Circuit Breaker Size Calculator
ANA›Life Services Authority›National Calculator Authority›Circuit Breaker Size Calculator
.calc-container { max-width: 640px; margin: 2rem 0; padding: 1.5rem; background: #fff; border: 1px solid #ddd; border-radius: 8px; box-shadow: 0 1px 3px rgba(0,0,0,0.06); font-family: system-ui, -apple-system, sans-serif; } .calc-container h3 { font-family: Georgia, serif; font-size: 1.15rem; color: #1a1a1a; margin-bottom: 1rem; padding-bottom: 0.5rem; border-bottom: 2px solid var(--ac, #3d5a80); } .calc-row { display: flex; align-items: center; gap: 0.75rem; margin-bottom: 0.75rem; flex-wrap: wrap; } .calc-row label { min-width: 160px; font-size: 0.9rem; color: #333; font-weight: 500; } .calc-row input[type="number"], .calc-row select { flex: 1; min-width: 120px; max-width: 200px; padding: 0.5rem 0.6rem; border: 1px solid #ccc; border-radius: 4px; font-size: 0.9rem; font-family: system-ui, sans-serif; color: #1a1a1a; background: #fafaf8; } .calc-row input:focus, .calc-row select:focus { outline: none; border-color: var(--ac, #3d5a80); box-shadow: 0 0 0 2px rgba(26,74,138,0.12); } .calc-row .unit { font-size: 0.82rem; color: #888; min-width: 30px; } .calc-btn { display: inline-block; margin-top: 0.5rem; padding: 0.55rem 1.5rem; background: var(--ac, #3d5a80); color: #fff; border: none; border-radius: 4px; font-size: 0.9rem; font-weight: 600; cursor: pointer; font-family: system-ui, sans-serif; } .calc-btn:hover { opacity: 0.9; } .calc-result { margin-top: 1.25rem; padding: 1rem 1.25rem; background: #f0f6fc; border-left: 3px solid var(--ac, #3d5a80); border-radius: 0 6px 6px 0; display: none; } .calc-result.visible { display: block; } .calc-result-label { font-size: 0.78rem; text-transform: uppercase; letter-spacing: 0.06em; color: #666; margin-bottom: 0.25rem; } .calc-result-value { font-size: 1.6rem; font-weight: 700; color: var(--ac, #3d5a80); } .calc-result-detail { font-size: 0.85rem; color: #555; margin-top: 0.5rem; line-height: 1.5; } .calc-note { margin-top: 1rem; font-size: 0.8rem; color: #888; font-style: italic; } .calc-grid { display: grid; grid-template-columns: 1fr 1fr; gap: 0.75rem; margin-top: 0.75rem; } .calc-grid-item { padding: 0.6rem 0.8rem; background: #f8f9fa; border-radius: 4px; border: 1px solid #eee; } .calc-grid-item .label { font-size: 0.75rem; color: #888; text-transform: uppercase; letter-spacing: 0.04em; } .calc-grid-item .value { font-size: 1.1rem; font-weight: 600; color: #1a1a1a; } @media (max-width: 720px) { .calc-row { flex-direction: column; align-items: flex-start; gap: 0.3rem; } .calc-row label { min-width: auto; } .calc-row input[type="number"], .calc-row select { max-width: 100%; width: 100%; } .calc-grid { grid-template-columns: 1fr; } } .calc-chart { margin: 1rem 0; text-align: center; } .calc-chart svg { max-width: 100%; height: auto; } .calc-chart-legend { display: flex; flex-wrap: wrap; justify-content: center; gap: 0.6rem 1.2rem; margin-top: 0.6rem; font-size: 0.8rem; color: #555; } .calc-chart-legend span { display: inline-flex; align-items: center; gap: 0.3rem; } .calc-chart-legend i { display: inline-block; width: 10px; height: 10px; border-radius: 2px; font-style: normal; } .calc-related { max-width: 640px; margin: 2rem 0 1rem; padding: 1.25rem 1.5rem; background: #f8f9fa; border: 1px solid #e8e8e8; border-radius: 8px; } .calc-related h3 { font-family: Georgia, serif; font-size: 1rem; color: #1a1a1a; margin: 0 0 0.75rem; padding-bottom: 0.4rem; border-bottom: 2px solid var(--ac, #3d5a80); } .calc-related-list { list-style: none; padding: 0; margin: 0 0 0.75rem; display: grid; grid-template-columns: 1fr 1fr; gap: 0.4rem 1.5rem; } .calc-related-list li a { font-size: 0.88rem; color: var(--ac, #3d5a80); text-decoration: none; } .calc-related-list li a:hover { text-decoration: underline; } .calc-browse-all { margin: 0.5rem 0 0; font-size: 0.9rem; font-weight: 600; } .calc-browse-all a { color: var(--ac, #3d5a80); text-decoration: none; } .calc-browse-all a:hover { text-decoration: underline; } @media (max-width: 720px) { .calc-related-list { grid-template-columns: 1fr; } }
Circuit Breaker Size Calculator
Determine the correct circuit breaker size for residential and commercial electrical circuits based on NEC 80% continuous load rule and wire ampacity ratings.
Total Load Current (Amps)
Load Type
Continuous Load (3+ hours) Non-Continuous Load
Wire Gauge (AWG)
-- Select Wire Gauge -- 14 AWG (15A max) 12 AWG (20A max) 10 AWG (30A max) 8 AWG (40A max) 6 AWG (55A max) 4 AWG (70A max) 3 AWG (85A max) 2 AWG (95A max) 1 AWG (110A max) 1/0 AWG (125A max) 2/0 AWG (145A max) 3/0 AWG (165A max) 4/0 AWG (195A max)
System Voltage (V)
120V (Single Phase) 240V (Single Phase) 208V (Three Phase) 277V (Single Phase Commercial) 480V (Three Phase)
Phase
Single Phase Three Phase
Calculate Breaker Size
function cirCalc() { var loadCurrent = parseFloat(document.getElementById('cir-load-current').value); var loadType = document.getElementById('cir-load-type').value; var wireGauge = document.getElementById('cir-wire-gauge').value; var voltage = parseFloat(document.getElementById('cir-voltage').value); var phase = document.getElementById('cir-phase').value; var resultDiv = document.getElementById('cir-result');
// --- Input Validation --- if (isNaN(loadCurrent) || loadCurrent ⚠ Please enter a valid load current greater than 0.'; return; } if (!wireGauge) { resultDiv.style.display = 'block'; resultDiv.innerHTML = '⚠ Please select a wire gauge.'; return; } if (isNaN(voltage) || voltage ⚠ Please select a valid system voltage.'; return; }
// --- Wire Ampacity Table (NEC 310.15, 60°C/75°C copper, conduit) --- var wireAmpacity = { '14': 15, '12': 20, '10': 30, '8': 40, '6': 55, '4': 70, '3': 85, '2': 95, '1': 110, '1/0': 125, '2/0': 145, '3/0': 165, '4/0': 195 };
// --- Standard Breaker Sizes (NEC 240.6(A)) --- var standardBreakers = [15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100, 110, 125, 150, 175, 200, 225, 250, 300, 350, 400];
var maxWireAmpacity = wireAmpacity[wireGauge];
// --- NEC 210.19(A)(1) & 215.2(A)(1): Continuous Load Rule --- // For continuous loads: breaker must be sized at 125% of load current // For non-continuous: breaker sized at 100% of load current var designCurrent; var multiplier; if (loadType === 'continuous') { multiplier = 1.25; designCurrent = loadCurrent * 1.25; } else { multiplier = 1.0; designCurrent = loadCurrent; }
// --- Find next standard breaker size >= designCurrent --- var recommendedBreaker = null; for (var i = 0; i = designCurrent) { recommendedBreaker = standardBreakers[i]; break; } } if (recommendedBreaker === null) { resultDiv.style.display = 'block'; resultDiv.innerHTML = '⚠ Load current exceeds standard breaker sizes (400A max). Consult a licensed electrician.'; return; }
// --- Wire Ampacity Check --- var wireOk = recommendedBreaker '; html += 'Load Current' + loadCurrent.toFixed(2) + ' A'; html += 'Load Type' + (loadType === 'continuous' ? 'Continuous (×1.25 NEC factor)' : 'Non-Continuous (×1.00)') + ''; html += 'Design Current' + designCurrent.toFixed(2) + ' A (' + loadCurrent.toFixed(2) + ' × ' + multiplier.toFixed(2) + ')'; html += 'Recommended Breaker Size' + recommendedBreaker + ' A'; html += 'Wire Gauge Selected' + wireGauge + ' AWG (max ' + maxWireAmpacity + ' A)'; html += 'Wire Adequacy' + wireStatus + ''; html += 'System Voltage' + voltage + ' V (' + (phase === 'three' ? 'Three Phase' : 'Single Phase') + ')'; html += 'Estimated Load Power' + power.toFixed(0) + ' W (' + powerKW.toFixed(2) + ' kW)'; html += '';
if (!wireOk) { html += ''; html += 'Action Required: Upgrade wire gauge to support a ' + recommendedBreaker + 'A breaker. '; html += 'Using undersized wire is a fire hazard and violates NEC code.
'; } else { html += ''; html += '✓ Circuit is properly sized. Install a ' + recommendedBreaker + 'A breaker with ' + wireGauge + ' AWG wire.
'; }
resultDiv.style.display = 'block'; resultDiv.innerHTML = html; }
#### Formulas Used
Continuous Load (NEC 210.19(A)(1) & 215.2(A)(1)):
Design Current = Load Current × 1.25 Breaker Size = Next standard size ≥ Design Current
Non-Continuous Load:
Design Current = Load Current × 1.00 Breaker Size = Next standard size ≥ Design Current
Power (Single Phase):
P = V × I
Power (Three Phase):
P = √3 × V × I ≈ 1.732 × V × I
#### Assumptions & References
- Wire ampacity values are based on NEC Table 310.15(B)(16) for copper conductors at 75°C in conduit (most common residential/commercial installation).
- The 125% continuous load factor is required by NEC 210.19(A)(1) and 215.2(A)(1) for loads energized for 3 or more hours.
- Standard breaker sizes follow NEC 240.6(A): 15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100, 110, 125, 150, 175, 200, 225, 250, 300, 350, 400A.
- The breaker size must not exceed the wire ampacity per NEC 240.4 to protect the conductor from overcurrent.
- Ambient temperature derating and conduit fill derating factors are not applied here — consult NEC 310.15(B)(2) and (3) for those adjustments.
- Reference: NFPA 70: National Electrical Code (NEC), 2023 Edition.
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