Use 12 AWG copper wire protected by a 20 amp breaker for standard 120V or 240V residential branch circuits. This is the definitive, code-compliant baseline for wiring 20A receptacles, appliances, and lighting runs.
- Material: Solid copper conductors.
- Insulation: THHN/THWN-2 (in conduit) or NM-B (Romex).
- Terminations: 75°C rated breaker lugs and devices.
- Ambient Temperature: 30°C (86°F) or lower.
- Raceway Fill: No more than three current-carrying conductors in a single conduit or cable.
Why 12 AWG? The NEC Ampacity Table Breakdown
To understand why 12 AWG is the correct pick, we have to look at how the National Electrical Code (NEC) marries wire insulation limits to overcurrent protection. Sizing isn't just about what the wire can physically handle before melting; it is about protecting the weakest link in the circuit, which is usually the termination point at the breaker or receptacle.
According to NFPA 70 (NEC) Table 310.16, a 12 AWG copper conductor has different ampacity ratings depending on the temperature column you use:
| Wire Size (AWG) | 60°C Column (NM-B / Romex) | 75°C Column (THHN in Conduit) | 90°C Column (Derating Only) |
|---|---|---|---|
| 14 AWG | 15 Amps | 20 Amps | 25 Amps |
| 12 AWG | 20 Amps | 25 Amps | 30 Amps |
| 10 AWG | 30 Amps | 35 Amps | 40 Amps |
Why not use 14 AWG?
Looking at the 75°C column above, you might think 14 AWG (rated 20A) could handle a 20A breaker. It cannot. NEC Section 240.4(D) contains a specific "small conductor rule" that strictly limits the overcurrent protection for 14 AWG copper to 15 amps, regardless of the insulation's higher temperature rating. If you put 14 AWG on a 20A breaker, you are violating code and creating a fire hazard.
The NM-B (Romex) vs. THHN Trap
If you are running yellow NM-B cable (commonly called Romex), NEC 334.80 mandates that you must use the 60°C column for ampacity, even though the individual wires inside the sheath have 90°C insulation. In the 60°C column, 12 AWG is rated exactly 20A. Therefore, a 20A breaker (like a standard Eaton BR120 or Square D HOM220) is the maximum allowable protection.
If you are pulling individual THHN/THWN-2 wires through EMT conduit, you are allowed to use the 75°C column (assuming your breaker lugs are rated 75°C, which nearly all modern ones are). Here, 12 AWG is rated 25A. However, because standard 20A duplex receptacles are designed for 20A circuits, we still pair the 12 AWG wire with a 20A breaker to match the load and device ratings.
Decision Tree: Standard Runs vs. Long Distance Voltage Drop
Ampacity tables assume a relatively short run. When wire gets long, resistance increases, causing voltage drop. While the NEC doesn't strictly enforce voltage drop as a hard rule for most residential branch circuits, NEC 210.19(A) Informational Note recommends keeping branch circuit voltage drop under 3% for reasonable efficiency. At 120V, a 3% drop is 3.6 volts.
Let's run the math on a 100-foot run of 12 AWG copper carrying a 16A load (80% of the breaker's capacity, representing a heavy continuous draw like a space heater or window AC).
K = 12.9 (copper), I = 16A, D = 100 ft, CM = 6530 (circular mils for 12 AWG)
VD = (2 × 12.9 × 16 × 100) / 6530 = 6.31 Volts (5.25% drop)
A 5.25% drop exceeds the 3% recommendation and can cause motors to overheat or lights to dim. Here is your decision path for sizing:
| Circuit Length (One-Way) | Expected Max Load | Required Wire Size | Breaker Size |
|---|---|---|---|
| Under 50 feet | Up to 16A continuous / 20A peak | 12 AWG Copper | 20 Amp |
| 50 to 85 feet | Up to 16A continuous / 20A peak | 12 AWG Copper | 20 Amp |
| 85 to 120 feet | 12A to 16A continuous | 10 AWG Copper | 20 Amp |
| Over 120 feet | 12A to 16A continuous | 8 AWG Copper | 20 Amp |
The Concrete Pick: For any standard run under 85 feet, buy 12 AWG. If you are wiring a detached garage, a long driveway gate, or a distant workshop outlet pushing past 85 feet, step up to 10 AWG to mitigate voltage drop. You can verify your specific scenario using the Southwire Voltage Drop Calculator.
Variables That Force an Upsize (Derating & Material)
The baseline assumptions at the top of this guide are exactly that: a baseline. Real-world jobsite conditions frequently force you to upsize your wire. Here is what changes the answer.
1. Bundling and Ambient Temperature (Derating)
NEC 310.15(C)(1) requires ampacity derating when you bundle more than three current-carrying conductors in a single raceway. If you pull four 12 AWG THHN wires through a single 1/2-inch EMT conduit (e.g., two separate 120V circuits sharing a neutral or just multiple hot/neutral pairs), you must derate the 90°C ampacity (30A) by 80%.
30A × 0.80 = 24A. You can still use 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 20A trip curve, and thermal buildup becomes a real issue. Rule of thumb: If you are pulling more than 3 current-carrying conductors in a conduit, upsize to 10 AWG.
2. High Ambient Temperatures
If your conduit runs through an attic in the American Southwest where ambient temperatures regularly exceed 113°F (45°C), Table 310.15(B)(1) forces a temperature correction factor. At 50°C ambient, the 90°C column must be multiplied by 0.82. Again, when in doubt, pulling 10 AWG gives you the thermal headroom to absorb these correction factors without tripping the breaker prematurely.
3. Aluminum vs. Copper
Aluminum wire is cheaper, but it has higher resistance and expands/contracts more than copper under load, leading to loose terminations and arc faults if not installed perfectly. If you must use aluminum for a 20A circuit, you must use 10 AWG Aluminum (rated 30A in the 75°C column). Furthermore, your receptacles and breakers must be explicitly rated CO/ALR. For 99% of residential 20A branch circuits, stick to copper to avoid termination failures.
Continuous Loads and the 80% Rule
A massive point of failure for DIYers is ignoring NEC 210.20(A), which governs continuous loads. A continuous load is defined as any load where the maximum current is expected to continue for three hours or more. Examples include commercial lighting, server racks, aquarium heaters, and hardwired EV chargers.
The code requires that the branch circuit rating (breaker and wire) be sized at no less than 125% of the continuous load.
- Scenario A: You have a 16A continuous load. 16A × 1.25 = 20A. You need a 20A breaker and 12 AWG wire.
- Scenario B: You have a 20A continuous load. 20A × 1.25 = 25A. You need a 25A (or 30A) breaker and 10 AWG wire. You cannot put a 20A continuous load on a standard 20A breaker and 12 AWG wire; the breaker will eventually nuisance-trip as the bimetallic strip heats up over time.
Always check the nameplate on hardwired appliances. If the manufacturer specifies a "Minimum Circuit Ampacity" (MCA) of 20A, they have already done the 125% math for you. Match the wire to the MCA, and the breaker to the "Maximum Overcurrent Protection" (MOCP) listed on the same plate.
When an Engineer or the AHJ Must Confirm
While 12 AWG on a 20A breaker covers the vast majority of residential and light-commercial receptacle and lighting circuits, there are edge cases where you must defer to a licensed professional or your local Authority Having Jurisdiction (AHJ).
- Motor Circuits: If your 20A circuit is dedicated to a large motor (like a well pump or heavy compressor), NEC Article 430 allows for breaker sizing up to 250% of the motor's full-load current to accommodate startup inrush. Wire sizing and breaker pairing here require specific calculations that override standard branch circuit rules.
- Service Entrance and Feeders: This guide applies to branch circuits. If you are sizing a feeder to a subpanel, the continuous load calculations and diversity factors change entirely.
- Local Amendments: Some municipalities have strict local amendments that mandate 12 AWG for all lighting circuits (banning 14 AWG entirely) or require AFCI/GFCI combinations that may introduce specific voltage drop sensitivities. Always pull a permit and have the rough-in inspected by your local AHJ before closing up drywall.
For standard 20A receptacle runs in your home, workshop, or garage, spool up the 12 AWG solid copper, terminate it on a quality 20A breaker, torque the lugs to the manufacturer's spec (usually 35-45 in-lbs), and you will have a safe, code-compliant circuit that will last the life of the building.
