You need 12 AWG copper wire for a 20 amp breaker. This is the absolute minimum size permitted by the NEC for standard residential branch circuits. While 10 AWG is also safe and often used for long runs to mitigate voltage drop, 14 AWG is strictly prohibited and creates a severe fire hazard on a 20A circuit.
Baseline Assumptions for This Guide
- Material: Copper (Aluminum requires different sizing, detailed below)
- Temperature Column: 75°C terminations (standard for modern breakers), 60°C for NM-B cable ampacity
- Ambient Temperature: 30°C (86°F) or lower
- Wiring Method: NM-B (Romex) in walls or THHN/THWN-2 in conduit
- Phase: Single-phase, 120V or 240V AC
The Baseline: Why 12 AWG Copper is the Standard
When determining what size wire for 20 amp breaker installations, the National Electrical Code (NEC) draws a hard line based on thermal limits and overcurrent protection. A breaker's job is to protect the wire, not the appliance. If you push 20 amps through a 14 AWG copper wire, the wire will overheat, melt its insulation, and potentially start a fire inside your walls long before a 20A breaker's thermal trip mechanism engages.
Under NEC 240.4(D), small conductors have strict overcurrent protection limits. Even though 14 AWG copper might show a 20A rating in the 75°C column of the ampacity tables, the NEC explicitly caps its breaker size at 15A. Therefore, 12 AWG is your starting point for 20A circuits.
| Wire Gauge (AWG) | Material | 60°C Column (Amps) | 75°C Column (Amps) | 90°C Column (Amps) | Max Standard Breaker |
|---|---|---|---|---|---|
| 14 AWG | Copper | 15A | 20A | 25A | 15A (NEC 240.4(D)) |
| 12 AWG | Copper | 20A | 25A | 30A | 20A (NEC 240.4(D)) |
| 10 AWG | Copper | 30A | 35A | 40A | 30A |
| 8 AWG | Copper | 40A | 50A | 55A | 40A |
| 12 AWG | Aluminum | 15A | 20A | 25A | 15A |
| 10 AWG | Aluminum | 25A | 30A | 35A | 25A |
Insulation matters: If you are using NM-B (standard indoor Romex), NEC 334.80 restricts you to the 60°C ampacity column, meaning 12 AWG NM-B is rated for exactly 20A. If you pull individual THHN wires in conduit, you can use the 90°C column for derating purposes, but the final termination ampacity at the breaker and receptacle is still governed by the 75°C or 60°C rating of those terminals, per NEC 110.14(C).
When 12 AWG Fails: Voltage Drop and Long Runs
Ampacity tables tell you what size wire will prevent a fire. They do not tell you what size wire will actually deliver usable voltage to your load. NEC 210.19(A) Informational Note recommends a maximum voltage drop of 3% for branch circuits to ensure reasonable efficiency.
Let's run a voltage drop check for a 120V circuit on a 20A breaker at a distance of 100 feet (which means 200 feet of total wire length for the hot and neutral).
- 12 AWG Copper: Resistance is ~1.93 ohms per 1,000 ft. At 20A over 200 ft, the drop is 7.72V. That is a 6.4% drop. Your 120V tool is now seeing 112V, which can cause motors to overheat and trip their internal thermal protectors.
- 10 AWG Copper: Resistance is ~1.21 ohms per 1,000 ft. The drop is 4.84V (4.0%). Better, but still above the 3% ideal.
- 8 AWG Copper: Resistance is ~0.764 ohms per 1,000 ft. The drop is 3.05V (2.5%). This passes the 3% recommendation.
| One-Way Distance | Wire Size for < 3% Drop | Wire Size for < 5% Drop | Practical Application |
|---|---|---|---|
| Under 50 ft | 12 AWG | 12 AWG | Standard interior rooms, kitchens, garages |
| 50 ft - 75 ft | 10 AWG | 12 AWG | Detached sheds, long hallway runs |
| 75 ft - 125 ft | 8 AWG | 10 AWG | Detached workshops, well pumps, long driveway lighting |
| Over 125 ft | 6 AWG or higher | 8 AWG | Agricultural buildings, remote gate motors |
The practical fix: If you are wiring a standard kitchen receptacle 40 feet from the panel, stick with 12 AWG. If you are running a 20A circuit to a detached workshop 100 feet away to run a table saw, pull 10 AWG or 8 AWG. You will also need to pigtail the 8 AWG wire down to 12 AWG at the receptacle, as most standard 20A duplex receptacles cannot physically accept 8 AWG solid wire under their terminal screws.
Derating Factors: What Changes the Answer
The baseline 12 AWG answer assumes perfect conditions. In the real world, heat buildup from bundling or high ambient temperatures forces you to increase your wire size. This is where the 90°C column of the ampacity table becomes useful for THHN/THWN-2 conductors in conduit.
1. Conductor Bundling in a Raceway
If you pull multiple circuits through the same conduit, the wires heat each other up. Under NEC 310.15(C)(1), if you have 4 to 6 current-carrying conductors in a single raceway, you must derate the ampacity to 80%.
If you are using 12 AWG THHN (90°C column = 30A), 80% of 30A is 24A. Since 24A is greater than your 20A breaker, 12 AWG THHN is still legally acceptable for 4-6 bundled wires. However, if you have 7 to 9 conductors (derated to 70%), 30A x 0.70 = 21A. You are now dangerously close to the limit, and bumping to 10 AWG is the professional move to prevent nuisance tripping and terminal degradation.
2. Aluminum vs. Copper
Never treat aluminum and copper interchangeably. Aluminum has higher resistance and expands/contracts more under thermal cycling. For a 20A breaker, 12 AWG aluminum is strictly prohibited (it is only rated for 15A). You must use a minimum of 10 AWG aluminum, but because aluminum terminations on standard 20A receptacles are often not rated for the thermal expansion of aluminum, most jurisdictions and professionals strongly prefer copper for all 15A and 20A branch circuits. Reserve aluminum for larger feeder wires (like 2 AWG for a 100A subpanel) where the cost savings outweigh the installation hassle.
3. High Ambient Temperatures
If your conduit runs through an unventilated attic in the Southwest US, the ambient temperature can easily exceed 110°F (43°C). You must apply temperature correction factors from NEC Table 310.15(B)(1). At 41-45°C ambient, the 90°C THHN wire must be derated to 87%. While 12 AWG THHN can survive this mathematically, the heat inside the attic combined with the load heat will degrade the breaker's internal thermal-magnetic trip curve over time. Bumping to 10 AWG provides a necessary thermal buffer.
When an Engineer or the AHJ Must Confirm
While the rules above cover 95% of residential and light commercial work, certain scenarios require you to step back and consult a Professional Engineer (PE) or your local Authority Having Jurisdiction (AHJ / electrical inspector).
Stop and Consult the AHJ When:
- Continuous Loads: The NEC defines a continuous load as one that runs for 3 hours or more (e.g., commercial lighting, server racks, EV chargers). Under NEC 210.20(A), a breaker must be sized at 125% of the continuous load. A 20A breaker can only handle 16A continuously. If your load is a true 20A continuous draw, you must upsize to a 25A breaker and use 10 AWG wire. Many inspectors will fail a 20A circuit if they know it feeds a continuous 20A load.
- Local Amendments: Some municipalities have strict local codes. For example, Chicago and surrounding areas require all wiring to be in metallic conduit (EMC) rather than NM-B cable, which changes fill calculations and heat dissipation profiles entirely.
- Mixed Temperature Ratings: If you are terminating a modern 75°C rated THHN wire into an older, legacy panelboard or a cheap receptacle only rated for 60°C, the entire circuit's ampacity drops to the lowest rated component in the chain. An inspector will check the termination ratings, not just the wire jacket.
Getting the wire size right for a 20A breaker is about balancing the hard limits of the NEC with the physical realities of voltage drop and heat. Stick to 12 AWG copper for standard interior runs under 50 feet, upsize to 10 AWG or 8 AWG for long distances, and always respect the temperature ratings of your terminations. When in doubt, pulling a larger wire is a one-time material cost; replacing melted terminals or fighting a voltage-starved motor is a recurring headache.
