The definitive amp chart for wire size in the United States is governed by NEC Table 310.16. For standard residential copper branch circuits terminating on 60°C or 75°C lugs, 14 AWG handles 15 amps, 12 AWG handles 20 amps, 10 AWG handles 30 amps, 8 AWG handles 40 amps, and 6 AWG handles 55 amps. However, pulling a single number from a chart without understanding temperature columns, terminal limitations, and derating factors is how wires melt inside conduit. This reference guide provides the complete baseline table, explains how to read the temperature columns, and details the adjustments that modify these base values in real-world installations.
How to Read the NEC Amp Chart for Wire Size
Before looking at the numbers, you must understand the three temperature columns: 60°C (140°F), 75°C (167°F), and 90°C (194°F). These columns represent the thermal rating of the wire's insulation (e.g., TW is 60°C, THHW is 75°C, THHN/THWN-2 is 90°C).
Which column applies to your installation? The National Electrical Code (NEC) section 110.14(C) dictates that for circuits rated 100 amps or less, or for wire sizes 14 AWG through 1 AWG, you must use the 60°C column to determine the final allowable ampacity, unless the equipment (breaker, receptacle, lug) is specifically listed and identified for 75°C. Most modern breakers and receptacles are rated 75°C, allowing you to use the 75°C column for sizing. For feeders and service conductors over 100 amps, the 75°C column is the default baseline.
The 90°C Column Rule: The 90°C column is almost never used to determine the final ampacity at the termination. It is strictly used as the starting point for derating calculations (adjusting for ambient heat or bundling). Your final derated ampacity cannot exceed the base ampacity of the 60°C or 75°C column, whichever applies to your termination.
The Small Conductor Rule (NEC 240.4(D)): Regardless of the insulation temperature rating, the NEC hard-caps the overcurrent protection device (breaker/fuse) size for small copper wires: 14 AWG is capped at 15A, 12 AWG at 20A, and 10 AWG at 30A. Even if you use 90°C THHN wire and apply no derating, you cannot put a 12 AWG wire on a 25A breaker.
Complete Wire Ampacity Table (NEC Table 310.16)
Source: NFPA 70 (National Electrical Code), Table 310.16. Assumptions: Ambient temperature of 30°C (86°F), not more than three current-carrying conductors in a raceway, cable, or earth.
| Wire Size (AWG/kcmil) | Copper 60°C (Amps) | Copper 75°C (Amps) | Copper 90°C (Amps) | Aluminum 75°C (Amps) | Aluminum 90°C (Amps) |
|---|---|---|---|---|---|
| 14 AWG (15A Circuits) | 15 | 20 | 25 | — | — |
| 12 AWG (20A Circuits) | 20 | 25 | 30 | — | — |
| 10 AWG (30A Circuits) | 30 | 35 | 40 | — | — |
| 8 AWG | 40 | 50 | 55 | 40 | 45 |
| 6 AWG | 55 | 65 | 75 | 50 | 60 |
| 4 AWG | 70 | 85 | 95 | 65 | 75 |
| 3 AWG | 85 | 100 | 110 | 75 | 85 |
| 2 AWG | 95 | 115 | 130 | 90 | 100 |
| 1 AWG | 110 | 130 | 145 | 100 | 115 |
| 1/0 AWG | 125 | 150 | 170 | 120 | 135 |
| 2/0 AWG | 145 | 175 | 195 | 135 | 150 |
| 3/0 AWG | 165 | 200 | 225 | 155 | 170 |
| 4/0 AWG (200A Services) | 195 | 230 | 260 | 180 | 205 |
Derating and Adjustments: When the Base Value Drops
The table above assumes ideal conditions: an ambient temperature of 30°C (86°F) and no more than three current-carrying conductors bundled in a conduit. When real-world conditions deviate, you must apply adjustment factors (derating) to the 90°C column value, then compare the result to the termination column (60°C or 75°C). The lower of the two numbers becomes your final allowable ampacity.
1. Bundling (NEC 310.15(C)(1))
When you pull four to six current-carrying conductors in a single raceway, the heat cannot dissipate. You must multiply the 90°C ampacity by 80%. For seven to nine conductors, the factor drops to 70%.
Worked Example: You are pulling four 12 AWG THHN (90°C) copper wires through a single conduit for two separate 20A circuits. The 90°C base ampacity for 12 AWG is 30A. Multiplying by the 80% bundling factor yields 24A (30A × 0.80). Because your breakers and receptacles are rated 75°C (base 25A), the 24A derated value is acceptable. However, if your terminations were only rated 60°C (base 20A), your wire is now limited to 20A, and you cannot increase the load.
2. Ambient Temperature (NEC 310.15(B)(1))
If your conduit runs through an attic in a hot climate where ambient temperatures reach 46°C (115°F), you must apply a temperature correction factor of 0.82 to the 90°C column. A 10 AWG THHN wire (90°C base = 40A) derates to 32.8A (40A × 0.82). It can still safely carry 30A, but you have lost your safety margin.
What This Amp Chart Cannot Tell You
An ampacity chart is a thermal limit guide, not a complete engineering specification. Relying on it blindly will cause you to fail inspections or experience equipment malfunction due to three missing variables:
- Voltage Drop: The NEC amp chart assumes the run is short enough that voltage drop is negligible. For runs exceeding 100 feet, a 12 AWG wire carrying 16 amps will drop over 5% of a 120V supply, starving motors and tripping sensitive electronics. According to Electrical Contractor Magazine and standard engineering practices (like IEEE 141), you should limit branch circuit voltage drop to 3%. For a 100-foot, 20A run, you must upsize to 10 AWG or 8 AWG copper, regardless of what the thermal amp chart says.
- Continuous Loads: If a load runs for three hours or more (like an EV charger, baseboard heater, or commercial lighting), NEC 210.20(A) requires you to multiply the continuous load by 125% to size the breaker and the wire. A 16A continuous load requires a wire and breaker sized for 20A (16A × 1.25 = 20A).
- Standard Breaker Sizing (NEC 240.6): If your derated wire ampacity calculates to an odd number like 83A, you are not required to find an 83A breaker. The NEC allows you to round up to the next standard overcurrent device size (90A), provided the load is non-continuous and the wire ampacity is at least equal to the non-continuous load.
Frequently Asked Questions
What size wire do I need for a 50-amp circuit?
For a standard 50-amp circuit (like an EV charger or RV outlet) using copper wire with 75°C rated terminations, you need 8 AWG copper, which has an allowable ampacity of 50A in the 75°C column. However, if your equipment is older or only rated for 60°C, you must use 6 AWG copper (55A at 60°C). Most electricians default to 6 AWG for 50-amp circuits to accommodate voltage drop on longer runs and to ensure compatibility with all termination ratings. If using aluminum, you must step up to 6 AWG aluminum (50A at 75°C) or 4 AWG aluminum (65A at 75°C for voltage drop buffer).
Can I use the 90°C column to size my breaker?
No. The 90°C column is strictly a starting point for derating calculations (adjusting for heat or bundling). NEC 110.14(C) explicitly forbids using the 90°C ampacity for termination sizing unless the equipment is specifically tested, listed, and identified for 90°C operation, which is exceptionally rare in residential and light commercial gear. Your final breaker size must be based on the 60°C or 75°C column limits.
Does the ground wire count when derating wire ampacity?
No. When calculating bundling derating factors under NEC 310.15(C)(1), equipment grounding conductors (bare copper or green insulated) are not counted as current-carrying conductors. They only carry current during a fault condition. Therefore, if you pull two hot wires, one neutral, and one ground through a conduit, you only count three current-carrying conductors, meaning no bundling derating is required.
How does aluminum wire sizing compare to copper?
Aluminum has a lower conductivity than copper, meaning it generates more heat for the same current. As a general rule, aluminum wire must be one to two AWG sizes larger than copper to carry the same ampacity. For example, to carry 100 amps at 75°C, you need 3 AWG copper, but you need 1/0 AWG aluminum. Aluminum is highly cost-effective for large feeders (like 200A service entrances using 4/0-4/0-2/0-4 mobile home feeder cable), but it requires specific anti-oxidant paste (like Noalox) and precise torque settings on lugs to prevent thermal expansion/contraction loosening over time.
