If you are sizing conductors for a standard residential or light commercial branch circuit, the base ampacities for copper wire (up to 3 current-carrying conductors in a raceway at 30°C ambient) are found in the 60°C or 75°C columns of NEC Table 310.16. For standard NM-B (Romex) cable, you must strictly use the 60°C column regardless of the wire's actual insulation rating.
This guide provides the exact data from the NFPA 70 National Electrical Code, breaks down how to read the temperature columns, and explains the derating math that changes these baseline numbers in real-world installations.
How to Read This Cable Amp Chart: Columns and Assumptions
Before you pick a wire gauge, you need to know which column of the cable amp chart applies to your specific installation. The NEC publishes ampacities across three temperature columns: 60°C (140°F), 75°C (167°F), and 90°C (194°F). These numbers represent the maximum temperature the wire insulation can safely handle before degrading.
- The 60°C Column: Use this for older installations, non-metallic sheathed cable (NM-B / Romex), and any equipment terminations not explicitly marked with a temperature rating. Per NEC 334.80, even if the individual conductors inside your Romex have 90°C THHN insulation, the cable assembly as a whole is limited to the 60°C column.
- The 75°C Column: This is the standard for most modern commercial and residential branch circuits and feeders. Most modern breakers, lugs, and terminal blocks are rated for 75°C. If you are pulling THHN/THWN-2 wire in conduit and terminating on 75°C rated lugs, use this column.
- The 90°C Column: You can almost never use this column to determine your final breaker size. Per NEC 110.14(C), your termination points (the breaker and the receptacle) must be rated for 90°C to use this column for ampacity, and they almost never are. However, the 90°C column is legally used as the starting baseline for derating calculations (adjusting for heat and bundling) before you apply the final breaker size.
NEC Table 310.16: Copper and Aluminum Ampacities
The following cable amp chart is derived directly from NEC Table 310.16 (formerly 310.15(B)(16)). It assumes an ambient temperature of 30°C (86°F) and not more than three current-carrying conductors bundled in a single raceway or cable. Bookmark this section for quick job-site lookups.
| AWG / kcmil | Copper 60°C | Copper 75°C | Copper 90°C | Aluminum 60°C | Aluminum 75°C | Aluminum 90°C |
|---|---|---|---|---|---|---|
| 14 AWG | 15A* | 20A* | 25A* | - | - | - |
| 12 AWG | 20A* | 25A* | 30A* | 15A* | 20A* | 25A* |
| 10 AWG | 30A* | 35A* | 40A* | 25A* | 30A* | 35A* |
| 8 AWG | 40A | 50A | 55A | 30A | 40A | 45A |
| 6 AWG | 55A | 65A | 75A | 40A | 50A | 60A |
| 4 AWG | 70A | 85A | 95A | 55A | 65A | 75A |
| 3 AWG | 85A | 100A | 110A | 65A | 75A | 85A |
| 2 AWG | 95A | 115A | 130A | 75A | 90A | 100A |
| 1 AWG | 110A | 130A | 145A | 85A | 100A | 115A |
| 1/0 AWG | 125A | 150A | 170A | 100A | 120A | 135A |
| 2/0 AWG | 145A | 175A | 195A | 115A | 135A | 150A |
| 3/0 AWG | 165A | 200A | 225A | 130A | 155A | 170A |
| 4/0 AWG | 195A | 230A | 260A | 150A | 180A | 205A |
*Asterisks denote sizes subject to NEC 240.4(D) small conductor rules, which cap overcurrent protection at 15A for 14 AWG, 20A for 12 AWG, and 30A for 10 AWG copper, regardless of the 75°C or 90°C column values.
Derating Factors: When the Chart Lies to You
The baseline numbers above assume perfect conditions: 30°C (86°F) ambient air and no more than three current-carrying conductors in the conduit. If you violate either assumption, the wire cannot dissipate heat efficiently, and you must apply derating factors (adjustment and correction) per OSHA and NEC safety guidelines.
1. Bundling (More than 3 Current-Carrying Conductors)
When you pull 4 to 6 current-carrying conductors in a single raceway, you must multiply the base ampacity by 80%. For 7 to 9 conductors, the multiplier drops to 70%. Note that a neutral wire carrying only unbalanced load does not count, but a neutral on a multi-wire branch circuit (MWBC) or a 3-phase non-linear load circuit does count.
2. Ambient Temperature Correction
If your conduit runs through a hot attic (e.g., 50°C / 122°F), you must apply a temperature correction factor from the bottom of Table 310.16. At 50°C, the correction factor for 90°C insulation is 0.82.
You are pulling four 10 AWG THHN current-carrying conductors through an attic with an ambient temperature of 50°C (122°F).
1. Start with the 90°C column for 10 AWG Copper: 40A.
2. Apply bundling derating (4-6 conductors = 80%): 40A × 0.80 = 32A.
3. Apply temperature correction (50°C ambient, 90°C insulation = 0.82): 32A × 0.82 = 26.24A.
4. Your final derated ampacity is 26.24A. Because NEC 240.4(D) limits 10 AWG to a 30A breaker, and your derated value is below 30A, you must step up to 8 AWG wire or reduce the breaker to 25A (if your load permits).
What This Cable Amp Chart Cannot Tell You
While Table 310.16 is the bible for thermal limits, it is not a complete design tool. Relying on it blindly will cause you to fail inspections or experience nuisance tripping. Here is what the chart leaves out:
- Voltage Drop: The NEC ampacity chart does not account for voltage drop over distance. A 14 AWG wire might be legally allowed to carry 15A for 200 feet according to Table 310.16, but the voltage drop will exceed the recommended 3% limit, causing motors to overheat and electronics to brown out. You must cross-reference NEC Chapter 9, Table 8 (conductor resistance) to calculate voltage drop for runs over 50 feet.
- Short-Circuit Let-Through Current: Ampacity measures continuous thermal handling. It does not tell you if the wire can survive the magnetic and thermal stress of a 10,000A short circuit before the breaker trips. That requires coordination with the breaker's let-through energy (I²t) ratings.
- Physical Fill Limits: Just because three 4 AWG wires fit the ampacity requirement doesn't mean they will physically fit in a 1/2-inch EMT conduit. You must always check NEC Chapter 9, Table 1 for maximum conduit fill percentages (40% for 3 or more wires).
FAQ: Cable Amp Chart Questions Answered
Does a standard cable amp chart apply to DC solar wiring?
Yes, the base thermal physics in Table 310.16 apply to DC current, but NEC Article 690 (Solar Photovoltaic Systems) imposes strict multiplier rules. Because solar arrays produce continuous current for 3+ hours, you must multiply the maximum circuit current by 125% before sizing the wire and breaker. Furthermore, DC solar runs on rooftops are subject to extreme temperature adders (NEC 310.15(B)(2)(c)), requiring heavy derating based on the conduit's height above the roof surface.
Why is my 12 AWG wire limited to 20A if the 90°C column says 30A?
This is due to NEC 240.4(D), known as the 'small conductor rule.' The code writers recognized that 14, 12, and 10 AWG wires are frequently used in residential branch circuits where high-fault currents and physical damage are risks. Therefore, regardless of how advanced the insulation is (even if it's 90°C THHN), the overcurrent protective device (breaker) cannot exceed 15A for 14 AWG, 20A for 12 AWG, and 30A for 10 AWG copper. The 90°C column for these sizes is only used as a starting point for derating math.
How do I calculate ampacity for aluminum feeder cables to a subpanel?
Use the Aluminum columns in the chart above. For a standard 100A residential subpanel feeder, you need 2/0 AWG Aluminum in the 75°C column (rated 135A, which covers the 100A breaker and continuous load math). Ensure your panel lugs are rated for 75°C, which almost all modern Square D, Eaton, and Siemens load centers are. Never use the 90°C column for aluminum terminations unless the specific equipment datasheet explicitly states the lugs are tested and listed for 90°C aluminum.
