Wire & Cable calculator
Cable Bundling Derating Calculator
Professional cable bundling derating calculator per NEC 310.15(C)(1). Calculates adjusted ampacity when more than 3 current-carrying conductors are installed together in a raceway, cable, or bundle. Applies both bundling adjustment factors and NEC Table 310.15(B)(1) temperature correction factors for ambient temperatures other than 30°C (86°F). Supports copper and aluminum conductors with 60°C, 75°C, and 90°C insulation ratings.
Updated July 10, 2026
Example Calculations
How to Use
NEC Conductor Derating: Why, When, and How
When current-carrying conductors are bundled together, each conductor generates heat (I²R losses) that warms its neighbors. More conductors in a confined space means more mutual heating and less ability to dissipate that heat to the surrounding air. NEC 310.15(C)(1) requires reducing — "derating" — the allowable ampacity to prevent insulation damage and fire hazards.
NEC 310.15(C)(1) Adjustment Factors
| Current-Carrying Conductors | Adjustment Factor | Effective Ampacity Loss |
|---|---|---|
| 1–3 | 1.00 (100%) | None — baseline condition |
| 4–6 | 0.80 (80%) | 20% reduction |
| 7–9 | 0.70 (70%) | 30% reduction |
| 10–20 | 0.50 (50%) | 50% reduction |
| 21–30 | 0.45 (45%) | 55% reduction |
| 31–40 | 0.40 (40%) | 60% reduction |
| 41+ | 0.35 (35%) | 65% reduction |
Temperature Correction Factors — NEC Table 310.15(B)(1)
Ambient temperature above 30°C (86°F) further reduces allowable ampacity. Apply both factors when conditions require:
| Ambient °C (°F) | 60°C Wire | 75°C Wire | 90°C Wire |
|---|---|---|---|
| 21–25°C (70–77°F) | 1.08 | 1.05 | 1.04 |
| 26–30°C (79–86°F) | 1.00 | 1.00 | 1.00 |
| 31–35°C (88–95°F) | 0.91 | 0.94 | 0.96 |
| 36–40°C (97–104°F) | 0.82 | 0.88 | 0.91 |
| 41–45°C (106–113°F) | 0.71 | 0.82 | 0.87 |
| 46–50°C (115–122°F) | 0.58 | 0.75 | 0.82 |
| 51–55°C (124–131°F) | 0.41 | 0.67 | 0.76 |
The Combined Derating Formula
Derated Ampacity = Base Ampacity × Bundling Factor × Temperature Factor
Worked Example: Control Panel with 12 Current-Carrying Conductors at 45°C
An industrial control panel contains 12 current-carrying #10 AWG THHN conductors. The panel interior reaches 45°C (113°F) during peak operation.
- Base ampacity: #10 THHN copper at 90°C column = 40A (NEC Table 310.16)
- Bundling factor: 12 conductors → 0.50 (50%)
- Temperature factor: 45°C for 90°C wire → 0.87
- Derated ampacity: 40A × 0.50 × 0.87 = 17.4A
This is a dramatic reduction — from 40A to 17.4A, a 56.5% loss. The 90°C wire rating helps here: if using 75°C-rated THWN wire (base 35A), the derated value would be 35 × 0.50 × 0.82 = 14.4A — even worse. This is why the NEC 310.15(B)(2) "90°C trick" is so valuable: rate the wire at the 90°C column for derating purposes, then limit the final ampacity to the 75°C terminal rating.
What Counts as a "Current-Carrying Conductor"?
This is the most common source of confusion in derating calculations:
| Conductor Type | Count It? | NEC Reference |
|---|---|---|
| Phase conductors (L1, L2, L3) | ✅ Always | 310.15(F) |
| Neutral — balanced linear loads | ❌ No | 310.15(F)(1) |
| Neutral — nonlinear loads (3rd harmonic) | ✅ Yes | 310.15(F)(2) |
| Equipment grounding conductor | ❌ No | 310.15(F) |
| Control wires (Class 1) | ✅ Yes | 310.15(C)(1) |
| Travelers (3-way/4-way switches) | ✅ Yes | Only one counts per switch loop |
Bundles Shorter Than 24 Inches
NEC 310.15(C)(1) exempts conductor bundles that are 24 inches (600mm) or shorter from derating. This is often relevant at panel boards, junction boxes, and wireways where conductors are bundled only at entry/exit points. However, the 24-inch exemption does not apply in raceways — if conductors are in conduit, they must be derated regardless of conduit length.
Common Applications
More applications. Open to review 5 additional use cases.
Frequently Asked Questions
When do I need to derate cables for bundling per NEC?
Per NEC 310.15(C)(1), ampacity adjustment is required whenever more than 3 current-carrying conductors are installed together in a raceway, cable, cable tray, or earth (direct burial). Bundles in free air shorter than 24 inches (600mm) are exempt — this commonly applies at panel boards and junction boxes where conductors are grouped only briefly. In conduit, the derating applies regardless of conduit length. Important: only count current-carrying conductors — equipment grounding conductors and neutral conductors carrying only unbalanced load current are excluded from the count.
Do I count neutral conductors for bundling derating?
It depends on the load type per NEC 310.15(F). For balanced linear loads (resistive heaters, incandescent lighting, motors), the neutral carries only the unbalanced current and does NOT count. For nonlinear loads that generate significant third-harmonic current (LED drivers, computer power supplies, VFDs), the neutral carries the sum of the third harmonics from all three phases and MUST be counted. In a building with predominantly fluorescent/LED lighting or computer loads, the neutral often carries more current than any phase conductor. When in doubt about the load type, count the neutral — it's conservative and avoids code violations.
Can I use the 90°C wire rating for derating and then limit to 75°C for terminals?
Yes — this is the "90°C trick" and it's fully code-compliant per NEC 310.15(B)(2). Start with the 90°C column ampacity from NEC Table 310.16 (e.g., #10 THHN = 40A), apply bundling and temperature correction factors, then verify the result does not exceed the 75°C column ampacity (30A for #10). Example: #10 THHN with 9 conductors at 40°C → 40 × 0.70 × 0.91 = 25.5A. Since 25.5A < 30A (75°C limit), the final ampacity is 25.5A. Without the 90°C trick using 75°C rating directly: 30 × 0.70 × 0.88 = 18.5A — the 90°C trick gives you 38% more ampacity.
How does cable tray derating differ from conduit derating?
Cable tray derating (NEC 392.80) uses different rules than conduit derating (310.15(C)). In cable trays, single-conductor cables are derated based on tray width and cable spacing, not by conductor count. Multiconductor cable trays follow NEC 392.80(A)(2) with specific fill limitations. Key differences: ladder-type cable trays with maintained spacing between cables often allow higher ampacities than conduit; ventilated channel trays perform better than solid-bottom trays; covered trays may require additional derating. Cable tray derating is typically more favorable than conduit derating because cables have more air circulation and heat dissipation area.
What happens if I need to apply both bundling and temperature derating simultaneously?
Multiply both factors together — they compound. This is the worst-case scenario common in hot environments with dense wiring (rooftop conduit, hot industrial facilities, engine rooms). Example: 6 current-carrying #6 AWG THHN conductors at 50°C ambient → base 75A (90°C column) × 0.80 (bundling) × 0.82 (temperature) = 49.2A. The combined derating reduces ampacity by 34.4%. To mitigate: (1) use 90°C rated wire to start with higher base ampacity; (2) reduce the number of conductors per raceway by adding parallel conduit runs; (3) use larger wire sizes; (4) add ventilation to reduce ambient temperature. The calculator applies both factors automatically and shows both individual and combined impacts.
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