The NEC amp table (officially Table 310.16 in the 2023 National Electrical Code) defines the allowable ampacities for insulated conductors rated 0 through 2000 volts. For standard residential and light commercial copper branch circuits (14, 12, and 10 AWG), you must use the 60°C column for final breaker sizing, even if your wire insulation (like THHN) is rated for 90°C. A 12 AWG copper wire is rated for 20 amps; a 10 AWG is rated for 30 amps; a 6 AWG is rated for 55 amps at standard termination temperatures.

How to Read the NEC Amp Table (NEC 310.16)

Before pulling wire, you need to understand how this chart is structured. The table is divided by conductor material (copper vs. aluminum) and then by insulation temperature rating (60°C, 75°C, and 90°C). The values assume an ambient air temperature of 30°C (86°F) and no more than three current-carrying conductors bundled together in a raceway or cable. If your installation deviates from these baseline conditions, the base values must be mathematically reduced (derated).

Bookmark Quick-Jumps: The most queried rows for DIY and residential jobs are 12 AWG (20A circuits for standard receptacles) and 6 AWG (55A/60A circuits for ranges, EV chargers, and subpanels). Keep these two rows memorized to save time on the jobsite.
Table 310.16: Allowable Ampacities of Insulated Copper Conductors
Source: NFPA 70 (NEC 2023), Table 310.16. Rated 0-2000V, 30°C Ambient.
AWG / kcmil 60°C (140°F) Column 75°C (167°F) Column 90°C (194°F) Column
14 15 A 20 A 25 A
12 (Most Queried) 20 A 25 A 30 A
10 30 A 35 A 40 A
8 40 A 50 A 55 A
6 (Most Queried) 55 A 65 A 75 A
4 70 A 85 A 95 A
3 85 A 100 A 115 A
2 95 A 115 A 130 A
1 110 A 130 A 145 A
1/0 125 A 150 A 170 A
2/0 145 A 175 A 195 A
3/0 165 A 200 A 225 A
4/0 195 A 230 A 260 A

Which Temperature Column Applies to Your Installation?

The most common mistake made by hobbyists and junior electricians is sizing wire using the 90°C column because they bought THHN wire, which has a 90°C insulation rating. This will fail inspection. The correct column is dictated by the termination points (breakers, lugs, and receptacles), not just the wire insulation, per NEC 110.14(C).

  • The 60°C Column: Mandatory for circuits rated 100 amps or less, or for wire sizes 14 AWG through 1 AWG. Standard residential breakers and receptacles are tested and rated for 60°C terminations. Even if your THHN wire can handle 90°C, the breaker lug cannot. Therefore, a 12 AWG wire is strictly limited to 20A.
  • The 75°C Column: Used for circuits rated over 100 amps, or wire sizes 1/0 AWG and larger. Most modern commercial panelboards and heavy-duty disconnects feature 75°C rated lugs.
  • The 90°C Column: You almost never use this column for final breaker sizing. Its primary purpose is to provide the base ampacity for derating calculations before you apply the final termination temperature limit.

Derating: How Ambient Heat and Bundling Modify the Base Value

The base values in the amp table assume perfect conditions: 30°C (86°F) ambient air and a maximum of three current-carrying conductors in a conduit. When conditions worsen, the wire cannot dissipate heat as efficiently, and you must derate the 90°C ampacity.

Safety Caveat: Never run NM-B (Romex) cable through hot insulation or bundle it tightly in bored holes without accounting for heat buildup. NM-B is permanently limited to the 60°C column regardless of derating math, per NEC 334.80.

Worked Numeric Example: You are pulling six current-carrying conductors (two 3-phase circuits) through a conduit in an attic where the ambient temperature reaches 113°F (45°C). You need to supply a 20A load.

  1. Start at the 90°C column: 12 AWG THHN = 30A.
  2. Apply Ambient Correction (45°C): The correction factor from NEC Table 310.15(B)(1) is 0.87. (30A × 0.87 = 26.1A)
  3. Apply Bundling Correction (6 conductors): The adjustment factor from NEC Table 310.15(C)(1) is 0.80. (26.1A × 0.80 = 20.88A)
  4. Final Check: The derated ampacity is 20.88A. Because 20.88A is greater than your 20A load, 12 AWG THHN is acceptable. However, you still protect it with a 20A breaker because the termination limit (60°C column) caps 12 AWG at 20A anyway.

What the Amp Table Cannot Tell You (And How to Decide)

While NFPA 70 Table 310.16 is the bible for thermal limits, it is blind to three critical real-world factors. If you ignore these, your circuit will either trip prematurely, overheat at the load, or physically fail to terminate.

  • Voltage Drop: The amp table does not account for distance. A 10 AWG wire can safely carry 30A thermally, but if you run it 250 feet to a 240V compressor, you will lose over 10V. For runs over 100 feet, calculate voltage drop using Chapter 9, Table 8 resistance values, and typically upsize the wire by one or two AWG steps.
  • Short-Circuit Withstand: The table assumes steady-state current. Under a massive short-circuit fault, smaller wires can vaporize before the breaker's magnetic trip clears the fault in milliseconds. For high available fault current services (e.g., 22kA+ at the panel), equipment grounding conductors may need upsizing per NEC Table 250.122.
  • Physical Lug Fit: A 4/0 AWG wire might be electrically rated for a 200A residential service, but the physical strands might not compress or bend neatly into a compact 200A main breaker lug without specialized pin terminals or reducing the strand count (which violates code).

Concrete Decision Path: Sizing Your Next Circuit

Stop guessing. Use this decision tree to terminate your wire sizing process with a concrete pick. This path assumes standard copper THHN/THWN-2 in conduit or standard NM-B, at standard 30°C ambient, with no more than 3 current-carrying conductors.

If Your Continuous + Non-Continuous Load Is... And Your Breaker / Termination Rating Is... Pick This Copper AWG (Default Pick)
≤ 15 Amps 15A (Standard 120V) 14 AWG (Min. 60°C col)
≤ 20 Amps 20A (Standard 120V/240V) 12 AWG (Min. 60°C col)
≤ 30 Amps 30A (Dryer/RV/Tools) 10 AWG (Min. 60°C col)
≤ 40 Amps 40A (Range/EV Level 2) 8 AWG (Min. 60°C col)
≤ 55 Amps 60A (Subpanel/Heavy EV) 6 AWG (Min. 60°C col)
≤ 100 Amps 100A (Subpanel) 3 AWG (Min. 60°C col)
≤ 200 Amps 200A (Main Service) 2/0 AWG (Use 75°C col)

The Default Recommendation: For any standard 120V/240V residential branch circuit under 60 amps, default to the 60°C column of the amp table. Buy THHN/THWN-2 for conduit pulls to give yourself thermal headroom for derating, but buy NM-B for interior wall cavities and strictly adhere to the 60°C limits printed in the first column. If your run exceeds 100 feet, upsize one AWG to mitigate voltage drop before finalizing your material list.