The definitive wire type chart for US residential and commercial wiring is NEC Table 310.16 (formerly 310.15(B)(16)). It dictates the allowable ampacity of insulated conductors based on wire gauge, conductor material, and insulation temperature rating. If you are sizing a branch circuit, feeding a subpanel, or pulling wire through conduit, this single table is the starting point for every calculation.

However, reading the chart incorrectly is one of the most common causes of failed electrical inspections and overheated terminations. The numbers in the chart are not absolute limits; they are baseline values that must be adjusted for your specific installation environment.

The Master Wire Type Chart (NEC Table 310.16)

Before looking at the numbers, understand how to read the columns. The table is divided by conductor material (Copper vs. Aluminum) and temperature rating (60°C, 75°C, and 90°C). The temperature rating is determined by the insulation type printed on the wire jacket. For example, standard NM-B (Romex) is rated for 60°C, while THHN/THWN-2 pulled in conduit is rated for 90°C.

Table 310.16: Allowable Ampacities for Insulated Copper Conductors (Up to 3 Current-Carrying Conductors in Raceway, 30°C Ambient)
AWG / kcmil 60°C (140°F)
TW, UF		 75°C (167°F)
THW, THWN, XHHW		 90°C (194°F)
THHN, THWN-2, XHHW-2
14 15 20 25
12 20 25 30
10 30 35 40
8 40 50 55
6 55 65 75
4 70 85 95
3 85 100 115
2 95 115 130
1 110 130 145
1/0 125 150 170

Source: NFPA 70 (National Electrical Code), Table 310.16. Values shown are for copper conductors. For aluminum, refer to the right side of the official NEC table.

Code Trap: Small Conductor Rule (NEC 240.4(D))
Notice that 14 AWG shows 25A in the 90°C column, and 12 AWG shows 30A. You cannot use these numbers for overcurrent protection. NEC 240.4(D) strictly limits 14 AWG to 15A, 12 AWG to 20A, and 10 AWG to 30A, regardless of the insulation's higher temperature rating. The higher 90°C values for these small wires are only used as a starting point for derating calculations.

Which Column Applies to Your Installation?

The most frequent mistake DIYers and junior electricians make is looking at a spool of THHN wire, seeing it is rated for 90°C, and sizing the breaker using the 90°C column. This violates NEC 110.14(C), which governs termination temperatures.

The rule is simple: The allowable ampacity of a circuit is limited by the lowest temperature rating of any connected component.

  • The 60°C Column: Applies to NM-B (Romex) cable, older equipment, and circuits rated 100A or less where the termination temperature is unmarked. If you are wiring a standard 15A or 20A receptacle with NM-B, you are locked into the 60°C column.
  • The 75°C Column: The modern standard for most residential and commercial breakers, lugs, and receptacles rated up to 100A. If you pull THHN in conduit to a modern 75°C-rated panel lug, you use this column for your final base ampacity.
  • The 90°C Column: Almost never used for final base ampacity at the termination. Its primary purpose is to provide a higher baseline for derating calculations before you apply the termination limit.
The "Weakest Link" Workflow: Always calculate your derated ampacity using the 90°C column (if using THHN/THWN-2), but then compare that result to the 75°C (or 60°C) column for your termination. The final allowable ampacity is the lower of the two numbers.

How Derating Modifies Your Base Ampacity

The values in the wire type chart assume two ideal conditions: an ambient temperature of 30°C (86°F) and no more than three current-carrying conductors bundled together. When real-world conditions violate these assumptions, the wire's ability to dissipate heat drops, and you must derate (reduce) the ampacity.

Derating is governed by two main NEC sections:

  1. Ambient Temperature Correction (NEC 310.15(B)(1)): If your conduit runs through a hot attic (e.g., 45°C / 113°F), you must multiply the base ampacity by a correction factor (0.87 for 90°C wire at 45°C).
  2. Adjustment Factors for Bundling (NEC 310.15(C)(1)): When you pull more than three current-carrying conductors in a single raceway, the mutual heating effect requires a reduction. For 4 to 6 conductors, you multiply the base ampacity by 80%. For 7 to 9 conductors, you multiply by 70%.

Worked Example: Sizing a 3-Phase Feeder

You are pulling a 3-phase, 4-wire feeder (4 current-carrying conductors) through a conduit in a standard 30°C environment to feed a 70A load. You want to use 4 AWG THHN copper.

  • Step 1 (Base 90°C): 4 AWG THHN in the 90°C column = 95A.
  • Step 2 (Derate for bundling): 4 current-carrying conductors require an 80% multiplier. 95A × 0.80 = 76A.
  • Step 3 (Check Terminations): The panel lugs are rated 75°C. 4 AWG in the 75°C column = 85A.
  • Step 4 (Final Verdict): Compare the derated 90°C value (76A) to the 75°C termination value (85A). The lower number wins. Your 4 AWG wire is legally limited to 76A, which safely covers your 70A load.

If you had blindly used the 75°C column without derating, you would have assumed 85A of capacity, potentially overheating the wires inside the conduit due to the bundling effect. For a comprehensive look at conductor properties and bundling physics, the Copper Development Association provides excellent engineering data on thermal dissipation in raceways.

What This Chart Cannot Tell You

NEC Table 310.16 is the law of the land for ampacity, but it is not a complete design tool. Relying solely on this wire type chart will leave you blind to three critical installation factors:

  • Voltage Drop: The NEC table tells you what size wire will prevent a fire; it does not guarantee your equipment will run properly. For long runs (typically over 100 feet), you must calculate voltage drop using NEC Chapter 9, Table 8 (Conductor Properties). A 3% drop is the standard recommendation for branch circuits, and 5% total for feeder plus branch.
  • Conduit Fill Capacity: Just because a wire has the right ampacity doesn't mean it will physically fit in your pipe. NEC Chapter 9, Table 1 dictates that conduit cannot be filled beyond 40% of its cross-sectional area for three or more wires. Jamming too much wire into a PVC or EMT conduit will damage the insulation during the pull and trap heat.
  • Physical Lug Sizing: A 1/0 AWG wire might be rated for 150A at 75°C, but the physical breaker lug you are terminating it into might only be mechanically rated to accept up to 2 AWG. Always check the manufacturer's spec sheet for the physical wire range of the termination hardware before buying your wire.

Bookmark this chart for your most common residential branch circuits (14, 12, and 10 AWG) and feeder pulls (6, 4, and 2 AWG), but always keep a copy of the full NEC Chapter 9 tables on hand for long runs and complex conduit pulls. Local AHJs (Authority Having Jurisdiction) have the final say on code interpretations, so when in doubt, verify your derating math with your local inspector before pulling wire.