The copper wire ampacity chart dictates the maximum continuous current a conductor can carry before its insulation begins to thermally degrade. In the United States, this data is governed by NEC Table 310.16 (formerly 310.15(B)(16)). The numbers below assume solid or stranded copper conductors, an ambient temperature of 30°C (86°F), and no more than three current-carrying conductors bundled in a raceway or cable.
How to read this table: The ampacity values are split into three temperature columns based on the insulation rating of the wire. 60°C covers older rubber and basic thermoplastic (like TW). 75°C covers most modern residential cables (like NM-B / Romex) and standard terminations. 90°C covers premium thermoplastics (like THHN/THWN-2 and XHHW) pulled in conduit. The correct column is rarely the highest one—selection depends entirely on the termination ratings of your breakers and devices, governed by NEC 110.14(C).
The Master Copper Wire Ampacity Chart (NEC Table 310.16)
| Wire Size (AWG/kcmil) | 60°C (140°F) Column | 75°C (167°F) Column | 90°C (194°F) Column |
|---|---|---|---|
| 14 AWG | 15 A | 20 A | 25 A |
| 12 AWG | 20 A | 25 A | 30 A |
| 10 AWG | 30 A | 35 A | 40 A |
| 8 AWG | 40 A | 50 A | 55 A |
| 6 AWG | 55 A | 65 A | 75 A |
| 4 AWG | 70 A | 85 A | 95 A |
| 3 AWG | 85 A | 100 A | 110 A |
| 2 AWG | 95 A | 115 A | 130 A |
| 1 AWG | 110 A | 130 A | 145 A |
| 1/0 AWG | 125 A | 150 A | 170 A |
- 15A Circuit (Lighting/General Receptacles): 14 AWG (Uses 60°C column limit per NEC 240.4(D))
- 20A Circuit (Kitchen/Bath/Laundry): 12 AWG (Uses 60°C column limit per NEC 240.4(D))
- 30A Circuit (Dryer/Water Heater): 10 AWG (Uses 60°C column limit per NEC 240.4(D))
- 40A Circuit (EV Charger/HVAC): 8 AWG (Uses 75°C column)
- 50A Circuit (Range/Subpanel Feeder): 6 AWG (Uses 75°C column; note 6 AWG is 55A at 60°C, so 75°C terminations are required)
- 100A Subpanel Feeder: 3 AWG (Uses 75°C column)
Which Temperature Column Actually Applies to Your Panel?
The most common mistake DIYers and junior apprentices make is looking at a spool of THHN wire, seeing the 90°C rating printed on the jacket, and using the 90°C column to size their breaker. This is a code violation and a fire hazard. The National Electrical Code (NEC) operates on a "weakest link" principle for terminations, detailed in NFPA 70 Article 110.14(C).
Here is the hard rule for selecting your column:
- Circuits rated 100A or less (or using 14-1 AWG wire): You must use the 60°C column, unless the equipment (breaker, lug, or receptacle) is explicitly marked and listed for 75°C. Most modern residential breakers and panelboards are rated 75°C, but standard 15A and 20A duplex receptacles are often only rated 60°C. If you are wiring a receptacle, the 60°C column governs the termination.
- Circuits rated over 100A (or using wire larger than 1 AWG): You must use the 75°C column, unless the equipment is specifically marked otherwise.
So why buy 90°C THHN wire at all? You use the 90°C column exclusively for derating calculations and as thermal headroom, not for the final termination ampacity. Furthermore, standard NM-B (Romex) cable is legally restricted to the 60°C column by NEC 334.80, even though the individual conductors inside the sheath might technically have 90°C insulation. Always match the cable assembly's overall rating, not just the inner wire's jacket.
Derating, Bundling, and What the Chart Cannot Tell You
The ampacity chart assumes ideal conditions: a 30°C ambient room temperature and no more than three current-carrying conductors (e.g., one hot, one neutral, one ground) in a single conduit. When real-world jobsite conditions deviate from this, the base ampacity drops.
How Derating Modifies the Base Value
When you pull four or more current-carrying conductors through a single raceway, they heat each other up. You must apply a derating factor from NEC Table 310.15(C)(1). This is where the 90°C column earns its keep.
You are pulling two 20A circuits (4 current-carrying hot/neutral wires + 1 ground) through a single EMT conduit. You want to use 12 AWG THHN.
Step 1: Find the 90°C base ampacity for 12 AWG. The chart above says 30A.
Step 2: Apply the derating factor for 4-6 conductors, which is 80%.
Step 3: Calculate the derated ampacity: 30A × 0.80 = 24A.
Step 4: Compare to the termination limit. Your 20A breaker is rated 75°C. The 75°C limit for 12 AWG is 25A. Since your derated value (24A) is less than the termination limit (25A), 12 AWG is no longer legally permitted for a 20A breaker in this bundle. You must upsize to 10 AWG THHN to maintain code compliance.
The Small Conductor Override (NEC 240.4(D))
The chart shows that 14 AWG copper has a 60°C ampacity of 15A, and a 75°C ampacity of 20A. However, NEC 240.4(D) explicitly overrides the chart for small conductors to prevent overcurrent devices from failing to protect the wire during short, high-fault events. Regardless of your termination temperature rating, the maximum breaker sizes are strictly capped:
- 14 AWG: Maximum 15 Amps
- 12 AWG: Maximum 20 Amps
- 10 AWG: Maximum 30 Amps
What the Ampacity Chart Cannot Tell You
Ampacity only solves for thermal failure (insulation melting). It completely ignores voltage drop. According to the Copper Development Association, a 12 AWG copper wire can safely carry 20A indefinitely without overheating. But if that 20A load is located 120 feet away from the panel on a 120V circuit, the voltage at the receptacle will drop below 114V (a >5% drop). This will cause motors to overheat, LED drivers to flicker, and electronics to brownout.
For any branch circuit run exceeding 50 feet, or feeder runs exceeding 100 feet, you must calculate voltage drop independently of the ampacity chart. As a standard bench rule: keep branch circuit voltage drop under 3%, and total feeder-plus-branch drop under 5%. If your voltage drop calculation requires a larger wire than the ampacity chart, the larger wire size always wins. Always verify dead with a tested meter before terminating, and torque all lugs to the manufacturer's specified inch-pound rating using a calibrated torque screwdriver—loose lugs cause more residential fires than undersized wire.
