 230.42 / 310.16) // NEC requires service conductors rated at 100% of continuous load // ----------------------------------------------------------------------- var ampacityIdx = -1; for (var i = 0; i = load) { ampacityIdx = i; break; } } if (ampacityIdx === -1) ampacityIdx = wireTable.length - 1; // largest available
// ----------------------------------------------------------------------- // Step 2 – Voltage-drop-based minimum size // Single-phase: VD = (2 × K × I × L) / CM // Three-phase: VD = (1.732 × K × I × L) / CM // K = resistivity constant (circular-mil·ohms/ft) // Copper: K = 12.9 (at 75°C) // Aluminum: K = 21.2 (at 75°C) // Rearranged for required CM: // CM = (multiplier × K × I × L) / VD_volts // ----------------------------------------------------------------------- var K = (material === 'copper') ? 12.9 : 21.2; var multiplier = (phases === 1) ? 2.0 : 1.732; var vdVolts = voltage * (vdropPct / 100.0);
var requiredCM = (multiplier * K * load * distance) / vdVolts;
var vdropIdx = -1; for (var j = 0; j = requiredCM) { vdropIdx = j; break; } } if (vdropIdx === -1) vdropIdx = wireTable.length - 1;
// ----------------------------------------------------------------------- // Step 3 – Take the larger (more conservative) of the two requirements // ----------------------------------------------------------------------- var finalIdx = Math.max(ampacityIdx, vdropIdx); var finalWire = wireTable[finalIdx]; var finalAmpacity = (material === 'copper') ? finalWire[1] : finalWire[2]; var finalCM = finalWire[3];
// Actual voltage drop with selected wire var actualVD = (multiplier * K * load * distance) / finalCM; var actualVDPct = (actualVD / voltage) * 100;
// Governing factor var governing = (finalIdx === vdropIdx && vdropIdx >= ampacityIdx) ? "Voltage Drop" : "Ampacity (NEC 310.16)"; if (ampacityIdx === vdropIdx) governing = "Both Ampacity & Voltage Drop";
// ----------------------------------------------------------------------- // Build result HTML // ----------------------------------------------------------------------- var matLabel = material.charAt(0).toUpperCase() + material.slice(1); var phLabel = (phases === 1) ? "Single-phase" : "Three-phase";
var html = ''; html += 'Service Entrance Wire Size Results';
html += 'Recommended Wire Size'; html += '' + finalWire[0] + ' ' + matLabel + '';
html += 'Conductor Ampacity (75°C)'; html += '' + finalAmpacity + ' A';
html += 'Circular Mils (CM)'; html += '' + finalCM.toLocaleString() + ' CM';
html += 'Required CM (Voltage Drop)'; html += '' + Math.round(requiredCM).toLocaleString() + ' CM';
html += 'Actual Voltage Drop'; var vdColor = actualVDPct ' + actualVD.toFixed(2) + ' V (' + actualVDPct.toFixed(2) + '%)';
html += 'Governing Factor'; html += '' + governing + '';
html += 'Service Configuration'; html += '' + phLabel + ', ' + voltage + ' V';
html += 'Conductor Material'; html += '' + matLabel + ''; html += '';
// Ampacity-based wire note html += ''; html += 'Note: Size based on NEC Table 310.16 (75°C, ' + matLabel + '). '; html += 'Minimum service size per NEC 230.79 is typically 100A for single-family dwellings. '; html += 'Always verify with local AHJ and apply applicable derating factors for conduit fill, ambient temperature, and continuous loads.'; html += '
';
document.getElementById('ser_result').innerHTML = html; }
#### Formulas Used
Voltage Drop (Single-phase): VD (volts) = (2 × K × I × L) / CM
Voltage Drop (Three-phase): VD (volts) = (1.732 × K × I × L) / CM
Required Circular Mils: CM = (multiplier × K × I × L) / VDallowed
Where: K = Resistivity constant (Copper = 12.9, Aluminum = 21.2 CM·Ω/ft at 75°C) I = Load current (Amperes) L = One-way conductor length (feet) CM = Circular mils of conductor VDallowed = Voltage × (Max VD% / 100)
The final wire size is the larger of the ampacity-based size (NEC Table 310.16, 75°C column) and the voltage-drop-based size.
#### Assumptions & References
- Ampacity values from NEC Table 310.16 (2020 edition), 75°C insulation column (THWN, XHHW, USE-2).
- NEC 210.19(A) and 215.2(A) recommend maximum 3% voltage drop for branch circuits and feeders; 5% total from service to final outlet.
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