Circuit Breaker Sizing Calculator

ANA›Life Services Authority›National Calculator Authority›Circuit Breaker Sizing Calculator

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Circuit Breaker Sizing Calculator

Calculate the minimum required circuit breaker size for your electrical circuit based on load current, wire ampacity, and NEC safety factors.

Load Current (A)

Total continuous or non-continuous load current

Load Type

Non-Continuous Load Continuous Load (3+ hours) Mixed Load

Continuous loads require 125% derating per NEC 210.20

Continuous Load Current (A)

Portion of load that runs continuously (≥3 hours)

System Voltage (V)

120V (Single Phase) 240V (Single Phase) 208V (Three Phase) 277V (Single Phase) 480V (Three Phase)

Number of Phases

Single Phase Three Phase

Power Factor

Typical: 0.85–0.95 for motors, 1.0 for resistive loads

Wire Ampacity (A) (optional)

From NEC Table 310.16 — leave blank to skip wire check

Calculate Circuit Breaker Size

function cirCalc() { var resultDiv = document.getElementById('cir-result');

var loadCurrent = parseFloat(document.getElementById('cir-load-current').value); var loadType = document.getElementById('cir-load-type').value; var contCurrent = parseFloat(document.getElementById('cir-continuous-current').value) || 0; var voltage = parseFloat(document.getElementById('cir-voltage').value); var phases = parseInt(document.getElementById('cir-phases').value); var pf = parseFloat(document.getElementById('cir-power-factor').value); var wireAmpacity = parseFloat(document.getElementById('cir-wire-ampacity').value);

// --- Validation --- if (isNaN(loadCurrent) || loadCurrent ⚠ Please enter a valid load current greater than 0.'; resultDiv.style.display = 'block'; return; } if (isNaN(pf) || pf 1.0) { resultDiv.innerHTML = '⚠ Power factor must be between 0.10 and 1.00.'; resultDiv.style.display = 'block'; return; } if (loadType === 'mixed' && (isNaN(contCurrent) || contCurrent ⚠ Please enter a valid continuous load current.'; resultDiv.style.display = 'block'; return; } if (loadType === 'mixed' && contCurrent > loadCurrent) { resultDiv.innerHTML = '⚠ Continuous load current cannot exceed total load current.'; resultDiv.style.display = 'block'; return; }

// --- NEC Derated Current Calculation --- // NEC 210.20(A): CB must be ≥ 125% of continuous load + 100% of non-continuous load var nonContCurrent, designCurrent, deratingFactor;

if (loadType === 'non-continuous') { designCurrent = loadCurrent * 1.0; deratingFactor = 1.0; nonContCurrent = loadCurrent; contCurrent = 0; } else if (loadType === 'continuous') { designCurrent = loadCurrent * 1.25; deratingFactor = 1.25; nonContCurrent = 0; contCurrent = loadCurrent; } else { // mixed nonContCurrent = loadCurrent - contCurrent; designCurrent = (contCurrent * 1.25) + (nonContCurrent * 1.0); deratingFactor = designCurrent / loadCurrent; }

// --- Standard Breaker Sizes (NEC 240.6(A)) --- var standardSizes = [ 15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100, 110, 125, 150, 175, 200, 225, 250, 300, 350, 400, 450, 500, 600, 700, 800, 1000, 1200, 1600, 2000, 2500, 3000, 4000, 5000, 6000 ];

// Select next standard size at or above design current var recommendedBreaker = null; for (var i = 0; i = designCurrent) { recommendedBreaker = standardSizes[i]; break; } } if (recommendedBreaker === null) { resultDiv.innerHTML = '⚠ Load current exceeds standard breaker sizes. Consult a licensed electrical engineer.'; resultDiv.style.display = 'block'; return; }

// --- Apparent Power Calculation --- var sqrtPhases = (phases === 3) ? Math.sqrt(3) : 1.0; var apparentPower = loadCurrent * voltage * sqrtPhases; // VA var realPower = apparentPower * pf; // W

// --- Wire Ampacity Check --- var wireWarning = ''; if (!isNaN(wireAmpacity) && wireAmpacity > 0) { if (recommendedBreaker > wireAmpacity) { wireWarning = '' + '⚠ Wire Ampacity Warning: The recommended breaker (' + recommendedBreaker + 'A) exceeds your wire ampacity (' + wireAmpacity + 'A). Upgrade your wiring or reduce the load. Per NEC 240.4, conductors must be protected at or below their ampacity.'; } else { wireWarning = '' + '✓ Wire Check Passed: Wire ampacity (' + wireAmpacity + 'A) is sufficient for the recommended breaker (' + recommendedBreaker + 'A).'; } }

// --- Breaker Type Recommendation --- var breakerType = ''; if (recommendedBreaker ' + 'Load Current' + '' + loadCurrent.toFixed(2) + ' A' + 'Load Type' + '' + (loadType === 'non-continuous' ? 'Non-Continuous' : loadType === 'continuous' ? 'Continuous' : 'Mixed') + '' + 'NEC Derated Design Current' + '' + designCurrent.toFixed(2) + ' A' + 'Apparent Power (VA)' + '' + apparentPower.toFixed(1) + ' VA' + 'Real Power (W)' + '' + realPower.toFixed(1) + ' W (' + (realPower/1000).toFixed(3) + ' kW)' + 'Recommended Breaker Type' + '' + breakerType + '' + '✅ Minimum Breaker Size' + '' + recommendedBreaker + ' A' + '' + wireWarning + 'ⓘ Results are for estimation only. Always verify with a licensed electrician and local AHJ (Authority Having Jurisdiction).

';

resultDiv.style.display = 'block'; }

// Show/hide continuous current field based on load type document.getElementById('cir-load-type').addEventListener('change', function() { var group = document.getElementById('cir-continuous-group'); group.style.display = (this.value === 'mixed') ? 'block' : 'none'; });

#### Formulas Used

NEC Design Current (NEC 210.20(A)):

Apparent Power:

Real Power: P = S × PF (W)

Breaker Selection: Next standard size ≥ Idesign per NEC 240.6(A)

#### Assumptions & References

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References

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