The direct answer: 12 AWG copper wire has a base ampacity of 20A (60°C column), 25A (75°C column), and 30A (90°C column) per NEC Table 310.16. However, for standard residential branch circuits, the maximum overcurrent protection is strictly limited to 20A by NEC 240.4(D). If you are pulling individual THHN conductors in conduit for a commercial panel or calculating bundling derating adjustments, the higher temperature columns dictate your actual safe current limits.

Below is the complete reference data, followed by the exact rules for selecting the correct temperature column and applying adjustment factors on the jobsite.

The AWG 12 Ampacity Reference Table (NEC 310.16)

How to read this table: This data is sourced directly from NFPA 70 (National Electrical Code) Table 310.16. The columns represent the temperature rating of the wire's insulation and the equipment terminals it connects to. The values assume an ambient air temperature of 30°C (86°F) and not more than three current-carrying conductors in a raceway or cable. Use the bookmark-friendly rows below to quickly find your specific material and insulation type.

Table 310.16 Allowable Ampacities (Ambient 30°C, up to 3 conductors)
AWG Size Material 60°C (140°F)
TW, UF		 75°C (167°F)
RHW, THHW, THW, THWN, XHHW		 90°C (194°F)
THHN, THHW, THW-2, THWN-2, XHHW-2
14 AWG Copper 15A 20A 25A
12 AWG Copper 20A 25A 30A
10 AWG Copper 30A 35A 40A
12 AWG Aluminum / Copper-Clad 15A 20A 25A
NEC 240.4(D) Small Conductor Rule: Even though 12 AWG copper has a 90°C ampacity of 30A, NEC 240.4(D) explicitly caps the overcurrent device (breaker or fuse) for 12 AWG copper at 20 amps for standard branch circuits. You cannot put a 25A or 30A breaker on a standard 12 AWG residential circuit, regardless of the insulation rating.

Which Temperature Column Applies to Your Installation?

The most common mistake DIYers and junior apprentices make is looking at the 90°C column for THHN wire and assuming they can run 30A through a 12 AWG conductor. The column you must use is dictated by the weakest link in your circuit, which is almost always the termination point.

The 60°C Rule for Residential Terminals

Per NEC 110.14(C)(1)(a), equipment terminals rated 100 amps or less (which includes virtually every residential breaker, receptacle, and switch) are assumed to be rated for 60°C conductors unless explicitly marked otherwise. Therefore, if you are wiring a standard 120V/240V residential receptacle using NM-B (Romex) cable, you must use the 60°C column. For 12 AWG copper, this locks your base ampacity at 20A.

When You Can Use the 90°C Column

You are only permitted to use the 90°C column (30A for 12 AWG copper) in two specific scenarios:

  1. Derating Calculations: When applying adjustment factors for bundling wires in a conduit or correcting for high ambient temperatures, you start your math from the 90°C column. (More on this below).
  2. Equipment Rated for 75°C/90°C: If you are terminating into a commercial lug or a specific industrial breaker explicitly stamped '75°C' or 'AL/CU 75°C', you may use the 75°C column (25A). True 90°C terminations are exceptionally rare in standard field work.
Pro-Tip for NM-B Cable: NM-B (Romex) contains 90°C insulated wires inside, but the overall cable assembly is only rated for 60°C ampacity. Always size your breakers based on the 60°C column when using NM-B.

Adjustment Factors: How Derating Modifies Base Ampacity

Ampacity tables assume ideal conditions: 30°C ambient air and no more than three current-carrying conductors bundled together. When you pull multiple circuits through a single conduit, the wires heat each other up. NEC 310.15(C)(1) requires you to apply a derating multiplier to the wire's ampacity.

This is where the 90°C column becomes highly valuable. When calculating derating, the NEC allows you to use the 90°C ampacity as your starting baseline, provided your final derated ampacity is still high enough to support the circuit's load and the termination limits.

Worked Derating Example: 12 AWG THHN in Conduit

Imagine you are pulling three 20A circuits (1 hot, 1 neutral per circuit = 6 current-carrying conductors total) through a single EMT conduit using 12 AWG THHN copper wire.

  • Step 1: Find the base 90°C ampacity. Per the table above, 12 AWG copper at 90°C is 30A.
  • Step 2: Find the adjustment factor. NEC Table 310.15(C)(1) states that for 4 to 6 current-carrying conductors, the adjustment factor is 80%.
  • Step 3: Calculate the derated ampacity. 30A × 0.80 = 24A.

The Verdict: Because the derated ampacity (24A) is still greater than the required circuit load (20A) and the termination limit (20A), 12 AWG THHN is perfectly legal and safe for this installation. If you had mistakenly started your math from the 60°C column (20A × 0.80 = 16A), you would have incorrectly concluded that you needed to upsize to 10 AWG wire, wasting time and money.

For more detailed bundling scenarios and ambient temperature correction factors, reference the comprehensive Cerrowire Ampacity Charts, which provide excellent field-ready matrices for these calculations.

What the Ampacity Table Cannot Tell You

While NEC Table 310.16 is the bible for thermal limits (preventing the wire insulation from melting or catching fire), it completely ignores two critical real-world constraints that will ruin your installation if overlooked.

1. Voltage Drop (NEC Chapter 9)

Ampacity tables assume the wire can handle the heat, but they do not account for the resistance of the copper over long distances. If you run a 12 AWG copper wire 150 feet to a 15A space heater, the wire will not overheat, but the voltage at the receptacle will drop significantly, potentially damaging the appliance or causing inefficient operation.

  • Rule of Thumb: Limit voltage drop to 3% for branch circuits and 5% total from the service entrance to the furthest outlet.
  • The Fix: For long runs, you must upsize the wire based on voltage drop calculations (using NEC Chapter 9, Table 8 for DC resistance), not just ampacity. A 150-foot run for a 20A circuit typically requires upsizing to 10 AWG or even 8 AWG copper, despite 12 AWG being thermally rated for the load.

2. Physical Terminal Fit and Local AHJ Amendments

The table tells you 10 AWG wire can carry 30A, but it does not tell you that the physical lug on a standard 15A/20A duplex receptacle cannot safely accept more than one 12 AWG wire, and often struggles to accommodate 10 AWG solid copper without bending the screw or stripping the threads. Always verify the physical gauge limits stamped on the device yoke.

Furthermore, the NEC is a model code. Your local Authority Having Jurisdiction (AHJ) or municipal inspector may have local amendments that restrict 12 AWG usage in certain dwelling units or require 14 AWG minimums for specific lighting circuits. Always treat NEC-style guidance as the baseline and defer to your local inspector for final compliance determinations.