NEC Box Fill Calculator
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.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; } }
NEC Box Fill Calculator
Calculate the minimum required box volume per NEC Article 314.16 based on conductors, devices, and fittings inside the box.
Conductor Size & Volume Allowance Largest Conductor AWG Size:
18 AWG — 2.0 in³ 16 AWG — 2.25 in³ 14 AWG — 2.5 in³ 12 AWG — 2.75 in³ 10 AWG — 3.0 in³ 8 AWG — 3.5 in³ 6 AWG — 5.0 in³
Volume per conductor per NEC Table 314.16(B)
Conductor Counts
Hot Conductors (count):
Neutral Conductors (count):
Equipment Grounding Conductors (count):
All grounds combined count as ONE conductor (largest ground size used).
Devices & Fittings
Yoke-Mounted Devices (switches, receptacles — count):
Each device = 2× the volume allowance of the largest conductor.
Internal Cable Clamps Present?
Yes — add 1× conductor volume No
One or more internal clamps = 1× volume allowance of largest conductor.
Fixture Studs or Hickeys Present?
Yes — add 1× conductor volume No
One or more studs/hickeys = 1× volume allowance of largest conductor.
Calculate Required Box Fill
#### Formula — NEC Article 314.16(B)
Total Fill (in³) = Conductor Fill + Device Fill + Ground Fill + Clamp Fill + Stud Fill
- Conductor Fill: (Hot count + Neutral count) × Volume per conductor
- Ground Fill: If any grounds present → 1× Volume per conductor (largest ground)
- Device Fill: Device count × 2 × Volume per conductor (largest conductor)
- Clamp Fill: If internal clamps present → 1× Volume per conductor
- Stud Fill: If fixture studs/hickeys present → 1× Volume per conductor
Volume allowances per NEC Table 314.16(B): 18 AWG=2.0 in³ | 16 AWG=2.25 in³ | 14 AWG=2.5 in³ | 12 AWG=2.75 in³ | 10 AWG=3.0 in³ | 8 AWG=3.5 in³ | 6 AWG=5.0 in³
#### Assumptions & References
- Based on NEC 2020 Article 314.16 — Conductors in Outlet, Device, and Junction Boxes.
- All equipment grounding conductors (regardless of count) are counted as a single conductor using the volume of the largest grounding conductor present.
- Each single yoke/strap device counts as two conductors at the volume of the largest conductor connected to that device.
- Conductors that originate and terminate within the box (e.g., pigtails ≤ 6 in) are not counted.
function necCalc() { var resultDiv = document.getElementById("nec-result");
// --- Inputs --- var volPerConductor = parseFloat(document.getElementById("nec-wire-size").value); var hotCount = parseInt(document.getElementById("nec-hot-conductors").value) || 0; var neutralCount = parseInt(document.getElementById("nec-neutral-conductors").value) || 0; var groundCount = parseInt(document.getElementById("nec-ground-conductors").value) || 0; var deviceCount = parseInt(document.getElementById("nec-devices").value) || 0; var hasClamps = parseInt(document.getElementById("nec-internal-clamps").value); var hasStuds = parseInt(document.getElementById("nec-fixture-studs").value);
// --- Validation --- if (hotCount ⚠ All counts must be zero or positive."; return; } if (hotCount + neutralCount + groundCount + deviceCount === 0 && hasClamps === 0 && hasStuds === 0) { resultDiv.innerHTML = "⚠ Please enter at least one conductor, device, or fitting."; return; }
// --- NEC 314.16(B) Calculations --- // (B)(1) Conductor Fill var conductorFill = (hotCount + neutralCount) * volPerConductor;
// (B)(2) Clamp Fill — all internal clamps = 1 conductor volume var clampFill = hasClamps * volPerConductor;
// (B)(3) Support Fittings Fill — studs/hickeys = 1 conductor volume var studFill = hasStuds * volPerConductor;
// (B)(4) Device or Equipment Fill — each device = 2× largest conductor volume var deviceFill = deviceCount * 2 * volPerConductor;
// (B)(5) Equipment Grounding Conductor Fill — all grounds = 1 conductor volume var groundFill = (groundCount > 0) ? volPerConductor : 0;
var totalFill = conductorFill + clampFill + studFill + deviceFill + groundFill;
// --- AWG label for display --- var awgLabels = { "2.0":"18 AWG","2.25":"16 AWG","2.5":"14 AWG", "2.75":"12 AWG","3.0":"10 AWG","3.5":"8 AWG","5.0":"6 AWG" }; var awgLabel = awgLabels[volPerConductor.toString()] || volPerConductor + " in³";
// --- Output --- resultDiv.innerHTML = "### ✓ Required Minimum Box Fill " + "" + "" + "Fill Component" + "Calculation" + "Volume (in³)" + "" + "" + "Conductor Fill (" + awgLabel + ")" + "(" + hotCount + " hot + " + neutralCount + " neutral) × " + volPerConductor + "" + "" + conductorFill.toFixed(2) + "" + "" + "" + "Ground Fill" + "" + (groundCount > 0 ? groundCount + " ground(s) → 1 × " + volPerConductor : "No grounds") + "" + "" + groundFill.toFixed(2) + "" + "" + "" + "Device Fill" + "" + deviceCount + " device(s) × 2 × " + volPerConductor + "" + "" + deviceFill.toFixed(2) + "" + "" + "" + "Clamp Fill" + "" + (hasClamps ? "Present → 1 × " + volPerConductor : "None") + "" + "" + clampFill.toFixed(2) + "" + "" + "" + "Stud / Hickey Fill" + "" + (hasStuds ? "Present → 1 × " + volPerConductor : "None") + "" + "" + studFill.toFixed(2) + "" + "" + "" + "TOTAL REQUIRED BOX VOLUME" + "" + totalFill.toFixed(2) + " in³" + "" + "" + "" + "ⓘ The selected box must have a listed volume ≥ " + totalFill.toFixed(2) + " in³ " + "per NEC 314.16(A). Common metal boxes: 4" square deep = 21.0 in³, " + "4" square shallow = 18.0 in³, single-gang device box = 18.0 in³ (varies by depth)." + "
"; }
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