Wire Ampacity Calculator
// CALCULATE MAXIMUM CURRENT CARRYING CAPACITY BASED ON NEC TABLE 310.16 WITH TEMPERATURE AND BUNDLING DERATING FACTORS //
Allowable Ampacities of Insulated Conductors (NEC Table 310.16)
Based on not more than three current-carrying conductors in raceway, cable, or earth, ambient temperature of 30°C (86°F).
| AWG/kcmil | Copper | Aluminum | ||||
|---|---|---|---|---|---|---|
| 60°C | 75°C | 90°C | 60°C | 75°C | 90°C | |
| 14 AWG | 15 | 20 | 25 | - | - | - |
| 12 AWG | 20 | 25 | 30 | 15 | 20 | 25 |
| 10 AWG | 30 | 35 | 40 | 25 | 30 | 35 |
| 8 AWG | 40 | 50 | 55 | 35 | 40 | 45 |
| 6 AWG | 55 | 65 | 75 | 40 | 50 | 55 |
| 4 AWG | 70 | 85 | 95 | 55 | 65 | 75 |
| 2 AWG | 95 | 115 | 130 | 75 | 90 | 100 |
| 1 AWG | 110 | 130 | 145 | 85 | 100 | 115 |
| 1/0 AWG | 125 | 150 | 170 | 100 | 120 | 135 |
| 2/0 AWG | 145 | 175 | 195 | 115 | 135 | 150 |
| 3/0 AWG | 165 | 200 | 225 | 130 | 155 | 175 |
| 4/0 AWG | 195 | 230 | 260 | 150 | 180 | 205 |
Conductor Bundling Adjustment Factors (NEC 310.15(C)(1))
1-3
Conductors
100%
4-6
Conductors
80%
7-9
Conductors
70%
10-20
Conductors
50%
21-30
Conductors
45%
31-40
Conductors
40%
41+
Conductors
35%
* Applies to more than three current-carrying conductors in a raceway or cable. Neutral conductors are not counted if they carry only unbalanced current.
What is Wire Ampacity?
Ampacity, short for ampere capacity, is the maximum electrical current a conductor can carry continuously without exceeding its temperature rating. This is critical for electrical safety—exceeding ampacity causes overheating, melted insulation, and fire hazards.
Overheating Risk
Exceeding ampacity melts insulation and creates fire hazards
Temperature Based
Higher insulation temp ratings allow more current
NEC Compliant
Table 310.16 is the primary ampacity reference
NEC Table 310.16 Base Conditions
The NEC Table 310.16 values are based on specific conditions. When your installation differs, apply correction factors.
| Condition | Base Assumption | If Different |
|---|---|---|
| Conductor Count | ≤3 current-carrying conductors | Apply bundling derating factors |
| Ambient Temperature | 30°C (86°F) | Apply temperature correction factors |
| Installation Type | Raceway, cable, or direct burial | Free air has higher ampacity (Table 310.17) |
Insulation Temperature Ratings Explained
60°C
(140°F)
Types: TW, UF
Lowest rating, dry locations only
Limited modern applications
75°C
(167°F)
Types: THW, THWN, XHHW
Most common for commercial
Standard terminal rating
90°C
(194°F)
Types: THHN, THWN-2
Highest ampacity values
Often limited by 75°C terminals
Important: Terminal Temperature Limitation
Even with 90°C wire, NEC 110.14(C) limits ampacity to the lowest temperature rating in the circuit. Most equipment terminals are rated 75°C, so use 75°C column values unless terminals are rated higher.
Ambient Temperature Correction Factors
Base ampacity assumes 30°C (86°F) ambient. Apply these correction factors from NEC Table 310.15(B)(1) for different temperatures:
| Ambient Temp | 60°C Wire | 75°C Wire | 90°C Wire |
|---|---|---|---|
| 21-25°C (70-77°F) | 108% | 105% | 104% |
| 26-30°C (78-86°F) | 100% | 100% | 100% |
| 36-40°C (97-104°F) | 82% | 88% | 91% |
| 46-50°C (115-122°F) | 58% | 75% | 82% |
Common High-Temperature Locations
Attics
40-55°C in summer
Rooftops
Direct sun exposure
Boiler Rooms
Near heat sources
Industrial
Process equipment
Copper vs. Aluminum Conductors
Copper Advantages
- Higher conductivity (61% better than aluminum)
- Smaller wire size for same ampacity
- Better corrosion resistance at terminals
- Preferred for 14-6 AWG branch circuits
Aluminum Advantages
- 70% lighter than copper
- Significantly lower cost per ampere
- Preferred for feeders and service entrance
- Modern AA-8000 alloy has improved performance
Note: NEC does not permit aluminum conductors smaller than 12 AWG. For equivalent ampacity, aluminum requires approximately 2 AWG sizes larger than copper. Learn more in our copper vs aluminum guide.
Ampacity Calculation Examples
Example 1: Standard 20A Kitchen Circuit
12 AWG copper, 75°C insulation, EMT conduit, 30°C ambient
Base Ampacity
25A
Bundle Factor
100%
Temp Factor
100%
Final Ampacity
25A
Result: 12 AWG copper is adequate for a 20A breaker.
Example 2: Attic Fan Circuit (High Temperature)
14 AWG copper, 75°C insulation, attic at 45°C
Base Ampacity
20A
Bundle Factor
100%
Temp Factor
82%
Final Ampacity
16.4A
Result: 14 AWG only provides 16.4A—upgrade to 12 AWG for a 15A circuit in hot attics.
Example 3: Multiple Circuits in Conduit
12 AWG copper, 9 current-carrying conductors (3 × 20A circuits)
Base Ampacity
25A
Bundle Factor
70%
Temp Factor
100%
Final Ampacity
17.5A
Result: 12 AWG inadequate for 20A circuits! Upgrade to 10 AWG (35A × 0.70 = 24.5A).
Overcurrent Protection Requirements (NEC 240.4)
| Wire Size | Max Breaker (Copper) | Exception |
|---|---|---|
| 14 AWG | 15A | NEC 240.4(D) limits small conductors |
| 12 AWG | 20A | NEC 240.4(D) limits small conductors |
| 10 AWG | 30A | NEC 240.4(D) limits small conductors |
| 8 AWG+ | Per Ampacity | Next standard size up to 800A allowed |
What is the difference between ampacity and current rating?
Ampacity is the maximum continuous current a conductor can carry without exceeding its temperature rating. Current rating typically refers to the maximum current an overcurrent device (breaker or fuse) can handle. Wire ampacity must meet or exceed the circuit's current rating.
Why can't I use the 90°C column for my 90°C rated wire?
NEC 110.14(C) limits conductor ampacity based on the lowest temperature rating in the circuit, including equipment terminals. Most equipment is rated for 75°C, requiring use of 75°C ampacity values even with 90°C wire. The 90°C rating can be used for derating calculations but final ampacity is limited by terminations.
Do ground wires count toward conductor bundling?
No, equipment grounding conductors are not counted as current-carrying conductors for bundling derating purposes. Similarly, neutral conductors that carry only unbalanced current from other conductors of the same circuit are not counted.
Can I use 14 AWG aluminum wire?
The NEC does not provide ampacity values for aluminum conductors smaller than 12 AWG in Table 310.16. Aluminum 14 AWG is not typically used or available for building wiring applications due to concerns about reliability and connection issues at small sizes.
How does continuous load affect ampacity requirements?
For continuous loads (operating 3 hours or more), NEC 210.20 and 215.3 require the conductor ampacity to be at least 125% of the continuous load. This effectively means using only 80% of the conductor's ampacity for continuous loads. Our calculator provides base ampacity; multiply by 0.8 for continuous load applications.
For official ampacity standards and additional technical information, consult these authoritative sources: