The base ampacity of 12 AWG wire (copper) is 20 amps in the 60°C column, 25 amps in the 75°C column, and 30 amps in the 90°C column, according to NEC Table 310.16. However, regardless of the insulation's temperature rating, NEC 240.4(D) strictly limits the overcurrent protective device (breaker) for 12 AWG copper to a maximum of 20 amps.
While the table provides the raw thermal limits of the copper, real-world installation requires navigating terminal temperature ratings, conduit bundling derations, and voltage drop. Below is the complete reference data, the mathematical rules for modifying it, and a decision tree to finalize your branch circuit design.
The NEC 310.16 Ampacity Table for 12 AWG Wire
How to read this table: The National Electrical Code (NEC) categorizes wire ampacity by the thermal rating of the insulation, not just the copper itself. The 60°C column applies to older insulations and specific cable assemblies like NM-B (Romex). The 75°C column applies to most standard commercial wiring and device terminations. The 90°C column applies to modern THHN/THWN-2 wire in conduit, which is primarily used as a starting point for calculating derating adjustments. All values below assume an ambient temperature of 30°C (86°F) and copper conductors.
| Insulation Type | Temp Rating | Base Ampacity (Copper) | Common Use Case |
|---|---|---|---|
| NM-B (Romex), UF-B, TW | 60°C | 20 Amps | Residential interior branch circuits, underground feeder |
| THHN, THWN-2, XHHW-2 | 90°C | 30 Amps | Commercial conduit pulls, multi-wire branch circuits (MWBC) |
| RHW, THHW, THW, XHHW | 75°C | 25 Amps | Wet locations, older commercial conduit installations |
| Aluminum 12 AWG (All Types) | Varies | Not Permitted | NEC 310.16 does not list 12 AWG Al; minimum is 8 AWG Al for branch circuits |
Source: NFPA 70 National Electrical Code, Table 310.16. For manufacturer-specific thermal ratings, refer to the Southwire Ampacity Chart.
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 pull 30 amps through a 12 AWG conductor on a 30-amp breaker. This violates the 'weakest link' rule found in NEC 110.14(C).
Here is how this dictates your column selection in practice:
- Residential NM-B (Romex): Even though the individual conductors inside a modern NM-B cable have 90°C insulation, NEC 334.80 mandates that the ampacity must be derived from the 60°C column. Therefore, 12 AWG NM-B is strictly capped at 20 amps.
- Conduit with THHN to Standard Receptacles: Standard 20A duplex receptacles (like the Leviton 5362 or Hubbell 5362) and standard residential breakers (Square D Homeline, Eaton BR) have terminals rated for 75°C. Even if your wire is rated 90°C, the termination limits the circuit to the 75°C column (25 amps). However, NEC 240.4(D) overrides this and caps the breaker at 20 amps anyway.
- When the 90°C column actually matters: You only use the 90°C column (30 amps) as your starting baseline when you need to apply derating factors for conduit bundling or high ambient temperatures. As long as the final derated ampacity remains at or above 20 amps, you can still use a 20-amp breaker.
How Derating Modifies the Base 12 AWG Ampacity
When you pull multiple current-carrying conductors through a single raceway, or when the ambient temperature exceeds 30°C (86°F), the wires cannot dissipate heat effectively. NEC 310.15(C)(1) requires you to multiply the base ampacity by a derating factor.
Worked Numeric Example: Bundling in Conduit
You are pulling three separate 120V circuits (6 current-carrying conductors total, plus 3 grounds which do not count) through a single 3/4-inch EMT conduit using 12 AWG THHN.
- Identify Base Ampacity: 12 AWG THHN in the 90°C column = 30 amps.
- Find Derating Factor: NEC Table 310.15(C)(1) states that 4 to 6 current-carrying conductors require an 80% adjustment factor.
- Calculate Derated Ampacity: 30A × 0.80 = 24 amps.
- Apply Breaker Limit: Because 24 amps is greater than the 20-amp breaker limit mandated by 240.4(D), 12 AWG is still legally permitted for this installation.
When Derating Forces a Size-Up:
If you were pulling 10 to 20 current-carrying conductors in that same conduit, the derating factor drops to 50%.
30A × 0.50 = 15 amps.
Because 15 amps is less than the required 20-amp breaker rating, 12 AWG fails. You must step up to 10 AWG THHN (Base 40A × 0.50 = 20A) to safely carry the load.
Decision Tree: Sizing Your 12 AWG Branch Circuit
Use this decision matrix to lock in your exact materials for a standard 20-amp, 120-volt branch circuit. Do not guess; follow the path that matches your physical installation.
| Installation Scenario | Wire Type Required | Derating / Temp Column | Breaker Size | Final Concrete Pick |
|---|---|---|---|---|
| Standard interior residential wall (receptacles/lights) | NM-B (Romex) | 60°C Column (No derating) | 20 Amp | Southwire 12/2 NM-B + Square D HOM120 |
| Commercial conduit, single circuit (1-3 conductors) | THHN / THWN-2 | 90°C Base, capped at 75°C terminal | 20 Amp | 12 AWG THHN (Black/White) + Eaton BR120 |
| Conduit with 4-6 current-carrying conductors | THHN / THWN-2 | 90°C Base × 80% = 24A | 20 Amp | 12 AWG THHN + 20A Breaker (Valid) |
| Conduit with 10+ current-carrying conductors | Must step up to 10 AWG | 12 AWG fails (15A). 10 AWG × 50% = 20A | 20 Amp | 10 AWG THHN + 20A Breaker (Mandatory) |
| Outdoor underground burial (direct) | UF-B | 60°C Column (No derating) | 20 Amp | 12/2 UF-B + GFCI 20A Breaker |
What the Ampacity Table Cannot Tell You
NEC Table 310.16 only governs thermal limits (preventing the insulation from melting). It completely ignores voltage drop, which can destroy motors and cause electronics to brown out. According to the Copper Development Association, maintaining voltage within 3% of nominal is critical for equipment longevity.
The Voltage Drop Trap:
The resistance of 12 AWG solid copper is approximately 1.93 ohms per 1,000 feet at 75°C. If you run a 12 AWG circuit 100 feet from the panel to a garage workshop and pull a full 20-amp load (like a table saw and dust collector simultaneously), the voltage drop calculation is:
- Round-trip distance: 200 feet (Hot + Neutral)
- Resistance: 1.93 Ω/kft × 0.2 kft = 0.386 ohms
- Voltage Drop: 20A × 0.386 Ω = 7.72 volts
- Percentage Drop: 7.72V / 120V = 6.4%
A 6.4% drop vastly exceeds the 3% NEC Informational Note recommendation for branch circuits. Your 120V outlet will only deliver ~112V under heavy load, which can cause motor overheating and premature failure. The Fix: For any 12 AWG run exceeding 60 feet on a 20-amp circuit, or any run exceeding 40 feet on a continuous 16-amp load, you must abandon 12 AWG and pull 10 AWG copper to mitigate voltage drop, regardless of what the ampacity table permits.
