Quick answer: Adjusted Ampacity = Base Ampacity × Temperature Factor × Current-Carrying Conductor Adjustment Factor. Example: 10 AWG THHN with 90°C insulation, 45°C ambient, and 9 current-carrying conductors gives 40 A × 0.87 × 0.70 = 24.4 A before terminal and conductor-protection checks. Use the Ampacity Calculator when conductor material, terminal rating, rooftop exposure, or multiple conditions must stay with the result.
NEC edition note: NFPA 70 is adopted and amended by jurisdiction. Use the NEC edition enforced by the AHJ for the project location, then verify Article 310 table references, terminal ratings, equipment markings, and local amendments before final conductor or overcurrent decisions.
NEC derating chart/table short answer
Read the derating chart in this order: base ampacity, temperature correction, conductor-count adjustment. For the common 10 AWG THHN example, use the 90°C ampacity basis for adjustment, then apply the temperature and conductor-count factors:
10 AWG THHN adjusted ampacity = 40 A × 0.87 × 0.70 = 24.4 A
The 24.4 A value is the adjusted ampacity screen before terminal limits, small-conductor protection, equipment markings, local amendments, and AHJ review. Use the calculator when the installation has more than this single example.
Calculator Follow-Up
How do I derate conductor ampacity?
Adjusted Ampacity = Base Ampacity × Temperature Factor × Current-Carrying Conductor Adjustment Factor
| Derating Type | Reference Point | Edition Check | Factor Range |
|---|---|---|---|
| Temperature | Article 310 temperature table | Verify adopted NEC edition | 0.41 – 1.08 |
| Conductor Count | Article 310 adjustment factors | Verify adopted NEC edition | 0.35 – 1.00 |
| Rooftop Solar | Rooftop ambient-temperature rule | Verify adopted NEC edition | +14°C to +33°C adder |
| Combined | Multiply factors | Keep terminal limits separate | 0.20 – 1.00 |
→ Use the Ampacity Calculator for automatic derating.
After the Derating Result
Once adjusted ampacity is known, continue the installation workflow instead of stopping at the factor math:
| Next decision | Use |
|---|---|
| Conductor gauge after derating | Wire Size Calculator |
| Physical raceway fill | Conduit Fill Calculator |
| Long-run voltage drop | Voltage Drop Calculator |
| Device size and small-conductor limits | Breaker Sizing Calculator |
| Ambient-temperature record | Temperature Correction Chart |
| Bundling documentation | Conductor Bundling Derating Chart |
| Terminal rating handoff | Terminal Temperature Rating Chart |
| Use this guide as a planning reference. Final conductor and overcurrent selections still need the adopted NEC edition, terminal ratings, equipment markings, local amendments, and AHJ review. |
Why Derate Ampacity?
Heat Is the Enemy
Conductors have maximum operating temperatures. Derating accounts for:
- High ambient temperature - Less heat can be dissipated
- Multiple conductors - Mutual heating reduces capacity
- Rooftop installations - Solar heat increases ambient
Consequences of Not Derating
- Insulation breakdown
- Shortened conductor life
- Fire hazard
- Failed inspection or adopted-code conflict
Temperature Correction Factors
NEC Table 310.15(B)(1) - Temperature Correction
For ambient temperatures other than 30°C (86°F):
| Ambient °C | Ambient °F | 60°C | 75°C | 90°C |
|---|---|---|---|---|
| 21-25 | 70-77 | 1.08 | 1.05 | 1.04 |
| 26-30 | 78-86 | 1.00 | 1.00 | 1.00 |
| 31-35 | 87-95 | 0.91 | 0.94 | 0.96 |
| 36-40 | 96-104 | 0.82 | 0.88 | 0.91 |
| 41-45 | 105-113 | 0.71 | 0.82 | 0.87 |
| 46-50 | 114-122 | 0.58 | 0.75 | 0.82 |
| 51-55 | 123-131 | 0.41 | 0.67 | 0.76 |
| 56-60 | 132-140 | — | 0.58 | 0.71 |
| 61-65 | 141-149 | — | 0.47 | 0.65 |
| 66-70 | 150-158 | — | 0.33 | 0.58 |
| 71-75 | 159-167 | — | — | 0.50 |
| 76-80 | 168-176 | — | — | 0.41 |
Understanding Temperature Ratings
| Rating | Common Insulation Types |
|---|---|
| 60°C | TW, UF |
| 75°C | THW, THWN, RHW, XHHW (wet) |
| 90°C | THHN, THWN-2, XHHW-2, USE-2 |
Important: Even with 90°C wire, if terminals are rated 75°C, use 75°C ampacity!
Rooftop Temperature Adders
NEC 310.15(B)(2) - Rooftop Adders
For conduits exposed to sunlight on rooftops:
| Height Above Roof | Temperature Adder |
|---|---|
| 0" - 1/2" | +33°C (+60°F) |
| 1/2" - 3-1/2" | +22°C (+40°F) |
| 3-1/2" - 12" | +17°C (+30°F) |
| 12" - 36" | +14°C (+25°F) |
Example: 40°C ambient, conduit 2" above roof:
Adjusted ambient = 40°C + 22°C = 62°C
Then use temperature correction table with 62°C.
Current-Carrying Conductor Adjustment Factors
NEC Table 310.15(C)(1) - More Than 3 Conductors
When more than 3 current-carrying conductors are in conduit:
| Number of Conductors | Adjustment Factor |
|---|---|
| 4-6 | 0.80 |
| 7-9 | 0.70 |
| 10-20 | 0.50 |
| 21-30 | 0.45 |
| 31-40 | 0.40 |
| 41+ | 0.35 |
What Counts as Current-Carrying?
Count:
- Phase conductors (all)
- Neutral with harmonics (>50% unbalanced)
- Control circuit conductors if significant
Don't Count:
- Equipment grounding conductors
- Balanced neutral in 3-phase systems
- Spare conductors (not energized)
Combined Derating Example
Step-by-Step Calculation
Given:
- 6 × #8 AWG THHN conductors in conduit
- Ambient temperature 40°C
- Equipment terminals rated 75°C
Solution:
- Base ampacity (NEC Table 310.16):
- #8 THHN at 90°C = 55A
- But terminals 75°C, so use: 50A
- Temperature correction (40°C, 75°C column):
- Factor = 0.88
- Current-carrying conductor adjustment (6 conductors):
- Factor = 0.80
- Combined derating:
Adjusted = 50A × 0.88 × 0.80 = 35.2A
Result: The numeric ampacity screen is 35A per conductor before final equipment and AHJ review.
Quick Reference: Common Derating Scenarios
4-6 Conductors at Various Temperatures
| Wire Size | Base 75°C | 35°C | 40°C | 45°C |
|---|---|---|---|---|
| 12 AWG | 25A | 18.8A | 17.6A | 16.4A |
| 10 AWG | 35A | 26.3A | 24.6A | 23.0A |
| 8 AWG | 50A | 37.6A | 35.2A | 32.8A |
| 6 AWG | 65A | 48.9A | 45.8A | 42.6A |
| 4 AWG | 85A | 63.9A | 59.8A | 55.8A |
Based on: 75°C ampacity × temp factor × 0.80 current-carrying conductor adjustment factor
7-9 Conductors at Various Temperatures
| Wire Size | Base 75°C | 35°C | 40°C | 45°C |
|---|---|---|---|---|
| 12 AWG | 25A | 16.5A | 15.4A | 14.4A |
| 10 AWG | 35A | 23.0A | 21.6A | 20.1A |
| 8 AWG | 50A | 32.9A | 30.8A | 28.7A |
| 6 AWG | 65A | 42.8A | 40.0A | 37.3A |
| 4 AWG | 85A | 55.9A | 52.4A | 48.9A |
Based on: 75°C ampacity × temp factor × 0.70 current-carrying conductor adjustment factor
Special Derating Rules
Continuous Loads (NEC 210.20, 215.3)
For loads operating 3 hours or more continuously (e.g., HVAC, lighting, process equipment):
Conductor ampacity ≥ 125% of continuous load current
Or equivalently — when working from conductor ampacity to maximum allowed load:
Maximum continuous load = conductor adjusted ampacity × 0.80
Important: When both derating and the continuous-load rule apply, the 125% continuous factor is applied after temperature and current-carrying conductor adjustment — it is not a multiplicative derating factor but a minimum conductor sizing requirement:
Required conductor ampacity = (Continuous load × 1.25 + Non-continuous load) ÷ (Temp Factor × Conductor Adjustment Factor)
Example: 20 A continuous load, 40°C ambient, 6 conductors in conduit:
- Required base ampacity (before derating): (20 × 1.25) = 25 A minimum
- Derate: 25 A ÷ (0.88 × 0.80) = 25 A ÷ 0.704 = 35.5 A required
- Select: 8 AWG THHN (50 A at 75°C) — derated to 35.2 A ⊼ 35.5 A, so select 6 AWG (65 A) → derated to 45.8 A > 25 A ✓
Neutral Conductor Derating
3-Phase, 4-Wire Systems:
- Balanced loads: Neutral doesn't count
- 50%+ neutral current: Count as current-carrying
- Harmonic loads (VFDs, computers): Always count neutral
Bundled Cables
For cables bundled >24" without spacing:
- Apply the same current-carrying conductor adjustment factors when bundled cables require that review
- 24" minimum spacing = no derating required
Worked Examples
Example 1: Hot Attic Installation
Given:
- 3 × #10 AWG THHN in attic (50°C ambient)
- Feeding 30A circuit
Solution:
- Base ampacity #10 at 90°C: 40A
- Temperature factor (50°C, 90°C): 0.82
- Current-carrying conductor adjustment (3 conductors): 1.00
- Adjusted: 40A × 0.82 = 32.8A ✓
Result: 32.8A > 30A, so this numeric ampacity screen passes before final equipment and AHJ review.
Example 2: Rooftop HVAC
Given:
- 3 × #6 AWG THHN on roof
- Conduit 1" above roof deck
- 35°C ambient temperature
- 40A load
Solution:
- Rooftop adder (0-1/2" to 3.5"): +22°C
- Effective ambient: 35 + 22 = 57°C
- Base ampacity #6 at 90°C: 75A
- Temperature factor (57°C, 90°C): ~0.68
- Adjusted: 75A × 0.68 = 51A ✓
Result: 51A > 40A, so this numeric ampacity screen passes before final equipment and AHJ review.
Example 3: Multiple Circuit Conduit
Given:
- 12 current-carrying conductors
- Normal ambient (30°C)
- Need 20A per circuit
- Terminals rated 75°C
Solution:
- Current-carrying conductor adjustment factor (10-20): 0.50
- Temperature factor (30°C): 1.00
- Required base ampacity: 20A / 0.50 = 40A
- Select: #8 AWG (50A at 75°C) ✓
Verify: 50A × 1.00 × 0.50 = 25A > 20A ✓
Example 4: Hot Location with Many Wires
Given:
- 8 conductors in conduit
- 45°C ambient
- Need 15A per circuit
Solution:
- Current-carrying conductor adjustment factor (7-9): 0.70
- Temp factor (45°C, 75°C): 0.82
- Combined: 0.70 × 0.82 = 0.574
- Required base: 15A / 0.574 = 26.1A
- Select: #10 AWG (35A at 75°C)
Verify: 35A × 0.574 = 20.1A > 15A ✓
Derating Calculation Workflow
For Any Installation:
- Determine load current
- Identify conditions:
- Ambient temperature
- Number of conductors
- Continuous vs. non-continuous
- Terminal ratings
- Look up factors:
- Temperature correction
- Current-carrying conductor adjustment
- Calculate:
Required Ampacity = Load / (Temp × Conductor Adjustment)
- Select conductor from NEC tables
- Verify the calculation both ways
Common Mistakes to Avoid
| Mistake | Why It's Wrong | Correct Approach |
|---|---|---|
| Ignoring terminal ratings | May exceed equipment limits | Use lower of wire/terminal rating |
| Not counting harmonics | Neutral may overheat | Count neutral with harmonic loads |
| Forgetting rooftop adder | Actual temp much higher | Add temperature per table |
| Using 90°C for all | Terminals usually 75°C | Match to terminal rating |
Related Calculators
| Calculator | Use When... |
|---|---|
| Ampacity Calculator | Full derating calculations |
| Wire Size Calculator | Selecting conductor gauge after derating is known |
| Conduit Fill Calculator | Physical raceway fill |
| Voltage Drop Calculator | Long run sizing |
Summary
Temperature Correction:
- Use the temperature-correction table from the adopted NEC edition
- Match to insulation temperature rating
- Add rooftop temperature where the adopted edition and installation conditions require it
Current-Carrying Conductor Adjustment:
- 4-6 conductors: 80%
- 7-9 conductors: 70%
- 10-20 conductors: 50%
Combined:
Adjusted = Base × Temp Factor × Conductor Adjustment Factor
FAQ
When do I need to derate conductors?
Derate when: ambient exceeds 30°C, more than 3 current-carrying conductors in conduit, rooftop installation, or continuous loads (apply 125% rule).
Do I derate ground wires?
No, equipment grounding conductors are not current-carrying under normal conditions and do not count toward ampacity adjustment or need derating.
Which derating do I apply first?
Apply all factors by multiplication - the order doesn't matter mathematically. Calculate: Base × Temp × Conductor Adjustment = adjusted ampacity.
Can I use 90°C ampacity with 75°C terminals?
No. The limiting factor is the terminal. Use 75°C ampacity values, but you may use 90°C temperature correction factors since the wire itself can handle higher temps.
How do I handle neutral conductors?
For balanced 3-wire circuits, neutral doesn't count. For 4-wire with harmonic loads (LED, computers, VFDs), count neutral as current-carrying.