Wire Ampacity Calculator - Current Carrying Capacity
Calculate the current carrying capacity (ampacity) of electrical wires with temperature and conduit fill derating. NEC Table 310.16 compliant calculations for all conductor types and conditions.
Quick Reference: Common Wire Ampacities at 75°C
Copper Wire:
- 14 AWG:20A*
- 12 AWG:25A*
- 10 AWG:35A*
- 8 AWG:50A
- 6 AWG:65A
Aluminum Wire:
- 12 AWG:20A*
- 10 AWG:30A*
- 8 AWG:40A
- 6 AWG:50A
- 4 AWG:65A
Key Derating:
- 104°F: × 0.88
- 122°F: × 0.75
- 4-6 wires: × 0.80
- 7-9 wires: × 0.70
- *240.4(D) limits
Important: *Small conductor rule (NEC 240.4(D)) limits: 14 AWG to 15A, 12 AWG to 20A, 10 AWG to 30A maximum overcurrent protection regardless of calculated ampacity. Always apply derating factors for actual conditions.
Wire Ampacity Calculator
Calculate the current carrying capacity for copper and aluminum conductors
Input Parameters
Complete Guide to Wire Ampacity
Understanding Ampacity
Ampacity is the maximum current a conductor can carry continuously without damaging its insulation. This value depends on the conductor material, insulation temperature rating, ambient temperature, and the number of conductors bundled together. The NEC provides detailed tables and adjustment factors to ensure safe installations.
Ampacity Calculation Steps
- 1.Base Ampacity: Start with Table 310.16 for the conductor size and temperature rating.
- 2.Temperature Correction: Apply factors from Table 310.15(B)(1) for ambient above 86°F.
- 3.Conductor Adjustment: Apply Table 310.15(C)(1) for more than 3 conductors.
- 4.Final Ampacity: Base × Temperature × Fill = Derated Ampacity.
Temperature Ratings Explained
60°C (140°F) Rating
Older wire types like TW and UF. NM (Romex) cable limited to 60°C ampacity. Lower ampacity but adequate for most residential branch circuits.
75°C (167°F) Rating
Standard for most modern installations. THW, THWN, XHHW in wet locations. Most circuit breakers and equipment rated for 75°C maximum.
90°C (194°F) Rating
THHN, THWN-2, XHHW-2. Higher ampacity but limited by termination temperature. Useful for derating calculations starting from higher base.
The 75°C Rule
Even with 90°C rated wire, you must use 75°C ampacity at device terminations unless all components (breakers, lugs, devices) are rated and listed for 90°C. This is rarely the case in standard installations.
Real-World Ampacity Derating Examples
| Scenario | Temperature | Conductors | Wire | Base | Temp Factor | Fill Factor | Final | Notes |
|---|---|---|---|---|---|---|---|---|
| Attic wiring in summer | 140°F (60°C) | 3 | 12 AWG THHN | 30A (90°C) | 0.58 | 1.00 | 17.4A | Severe derating in hot attics |
| Conduit with 6 circuits | 86°F (30°C) | 12 | 10 AWG THHN | 40A (90°C) | 0.94 | 0.50 | 18.8A | 50% derating for 10-20 conductors |
| Outdoor conduit in sun | 95°F + 20°F sun | 6 | 8 AWG THHN | 55A (90°C) | 0.76 | 0.80 | 33.4A | Add 20°F for sunlight exposure |
| Underground direct burial | 68°F (20°C) | 3 | 2 AWG USE | 130A (90°C) | 1.04 | 1.00 | 135A | Higher ampacity in cool earth |
| Industrial high ambient | 104°F (40°C) | 9 | 4 AWG THHN | 95A (90°C) | 0.87 | 0.70 | 57.9A | Multiple derating factors apply |
Critical: These examples show how severely ampacity can be reduced. A 12 AWG wire rated for 30A at 90°C can be derated to just 17.4A in a hot attic—below the 20A circuit rating!
Temperature Correction Factors (NEC Table 310.15(B)(1))
| Ambient Temp | Correction Factors for Conductor Temperature Rating | |||||
|---|---|---|---|---|---|---|
| 60°C | 75°C | 90°C | 110°C | 125°C | 150°C | |
| 50°F(10°C) | 1.29 | 1.20 | 1.15 | 1.13 | 1.11 | 1.10 |
| 59°F(15°C) | 1.22 | 1.15 | 1.12 | 1.10 | 1.09 | 1.08 |
| 68°F(20°C) | 1.15 | 1.11 | 1.08 | 1.07 | 1.06 | 1.05 |
| 77°F(25°C) | 1.08 | 1.05 | 1.04 | 1.04 | 1.03 | 1.03 |
| 86°F(30°C) | 1.00 | 1.00 | 1.00 | 1.00 | 1.00 | 1.00 |
| 95°F(35°C) | 0.91 | 0.94 | 0.96 | 0.97 | 0.97 | 0.98 |
| 104°F(40°C) | 0.82 | 0.88 | 0.91 | 0.93 | 0.94 | 0.95 |
| 113°F(45°C) | 0.71 | 0.82 | 0.87 | 0.89 | 0.91 | 0.92 |
| 122°F(50°C) | 0.58 | 0.75 | 0.82 | 0.86 | 0.87 | 0.89 |
| 131°F(55°C) | 0.41 | 0.67 | 0.76 | 0.82 | 0.84 | 0.86 |
| 140°F(60°C) | — | 0.58 | 0.71 | 0.77 | 0.80 | 0.83 |
Hot Location Examples
- • Attics in summer: 120-140°F
- • Boiler rooms: 100-120°F
- • Outdoor desert: 110-125°F
- • Near heat sources: Add 20-40°F
Cool Location Benefits
- • Underground (68°F): +8% ampacity
- • Air-conditioned spaces: No derating
- • Outdoor winter: Potential increase
- • Water-cooled: Special calculations
Conduit Fill Adjustment Factors (NEC 310.15(C)(1))
| Number of Conductors | Adjustment Factor | Notes |
|---|---|---|
| 3 or less | 100% | No derating required |
| 4-6 | 80% | Minimal heat buildup |
| 7-9 | 70% | Moderate derating |
| 10-20 | 50% | Significant derating |
| 21-30 | 45% | Major heat concerns |
| 31-40 | 40% | Maximum fill conditions |
| 41+ | 35% | Special calculations required |
Counting Current-Carrying Conductors
Don't Count:
- • Equipment grounding conductors
- • Neutral in balanced 3-phase
- • Travelers between 3-way switches
Must Count:
- • All ungrounded (hot) conductors
- • Neutral with unbalanced loads
- • Neutral with harmonic loads
Example: Six 20A circuits in one conduit = 12 current-carrying conductors (not counting grounds). Apply 50% derating factor.
Special Ampacity Conditions
Rooftop installations
Add temperature adder based on height above roof
- 0-1/2 inch: Add 33°C (60°F)
- 1/2-3.5 inches: Add 22°C (40°F)
- 3.5-12 inches: Add 17°C (30°F)
- 12-36 inches: Add 14°C (25°F)
Wet locations
May require different insulation types
- THWN/THWN-2 for wet locations
- No ampacity reduction for wet-rated
- Consider corrosion protection
- Verify termination compatibility
High altitude
Derating may be required above 3000 feet
- Reduced cooling efficiency
- Consult manufacturer data
- Consider oversizing conductors
- Motor loads especially affected
Harmonic currents
Non-linear loads affect neutral sizing
- Neutral carries harmonic currents
- Count neutral as current-carrying
- Consider 200% neutral sizing
- Common with LED and electronic loads
Wire Insulation Types and Temperature Ratings
| Type | Dry Rating | Wet Rating | Application | Notes |
|---|---|---|---|---|
| TW | 60°C | 60°C | Residential wiring in dry locations | Older installations |
| THW | 75°C | 75°C | General purpose, wet or dry | Common in commercial |
| THWN | 75°C | 75°C | Conduit and raceway, wet/dry | Most common today |
| THHN | 90°C | — | Dry and damp locations only | High temperature rating |
| THWN-2 | 90°C | 90°C | All locations, high temp | Dual rated THHN/THWN |
| XHHW | 90°C | 75°C | Industrial, wet/dry locations | Chemical resistant |
| XHHW-2 | 90°C | 90°C | All locations, industrial | Best for harsh conditions |
| USE-2 | 90°C | 90°C | Underground service entrance | Direct burial rated |
Pro Tip: THWN-2 (dual rated THHN/THWN) is the most versatile choice for modern installations. It provides 90°C dry rating for ampacity calculations and maintains rating in wet locations.
Common Ampacity Mistakes to Avoid
Using 90°C ampacity with 75°C terminals
Consequence: Overheating at terminations, equipment damage
Solution: Always use the lowest temperature rating in the circuit
Forgetting ambient temperature derating
Consequence: Wire insulation breakdown, fire hazard
Solution: Apply temperature correction for ambients above 86°F
Not counting neutral as current-carrying
Consequence: Overheating with non-linear loads
Solution: Count neutral with unbalanced or harmonic loads
Ignoring conduit fill derating
Consequence: Excessive heat buildup in raceways
Solution: Apply Table 310.15(C)(1) for more than 3 conductors
Using wrong table for cable assemblies
Consequence: Incorrect ampacity values
Solution: Use 334.80 for NM cable, not Table 310.16
Not considering continuous loads
Consequence: Conductor overheating during extended operation
Solution: Apply 125% factor or use 90°C ampacity at 75°C terminals
Practical Ampacity Examples
Residential Kitchen
12 AWG THWN in conduit, 6 circuits (12 conductors), 86°F ambient
⚠️ Below 20A breaker rating! Need larger wire or fewer circuits per conduit.
Underground Feeder
2/0 AWG USE-2 direct burial, 3 conductors, 68°F earth
✓ Cool earth increases ampacity above table value.
Rooftop Equipment
6 AWG THWN in conduit on roof, 3 conductors, 95°F + 33°F adder
⚠️ Significant derating on hot roofs. Consider larger wire or different routing.
Ampacity Frequently Asked Questions
Q:What is ampacity in electrical terms?
A: Ampacity is the maximum current in amperes that a conductor can carry continuously under specific conditions without exceeding its temperature rating. It depends on conductor material, insulation type, ambient temperature, and installation method.
Q:How do I derate wire for temperature?
A: Multiply the table ampacity by the temperature correction factor. For example, 12 AWG THHN (30A at 90°C) in 104°F ambient: 30A × 0.87 = 26.1A. Always use the correction factor for your wire's temperature rating.
Q:What's the ampacity of 12 AWG THHN wire?
A: 12 AWG THHN has 30A ampacity at 90°C rating, but when terminated on 75°C devices (standard breakers), you must use 25A from the 75°C column. For 15A or 20A circuits, use the small conductor rule in 240.4(D).
Q:How many conductors before derating is required?
A: More than 3 current-carrying conductors in a raceway or cable require derating per NEC 310.15(C)(1). Equipment grounding conductors don't count. Neutrals may or may not count depending on the load type.
Q:Does conduit in sunlight require derating?
A: Yes, add temperature adders per Table 310.15(B)(2)(c): rooftop conduits need 33°F to 60°F added to ambient temperature based on height above roof. This significantly impacts ampacity in hot climates.
Q:What's the difference between 75°C and 90°C ampacity?
A: 90°C ampacity is about 15-20% higher than 75°C, but you can only use it when all components are 90°C rated. Most devices are 75°C rated, limiting you to 75°C ampacity at terminations even with 90°C wire.
Q:How do I calculate ampacity for parallel conductors?
A: Add the ampacities of individual conductors, then apply any derating factors. For example, two parallel 1/0 AWG copper (150A each at 75°C) = 300A total before derating. All parallel conductors must be identical.
Q:Does wire ampacity change with voltage?
A: No, ampacity is independent of voltage. A 12 AWG wire has the same 25A ampacity (at 75°C) whether used for 120V or 480V. Voltage affects insulation requirements, not current capacity.
Q:What is continuous load derating?
A: Continuous loads (3+ hours) require conductors sized at 125% of the load per NEC 210.19(A). Alternatively, you can use the 90°C ampacity for derating calculations when terminated on 75°C devices.
Q:How does altitude affect ampacity?
A: High altitude reduces cooling efficiency. Above 3000 feet, consider derating factors: 3000-5000 ft (0.96), 5000-7000 ft (0.93), 7000-9000 ft (0.90). Consult manufacturer data for specific requirements.
Q:Can I use NM cable ampacity from Table 310.16?
A: No, NM (Romex) cable ampacity comes from the 60°C column of Table 310.16 per NEC 334.80, not the 90°C rating of the conductors. This significantly reduces ampacity compared to individual THHN wires.
Q:How do harmonics affect ampacity?
A: Harmonic currents from non-linear loads (LED, electronic ballasts) cause neutral current even with balanced phases. Count the neutral as a current-carrying conductor for derating when harmonics exceed 50% of load.
Q:What's the ampacity of aluminum vs copper?
A: Aluminum has about 61% of copper's conductivity. For the same ampacity, aluminum wire must be 1-2 sizes larger. Example: 1 AWG aluminum (130A) equals 2 AWG copper (115A) at 75°C.
Q:Do I derate for conductors in separate conduits?
A: No, derating only applies to conductors in the same raceway or cable. Conductors in separate conduits dissipate heat independently. However, bundled conduits may require special consideration.
Q:How does wet location affect ampacity?
A: Wet location doesn't directly reduce ampacity if using wet-rated insulation (THWN, XHHW). However, the wet rating temperature may be lower (75°C vs 90°C for XHHW), affecting available ampacity.
Q:What's the 10% rule for ampacity?
A: NEC 310.15(A)(2) Exception: If the derated ampacity doesn't correspond to a standard overcurrent device rating, you can use the next higher standard size if the load doesn't exceed the derated ampacity.
Q:Can I use 90°C ampacity for voltage drop?
A: Yes, you can use 90°C ampacity for voltage drop calculations since this relates to conductor resistance, not termination temperature. This is beneficial for long runs where voltage drop controls sizing.
Q:How do I size conductors for motor loads?
A: Use 125% of motor full-load current from NEC tables, not nameplate. Apply this before any derating. For multiple motors, size for 125% of largest plus 100% of others.
Q:Does burial depth affect underground ampacity?
A: Yes, Table 310.16 assumes 30-inch burial depth. Shallower burial may require derating due to higher earth temperature. Deeper burial may allow higher ampacity in some conditions.
Q:What is ampacity correction vs adjustment?
A: Correction factors apply to ambient temperature (Table 310.15(B)(1)). Adjustment factors apply to number of conductors (Table 310.15(C)(1)). Both multiply together with base ampacity for final value.