Home Electrical Panel Capacity Calculator

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Home Electrical Panel Capacity Calculator

Estimate your electrical panel's total capacity, current load, and remaining headroom. Enter your panel's main breaker size and your major appliances/circuits to see if you have room for new loads.

### Panel Information

Main Breaker Size (Amps)

60A (Older/Small Home) 100A (Standard) 150A (Medium Home) 200A (Modern Standard) 320A (Large Home) 400A (Very Large Home)

Service Voltage

240V (Standard US Split-Phase) 120V (Single Phase)

### Major Appliances & Circuits

Enter the amperage for each circuit breaker (leave 0 if not present).

HVAC / Central Air Conditioner (Amps)

Electric Furnace / Heat Strips (Amps)

Electric Water Heater (Amps)

Electric Dryer (Amps)

Electric Range / Oven (Amps)

EV Charger (Amps)

Dishwasher (Amps)

Microwave / Garbage Disposal (Amps)

Refrigerator (Amps)

Lighting Circuits (Total Amps across all circuits)

General Outlet Circuits (Total Amps across all circuits)

Other / Miscellaneous (Amps)

### Demand Factor Settings

NEC Article 220 allows demand factors — not all loads run simultaneously at full capacity.

Calculation Method

Standard NEC Demand Factor Full Load (Conservative / Worst Case)

Calculate Panel Capacity

function homCalc() { // --- Gather Inputs --- var panelAmps = parseFloat(document.getElementById('hom-panel-amps').value); var voltage = parseFloat(document.getElementById('hom-voltage').value); var method = document.getElementById('hom-demand-method').value;

var hvac = parseFloat(document.getElementById('hom-hvac').value) || 0; var heat = parseFloat(document.getElementById('hom-heat').value) || 0; var waterHeater = parseFloat(document.getElementById('hom-waterheater').value) || 0; var dryer = parseFloat(document.getElementById('hom-dryer').value) || 0; var range = parseFloat(document.getElementById('hom-range').value) || 0; var ev = parseFloat(document.getElementById('hom-ev').value) || 0; var dishwasher = parseFloat(document.getElementById('hom-dishwasher').value) || 0; var microwave = parseFloat(document.getElementById('hom-microwave').value) || 0; var refrigerator = parseFloat(document.getElementById('hom-refrigerator').value) || 0; var lighting = parseFloat(document.getElementById('hom-lighting').value) || 0; var outlets = parseFloat(document.getElementById('hom-outlets').value) || 0; var other = parseFloat(document.getElementById('hom-other').value) || 0;

// --- Input Validation --- var errors = []; if (isNaN(panelAmps) || panelAmps 200) errors.push("HVAC amps must be between 0 and 200."); if (heat 200) errors.push("Heat amps must be between 0 and 200."); if (waterHeater 100) errors.push("Water heater amps must be between 0 and 100."); if (dryer 100) errors.push("Dryer amps must be between 0 and 100."); if (range 100) errors.push("Range amps must be between 0 and 100."); if (ev 200) errors.push("EV charger amps must be between 0 and 200."); if (dishwasher 60) errors.push("Dishwasher amps must be between 0 and 60."); if (microwave 60) errors.push("Microwave amps must be between 0 and 60."); if (refrigerator 40) errors.push("Refrigerator amps must be between 0 and 40."); if (lighting 300) errors.push("Lighting amps must be between 0 and 300."); if (outlets 300) errors.push("Outlet amps must be between 0 and 300."); if (other 500) errors.push("Other amps must be between 0 and 500.");

var resultDiv = document.getElementById('hom-result'); if (errors.length > 0) { resultDiv.style.display = 'block'; resultDiv.innerHTML = 'Please fix the following:' + errors.map(function(e){ return ''; }).join('') + ''; return; }

// --- Core Calculations --- // Panel total capacity in watts (kVA) // For split-phase 240V: Total VA = Amps × Voltage var panelCapacityVA = panelAmps * voltage; var panelCapacityKVA = panelCapacityVA / 1000;

// Convert all circuit amps to VA (using service voltage) // 240V circuits (large appliances) use full 240V // 120V circuits use 120V // We'll treat all inputs as breaker amps at the panel level // For a 240V panel: a 30A double-pole breaker = 30A × 240V = 7200VA // For simplicity and NEC standard load calc, we use voltage as entered

// Raw loads in amps (at panel voltage) var rawLoads = { "HVAC / Central AC": hvac, "Electric Furnace/Heat": heat, "Electric Water Heater": waterHeater, "Electric Dryer": dryer, "Electric Range/Oven": range, "EV Charger": ev, "Dishwasher": dishwasher, "Microwave/Disposal": microwave, "Refrigerator": refrigerator, "Lighting Circuits": lighting, "General Outlet Circuits": outlets, "Other/Miscellaneous": other };

// NEC Article 220 Demand Factors: // - Lighting & general outlets: first 3000VA @ 100%, next 117,000VA @ 35%, remainder @ 25% // - Electric range: NEC Table 220.55 (simplified: 8kW for single range) // - Dryer: 5000W or nameplate, whichever is larger, demand factor 100% for 1-4 units // - HVAC vs Heat: use larger of the two (NEC 220.60 non-coincident loads) // - EV: 100% of nameplate (NEC 625.42) // - Water heater, dishwasher, microwave: 100%

var demandAmps = 0; var demandBreakdown = {};

if (method === 'full') { // Full load — no demand factors for (var key in rawLoads) { demandBreakdown[key] = rawLoads[key]; } demandAmps = Object.values(demandBreakdown).reduce(function(a,b){ return a+b; }, 0);

} else { // Standard NEC Demand Factor Method

// 1. General lighting & outlets (NEC 220.42) // Convert to VA first, apply tiered demand, convert back to amps var genVA = (lighting + outlets) * voltage; var genDemandVA = 0; if (genVA 12kW: 8kW + 5% per kW over 12kW var rangeVA = range * voltage; var rangeKW = rangeVA / 1000; var rangeDemandKW = 0; if (rangeKW > 0) { if (rangeKW 0 ? 5000 : 0); var dryerDemandAmps = dryerDemandVA / voltage; demandBreakdown["Electric Dryer (NEC 220.54)"] = dryerDemandAmps;

// 5. Water heater — 100% demandBreakdown["Electric Water Heater (100%)"] = waterHeater;

// 6. EV Charger — 100% (NEC 625.42 continuous load) demandBreakdown["EV Charger (100% continuous)"] = ev;

// 7. Dishwasher, microwave, refrigerator — 100% demandBreakdown["Dishwasher (100%)"] = dishwasher; demandBreakdown["Microwave/Disposal (100%)"] = microwave; demandBreakdown["Refrigerator (100%)"] = refrigerator;

// 8. Other — 100% if (other > 0) demandBreakdown["Other/Miscellaneous (100%)"] = other;

// Total demand amps demandAmps = Object.values(demandBreakdown).reduce(function(a,b){ return a+b; }, 0); }

// Demand in watts/kW var demandVA = demandAmps * voltage; var demandKVA = demandVA / 1000;

// Available headroom var headroomAmps = panelAmps - demandAmps; var headroomVA = headroomAmps * voltage; var headroomKVA = headroomVA / 1000;

// Load percentage var loadPct = (demandAmps / panelAmps) * 100;

// NEC 80% continuous load rule: panel should not exceed 80% for continuous loads var necSafeAmps = panelAmps * 0.80; var necHeadroomAmps = necSafeAmps - demandAmps;

// Status var statusColor, statusMsg, statusIcon; if (loadPct 0) { var watts = (amps * voltage).toFixed(0); tableRows += '' + label + '' + amps.toFixed(1) + ' A' + watts + ' W'; } }

// Progress bar var barPct = Math.min(loadPct, 100); var barColor = loadPct '; html += '';

html += ''; html += '' + panelAmps + 'A'; html += 'Panel Capacity'; html += '' + panelCapacityKVA.toFixed(1) + ' kVA'; html += '';

html += ''; html += '' + demandAmps.toFixed(1) + 'A'; html += 'Calculated Demand'; html += '' + demandKVA.toFixed(2) + ' kVA'; html += '';

html += ''; html += '= 0 ? '#27ae60' : '#e74c3c') + ';">' + headroomAmps.toFixed(1) + 'A'; html += 'Available Headroom'; html += '' + headroomKVA.toFixed(2) + ' kVA'; html += '';

html += ''; html += '' + loadPct.toFixed(1) + '%'; html += 'Load Factor'; html += 'of panel capacity'; html += '';

html += '';

// Progress bar html += ''; html += ''; html += '0A80% (' + necSafeAmps.toFixed(0) + 'A NEC limit)' + panelAmps + 'A'; html += ''; html += ''; html += ''; html += ''; html += ''; html += '';

// Status message html += ''; html += '' + statusMsg + ''; html += '';

// NEC 80% rule note html += ''; html += 'NEC 80% Continuous Load Rule: For continuous loads (running 3+ hours), '; html += 'circuits should not exceed 80% of breaker rating. '; html += 'Your NEC-safe limit is ' + necSafeAmps.toFixed(0) + 'A. '; if (necHeadroomAmps >= 0) { html += 'You have ' + necHeadroomAmps.toFixed(1) + 'A of NEC-safe headroom remaining.'; } else { html += 'You are ' + Math.abs(necHeadroomAmps).toFixed(1) + 'A over the NEC-safe limit.'; } html += '';

// Breakdown table html += '#### Load Breakdown '; html += ''; html += ''; html += 'Circuit / Appliance'; html += 'Demand (A)'; html += 'Demand (W)'; html += ''; html += tableRows; html += ''; html += 'TOTAL DEMAND'; html += '' + demandAmps.toFixed(1) + ' A'; html += '' + (demandAmps * voltage).toFixed(0) + ' W'; html += ''; html += '';

// Upgrade recommendation if (loadPct > 80) { html += ''; html += '💡 Upgrade Recommendation:'; if (panelAmps '; }

resultDiv.style.display = 'block'; resultDiv.innerHTML = html; }

#### Formulas Used

Panel Capacity (VA): Panel Amps × Service Voltage

Total Demand (Full Load): Sum of all circuit breaker amperage × voltage

NEC Demand Factor — General Lighting & Outlets (NEC 220.42): First 3,000 VA @ 100% + Next 117,000 VA @ 35% + Remainder @ 25%

NEC Electric Range (NEC Table 220.55): Single range ≤ 12 kW → 8 kW demand; >12 kW → 8 kW + 5% per kW over 12 kW

NEC Electric Dryer (NEC 220.54): 5,000W minimum or nameplate, whichever is greater

NEC Non-Coincident Loads (NEC 220.60): Use the larger of HVAC or electric heat, not both

NEC 80% Continuous Load Rule: Continuous loads ≤ 80% × panel amperage

Load Factor (%): (Total Demand Amps ÷ Panel Amps) × 100

Available Headroom: Panel Amps − Total Demand Amps

#### Assumptions & References

Based on NEC (NFPA 70) Article 220 — Branch-Circuit, Feeder, and Service Load Calculations.
This calculator provides an estimate. Always consult a licensed electrician for actual panel assessments and upgrades.

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References

10 CFR § 434.401 — § 434.401 Electrical power and lighting systems.

Home Electrical Panel Capacity Calculator

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