For a standard 20 amp circuit, use 12 AWG copper wire with a 20-amp breaker. This applies to typical 120V/240V residential branch circuits using THHN/THWN-2 or NM-B (Romex) insulation. Never use 14 AWG on a 20A breaker, as it violates NEC 240.4(D) and creates a severe fire hazard.
- Material: Copper (Aluminum requires different sizing; see Section 4)
- Temperature Column: 60°C for NM-B (Romex); 75°C for THHN in conduit
- Ambient Temperature: 30°C (86°F) or lower
- Conduit Fill: Not more than 3 current-carrying conductors in a single raceway
The Baseline: 12 AWG Copper and the NEC 240.4(D) Rule
When determining what gauge wire for 20 amp circuit installations, the National Electrical Code (NEC) draws a hard line in the sand. Under normal conditions, 12 AWG copper wire is rated for 20 amps when used in standard residential branch circuits. However, the reason you cannot use a smaller wire (like 14 AWG) or a larger breaker (like 25A) is dictated by NEC Article 240.4(D), which covers small conductors.
NEC 240.4(D) explicitly caps the overcurrent protection for 14 AWG at 15 amps, 12 AWG at 20 amps, and 10 AWG at 30 amps. Even if your 12 AWG THHN wire has a printed ampacity of 25 amps in the 75°C column, you are legally and physically restricted to a 20-amp breaker for general branch circuits. This prevents the wire from overheating if a fault occurs just below the breaker's trip curve.
Ampacity Table: Why 12 AWG and Not 14 AWG?
To understand why 12 AWG is the minimum, we have to look at the standard ampacity charts based on NEC Table 310.16. The most common mistake DIYers make is looking at the 90°C column for THHN wire and assuming they can push more current through it. You cannot. The termination temperature rating of your breaker and receptacles (usually 75°C) and the NEC 60°C restriction for NM-B cable govern your final ampacity.
| Wire Gauge (AWG) | 60°C Column (NM-B / Romex) | 75°C Column (THHN in Conduit) | Max Breaker Size (NEC 240.4(D)) |
|---|---|---|---|
| 14 AWG | 15 Amps | 20 Amps | 15 Amps |
| 12 AWG | 20 Amps | 25 Amps | 20 Amps |
| 10 AWG | 30 Amps | 35 Amps | 30 Amps |
Why not 14 AWG? A 14 AWG wire will safely carry 15 amps. If you pull 18 amps through it (which a 20-amp breaker will allow before tripping), the wire's insulation will degrade, melt, and eventually arc or catch fire inside the wall cavity. The breaker is there to protect the wire, not the appliance. Therefore, 12 AWG is the absolute floor for a 20A circuit.
Voltage Drop: When to Upsize to 10 AWG
Ampacity tells you what size wire won't catch fire; voltage drop tells you what size wire will actually deliver usable power to your tool or appliance. The NEC recommends a maximum 3% voltage drop on branch circuits (Informational Note to NEC 210.19(A)).
Let's run a voltage drop check for a 12 AWG copper wire on a 120V circuit at a distance of 100 feet from the panel, pulling a full 20A load. Using the standard formula VD = (2 × K × I × D) / CM (where K=12.9 for copper, I=20A, D=100ft, and CM=6530 for 12 AWG):
- 12 AWG at 100 ft: 7.9V drop (6.5% drop). This exceeds the 3% recommendation and will cause motors to run hot and lights to dim.
- 10 AWG at 100 ft: 4.9V drop (4.1% drop). Better, but still slightly over 3% at a full 20A continuous pull.
- 8 AWG at 100 ft: 3.1V drop (2.6% drop). This is the correct size for a 100-foot run at maximum load.
The Practical Rule: For runs under 50 feet, stick with 12 AWG. For runs between 50 and 80 feet, 12 AWG is usually fine unless the load is a continuous 20A (like a heavy space heater). For runs over 80 feet, upsize to 10 AWG or 8 AWG depending on the exact load. You can verify your specific run using the Southwire Voltage Drop Calculator.
Derating, Bundling, and Aluminum: What Changes the Answer
The 12 AWG baseline assumes normal conditions. Three specific factors will force you to change your wire size:
1. Conductor Bundling (Derating)
When you pull multiple wires through a single conduit, they heat each other up. NEC Table 310.15(C)(1) requires you to derate the ampacity of the 90°C column (THHN) based on the number of current-carrying conductors. If you pull 4 to 6 current-carrying conductors in a conduit, you must multiply the 90°C ampacity (30A for 12 AWG) by 80%. That leaves you with 24A, which is still fine for a 20A breaker. However, if you bundle 7 to 9 conductors, the derating factor drops to 70%. 30A × 0.70 = 21A. You are now dangerously close to the limit, and any ambient temperature increase will push you over. Fix: Upsize to 10 AWG when bundling more than 6 wires.
2. High Ambient Temperatures
If your conduit runs through an attic in the US South where temperatures regularly exceed 104°F (40°C), you must apply temperature correction factors. At 41-50°C, 12 AWG THHN loses 15% of its capacity. Again, upsizing to 10 AWG provides the necessary thermal headroom.
3. Aluminum vs. Copper
Never treat aluminum and copper interchangeably. Aluminum has higher resistance and expands/contracts more under heat. For a 20A branch circuit, aluminum wire is almost never used because 12 AWG aluminum is not standard building wire, and 10 AWG aluminum is only rated for 30A at 75°C but is physically difficult to terminate on standard 20A duplex receptacles. For 20A branch circuits, always use copper. Aluminum is reserved for large feeder cables (like a 2-2-2-4 SER cable to a subpanel), which are sized for the subpanel's main breaker, not the individual 20A branch circuits inside it.
Decision Tree: Finalizing Your Wire and Breaker Pick
Use this decision path to lock in your exact materials for the job. Do not guess; follow the tree to your specific scenario.
| Scenario Condition | Wire Selection | Breaker Selection | Concrete Part Pick |
|---|---|---|---|
| Standard Receptacle Circuit (Kitchen, Garage, Bathroom; < 50ft run) |
12 AWG Copper NM-B (Romex) | 20A Single-Pole (120V) or Double-Pole (240V) | Southwire 250ft 12/2 NM-B + Eaton BR120 |
| Conduit Run / Wet Location (Outdoor, underground, or exposed garage walls) |
12 AWG Copper THHN/THWN-2 | 20A GFCI or AFCI Breaker (per NEC 210.8) | Southwire 12 AWG Stranded THHN + Eaton BR120GFT |
| Long Distance Run (Detached garage, workshop, 60ft - 100ft) |
10 AWG Copper THHN or 8 AWG NM-B | 20A Breaker (Do not upsize breaker, only wire) | Cerrowire 10 AWG THHN + Eaton BR120 |
| Continuous Load (Hardwired heater running 3+ hours at 16A+) |
10 AWG Copper THHN (125% rule applied) | 25A Breaker (if hardwired) or 20A (if receptacle) | 10 AWG THHN + Eaton BR125 (Hardwired only) |
When an Engineer or the AHJ Must Confirm
While the rules above cover 95% of residential and light commercial 20A circuits, you must defer to a licensed Professional Engineer (PE) or your local Authority Having Jurisdiction (AHJ / electrical inspector) in the following edge cases:
- Continuous Loads on Receptacles: NEC 210.21 restricts standard 20A receptacles to a maximum 20A breaker, but NEC 210.20(A) requires branch circuit conductors and overcurrent devices to be sized at 125% of continuous loads. If you are plugging in a piece of equipment that pulls a continuous 18A for more than 3 hours, you cannot legally put it on a standard 20A receptacle circuit. You need a hardwired connection on a 25A breaker with 10 AWG wire.
- Multi-Wire Branch Circuits (MWBC): If you are sharing a neutral between two 120V 20A circuits (using 12/3 NM-B), the handle-tied breaker must be a common-trip 2-pole 20A breaker (e.g., Eaton BQT22020). Local inspectors are highly critical of MWBCs if the neutral isn't properly pigtailed at every device.
- Solar and Battery Inverter Circuits: Inverter output circuits often have specific, non-standard continuous load calculations dictated by the manufacturer's installation manual, which supersedes general NEC branch circuit rules under NEC 110.3(B).
By sticking to 12 AWG copper for standard runs and upsizing to 10 AWG or 8 AWG strictly for voltage drop and derating, you ensure your 20-amp circuits are safe, code-compliant, and capable of delivering full power without tripping or overheating.
