Derating Calculator — Free Online Calculator

How to Use This Calculator

Enter the base ampacity of the conductor and select the number of current-carrying conductors bundled together.

The Formula Explained

NEC 310.15(C)(1) requires reducing conductor ampacity when more than 3 current-carrying conductors are bundled in a raceway or cable. More conductors generate more heat, requiring each to carry less current.

Conductor Derating: Real-World Ampacity

The ampacity values in NEC Table 310.16 are based on standardized test conditions: 30°C ambient temperature (86°F) and no more than 3 current-carrying conductors in a single raceway. Real installations often violate both assumptions. Attics exceed 50°C in summer, commercial conduits often contain 6+ current-carrying conductors, and mechanical rooms run 40°C+ year-round. Derating factors adjust the base ampacity to account for these conditions, giving the actual safe current the conductor can carry without exceeding its insulation temperature rating.

The two main derating factors are ambient temperature (NEC Table 310.15(B)(2)(a)) and bundling (NEC Table 310.15(B)(3)(a)). They multiply: if ambient gives 0.87 factor and bundling gives 0.80 factor, total adjusted ampacity is 0.87 × 0.80 = 0.696 × base value. A 12 AWG THHN copper at 90°C base ampacity of 30A becomes 30 × 0.696 = 20.9 A adjusted. Round down to next standard breaker protection: 20A (and the circuit still needs the standard 20A breaker, so 12 AWG THHN is just barely adequate under these conditions).

Worked Example: Attic Feeder Derating

A 60A subfeeder runs through an attic in Phoenix, Arizona. Summer attic temperature: 55°C (131°F). Conductor: 6 AWG THHN copper (90°C insulation), 3 current-carrying conductors (black, red, white neutral — the neutral counts because loads are unbalanced line-to-neutral).

Base ampacity of 6 AWG THHN at 90°C: 75 amps (NEC Table 310.16). Ambient derating at 55°C: 0.71 factor (Table 310.15(B)(2)(a) for 90°C insulation). No bundling derating needed (only 3 CCCs). Adjusted ampacity: 75 × 0.71 = 53.25 A.

Problem: the 60A breaker requires conductor ampacity of 60A minimum. Adjusted 53.25 A is below 60A — this is a code violation. Solutions: (a) upsize to 4 AWG THHN (95A base × 0.71 = 67.5A adjusted, acceptable), (b) reroute the conduit below the insulation where it is cooler, or (c) reduce the feeder to 50A max (53.25A rounds down to 50A standard size).

This is a common problem in Southwestern US residential installations and is often missed by DIY installers who do not know the derating rules. Professional installers in hot climates typically use 90°C insulation and upsize conductors accordingly.

Worked Example: Commercial Branch Circuits in Conduit

A commercial installation has 8 current-carrying conductors in one 1-inch EMT conduit in a 30°C ambient mechanical room. Each circuit is protected by 20A breaker and wired with 12 AWG THHN.

Base ampacity: 12 AWG THHN at 90°C = 30A. No ambient derating (30°C = base). Bundling derating: 8 CCCs = 0.70 factor (NEC Table 310.15(B)(3)(a), 7-9 conductors). Adjusted ampacity: 30 × 0.70 = 21 A.

Just above the 20A breaker rating, so 12 AWG is acceptable for 20A protection. But add any ambient above 30°C and it fails. If the mechanical room ever hits 40°C (common near boilers or in equipment rooms), ambient factor is 0.91, and adjusted ampacity becomes 30 × 0.70 × 0.91 = 19.1 A — below 20A, code violation.

For bundled commercial circuits in conduit, 10 AWG is often the better choice even though 12 AWG technically works at base conditions. The larger wire provides derating headroom for ambient temperature variations and future circuit additions.

Derating Mistakes

1. Counting the neutral wrong. Neutrals carrying unbalanced current count as current-carrying conductors. In 3-phase 4-wire systems with nonlinear loads (harmonic currents), the neutral may need to be counted even if balanced.

2. Using the wrong ambient temperature column. Tables have separate columns for 60°C, 75°C, and 90°C insulation. Use the column matching your conductor insulation, not the conductor size alone.

3. Forgetting that the 60°C column often applies. Even if conductor insulation is 90°C, the terminals on breakers and equipment are typically 60°C (small) or 75°C (100A+). The final ampacity is limited by the lower of conductor derated value and terminal rating.

4. Not applying derating to the 125% continuous load rule. The 125% continuous load calculation uses the derated ampacity, not the base ampacity. A derated 19A conductor cannot serve a 16A continuous load (16 × 1.25 = 20A required).

5. Ignoring harmonic currents on neutral. Nonlinear loads (LED drivers, VFDs, computer power supplies) generate triplen harmonics that add up in the neutral of a 3-phase system. NEC 310.15(E) provides guidance on neutral derating for these conditions.

Derating Factor Quick Reference

Ambient temperature correction for 75°C insulation (NEC Table 310.15(B)(2)(a)):

Ambient 26-30°C: 1.00 (no derating).

Ambient 31-35°C: 0.94.

Ambient 36-40°C: 0.88.

Ambient 41-45°C: 0.82.

Ambient 46-50°C: 0.75.

Ambient 51-55°C: 0.67.

Ambient 56-60°C: 0.58.

Ambient 61-70°C: 0.33.

Conductor count correction (NEC Table 310.15(B)(3)(a)):

1-3 current-carrying conductors: 1.00 (no derating).

4-6 CCCs: 0.80.

7-9 CCCs: 0.70.

10-20 CCCs: 0.50.

21-30 CCCs: 0.45.

31-40 CCCs: 0.40.

41+ CCCs: 0.35.

NEC References

NEC 310.15 — Ampacity adjustments and correction factors. NEC Table 310.15(B)(2)(a) — Ambient temperature correction factors. NEC Table 310.15(B)(3)(a) — Adjustment factors for more than 3 current-carrying conductors. NEC 310.15(E) — Neutral conductor as current-carrying conductor in harmonic-rich systems. NEC 310.16 — Main ampacity table (base values before derating).

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