The correct wire for a 20 amp circuit is 12 AWG copper wire protected by a 20-amp breaker. This applies to standard 120V or 240V branch circuits using THHN/THWN-2 or NM-B (Romex) insulation under normal residential conditions. Never use 14 AWG on a 20-amp breaker, as it creates a severe fire hazard by allowing the wire to overheat before the breaker trips.
- Material: Solid copper conductors (aluminum requires different sizing).
- Temperature Column: 75°C for THHN in conduit; 60°C for NM-B cable.
- Ambient Temperature: 30°C (86°F) or lower.
- Installation Method: Single circuit in a standard wall cavity or up to 3 current-carrying conductors in a raceway.
- Load Type: Non-continuous (operating for less than 3 hours at a time).
The Baseline: 12 AWG Copper and the NEC Ampacity Columns
When sizing the wire for a 20 amp circuit, hobbyists and apprentices often make the mistake of looking only at the 90°C column of the NEC ampacity tables because THHN wire is physically rated for 90°C. However, the National Electrical Code (NEC) restricts how you use that rating based on termination limits and cable types.
Per NEC 110.14(C), for circuits rated 100 amps or less, you must size the conductor based on the 60°C column unless the equipment (breaker and receptacle) is explicitly marked for 75°C. Most modern 20-amp breakers and commercial receptacles carry the 75°C marking, allowing you to use the 75°C column for THHN wire in conduit. However, if you are using NM-B (Romex) cable, NEC 334.80 strictly mandates that the ampacity be determined from the 60°C column, regardless of the termination markings.
Fortunately, 12 AWG copper is rated for 20 amps in the 60°C column and 25 amps in the 75°C column. Because NEC 240.4(D) specifically limits the overcurrent protection for 12 AWG copper to 20 amps, a 12 AWG wire and a 20-amp breaker are a perfect, code-compliant match.
| Wire Size (AWG) | 60°C Ampacity (NM-B) | 75°C Ampacity (THHN) | Max Standard Breaker | Common Use Case |
|---|---|---|---|---|
| 14 AWG | 15A | 20A | 15A | Lighting circuits, low-draw receptacles |
| 12 AWG | 20A | 25A | 20A | Kitchen/bath small appliance, standard 20A outlets |
| 10 AWG | 30A | 35A | 30A | Dryers, water heaters, long-run 20A upsizing |
| 8 AWG | 40A | 50A | 40A | Ranges, subpanel feeders, extreme distance upsizing |
Why not use 14 AWG? A 14 AWG wire has a 60°C ampacity of 15 amps. If you pull 19 amps through it, the wire will overheat, melt its insulation, and potentially ignite the surrounding framing. A 20-amp breaker will not trip at 19 amps, meaning the breaker fails to protect the wire. This is why NEC 240.4(D) strictly forbids placing 14 AWG on a 20-amp breaker.
When 12 AWG Isn't Enough: Voltage Drop and Long Runs
The ampacity tables in the NEC only tell you the maximum current a wire can carry before its insulation degrades or melts. They do not guarantee that your equipment will actually function correctly at the end of the run. When current flows through copper, it encounters resistance, resulting in a voltage drop. For branch circuits, the NEC recommends keeping voltage drop under 3% for optimal efficiency.
If you are running a 20-amp circuit to a detached garage, a shed, or a distant workshop outlet, 12 AWG wire will result in unacceptable voltage drop over long distances. Let's look at the math for a 150-foot run (300 feet total conductor length out and back) carrying a full 20-amp load at 120V.
Using the standard voltage drop formula VD = (2 × L × I × R) / 1000, where the resistance (R) of 12 AWG copper is roughly 1.93 ohms per 1,000 feet:
- VD = (2 × 150 × 20 × 1.93) / 1000 = 11.58 Volts dropped.
- Percentage Drop = (11.58 / 120) × 100 = 9.65%.
A 9.65% drop means your 120V tool is only seeing 108.4V. This will cause motors to overheat, trip their internal thermal protectors, and burn out prematurely. To fix this, you must upsize the wire, even though 12 AWG is technically rated for 20 amps.
| One-Way Distance | 12 AWG Drop | 10 AWG Drop | 8 AWG Drop | Required Wire for <3% Drop |
|---|---|---|---|---|
| 25 feet | 1.6% (OK) | 1.0% | 0.6% | 12 AWG |
| 50 feet | 3.2% (Marginal) | 2.0% (OK) | 1.3% | 10 AWG (Recommended) |
| 100 feet | 6.4% (Fail) | 4.0% (Marginal) | 2.5% (OK) | 8 AWG |
| 150 feet | 9.6% (Fail) | 6.0% (Fail) | 3.8% (Marginal) | 6 AWG (for strict 3%) |
When upsizing for voltage drop, the breaker size remains 20 amps. You are simply using a thicker wire to reduce resistance. You can use a tool like the Southwire Voltage Drop Calculator to verify your exact run length and load before pulling wire.
Derating Factors: Bundling, Ambient Heat, and Aluminum
The baseline assumption of 12 AWG for 20 amps falls apart if your installation environment deviates from the norm. The NEC requires you to apply correction and adjustment factors that effectively reduce the ampacity of the wire.
Conductor Bundling (Raceway Fill)
If you are pulling multiple circuits through a single piece of conduit, the wires heat each other up. Per NEC 310.15(C)(1), if you have between 4 and 6 current-carrying conductors in a raceway, you must multiply the wire's ampacity by 80%. If you have 7 to 9 conductors, the multiplier drops to 70%.
For 12 AWG THHN (75°C column = 25A), a 70% derating factor yields 17.5 amps. Because 17.5 amps is less than your 20-amp breaker, you must upsize to 10 AWG THHN (35A × 0.70 = 24.5A) to safely protect the circuit.
High Ambient Temperatures
Running NM-B or THHN through an unconditioned attic in the middle of summer introduces severe ambient heat. If the attic reaches 110°F (43°C), you must apply a temperature correction factor. For THHN (90°C rating used for derating purposes), the correction factor at 105°F-113°F is 0.87. While 12 AWG THHN can usually absorb this hit and still carry 20 amps, NM-B cable (limited to the 60°C column) derates much faster and may require upsizing to 10 AWG if the ambient temperature exceeds standard limits.
The Aluminum Trap
Aluminum wire is cheaper and lighter than copper, but it has higher resistance and expands/contracts more under thermal cycling. Never use 12 AWG aluminum for a 20-amp circuit. Aluminum branch circuits typically start at 8 AWG or 6 AWG. Furthermore, aluminum requires specific anti-oxidant paste (like Noalox) and precise torque settings on terminations to prevent high-resistance connections that lead to arcing and fires. For standard 20-amp residential branch circuits, stick to copper.
Continuous Loads and When to Involve the AHJ
A "continuous load" is defined by the NEC as any load where the maximum current is expected to continue for 3 hours or more. Examples include commercial lighting, server racks, space heaters, and EV chargers.
For continuous loads, NEC 210.20(A) requires the overcurrent device to be rated at 125% of the continuous load. Conversely, this means you can only load a standard breaker to 80% of its rating. A 20-amp breaker can only handle a 16-amp continuous load. If your specific application requires a true, uninterrupted 20-amp draw for hours at a time, a 20-amp breaker and 12 AWG wire are illegal and unsafe. You must upsize to a 25-amp breaker (if the receptacle allows) or a 30-amp breaker with 10 AWG wire and a 30-amp receptacle configuration.
- Your calculated voltage drop exceeds 5% (feeder + branch combined).
- You are routing conductors through environments where ambient temperatures routinely exceed 104°F (40°C).
- You are dealing with non-standard continuous loads (like hardwired 20A commercial equipment) that push the 80% derating rule to its mathematical limits.
- Local amendments override standard NEC practices (some municipalities strictly forbid aluminum branch wiring or have specific conduit fill limits).
Always remember that while the NEC provides the baseline safety standard, your local electrical inspector has the final say on what constitutes a safe, compliant installation in your specific jurisdiction.
Sizing the wire for a 20 amp circuit is straightforward on paper: 12 AWG copper and a 20-amp breaker. But on the jobsite, you must account for the physical length of the run, the heat of the environment, and the number of wires sharing the same conduit. By verifying your voltage drop and checking your derating factors before you make your first termination, you ensure a circuit that is not only code-compliant, but built to last.
