For a standard 20-amp circuit, use 12 AWG copper wire protected by a 20-amp breaker. This is the absolute baseline for residential and commercial branch circuits. If your one-way run exceeds 50 feet at full load, you must calculate voltage drop and upgrade to 10 AWG copper to maintain safe, code-compliant delivery at the receptacle.
- Material: Copper (aluminum requires different sizing, covered below)
- Temperature Column: 75°C (standard for modern breakers and terminals)
- Ambient Temperature: 30°C (86°F)
- Installation: Up to 3 current-carrying conductors in a raceway, or standard NM-B (Romex) cable
The Core Sizing Rule: Why 12 AWG and Not 14 AWG?
A common mistake on the jobsite is assuming that because 14 AWG THHN wire has a 90°C ampacity of 25 amps, it can safely handle a 20-amp breaker. It cannot. The National Electrical Code (NEC) specifically restricts small conductors to prevent overheating before the breaker trips.
Under NEC Article 240.4(D), the overcurrent protection for 14 AWG copper is hard-capped at 15 amps, 12 AWG at 20 amps, and 10 AWG at 30 amps. Even if your math shows a 14 AWG wire won't melt at 18 amps, the breaker must trip at 15 amps. Therefore, 12 AWG is the absolute minimum size for a 20-amp circuit. Using 14 AWG on a 20-amp breaker is a direct code violation and a severe fire hazard.
Ampacity Table Breakdown: Temperature Columns and Insulation
Wire ampacity changes based on the insulation type and the temperature rating of the terminals it connects to. Per NEC 110.14(C), you must size your wire based on the lowest temperature rating in the circuit. Most residential NM-B (Romex) is rated for 60°C, while individual THHN/THWN-2 wires in conduit are rated for 75°C or 90°C at the terminals.
| AWG Size | 60°C Column (NM-B / Romex) | 75°C Column (THHN in Conduit) | 90°C Column (Derating Base) |
|---|---|---|---|
| 14 AWG | 15A (Max Breaker: 15A) | 20A (Max Breaker: 15A) | 25A |
| 12 AWG | 20A (Max Breaker: 20A) | 25A (Max Breaker: 20A) | 30A |
| 10 AWG | 30A (Max Breaker: 30A) | 35A (Max Breaker: 30A) | 40A |
Voltage Drop: When 12 AWG Isn't Enough
Ampacity tells you what size wire prevents a fire. Voltage drop tells you what size wire actually delivers usable power to your device. The NEC recommends a maximum 3% voltage drop on branch circuits for reasonable efficiency.
Let's run the exact math for a 20-amp load on a 120V circuit using 12 AWG copper:
- Allowable Drop (3% of 120V): 3.6 Volts
- Circular Mils (CM) for 12 AWG: 6,530
- K Factor (Copper at 75°C): ~12.9
Using the single-phase voltage drop formula: Distance = (Voltage Drop × CM) / (2 × K × Current)
Distance = (3.6 × 6530) / (2 × 12.9 × 20) = 23,508 / 516 = 45.5 feet.
If your one-way wire run from the panel to the furthest receptacle exceeds 50 feet and you expect to pull a full 20 amps, 12 AWG will yield a voltage drop greater than 3%. You must upgrade to 10 AWG copper. The breaker remains 20 amps; only the wire gets larger to reduce resistance.
Decision Tree: Finalizing Your Wire Pick
Use this decision matrix on the bench to lock in your exact material and gauge before cutting any wire.
| Scenario Condition | Required Action | Final Concrete Pick |
|---|---|---|
| Standard residential run, under 50 ft, non-continuous load. | Apply baseline NEC 240.4(D) rules. | 12 AWG Copper (NM-B or THHN) |
| Run exceeds 50 feet at or near full 20A load. | Upsize to mitigate >3% voltage drop. | 10 AWG Copper (Keep 20A breaker) |
| Pulling 4 to 6 current-carrying conductors in one conduit. | Apply 80% derating to 90°C column (30A × 0.8 = 24A). | 12 AWG Copper THHN (24A > 20A) |
| Pulling 10 to 20 current-carrying conductors in one conduit. | Apply 50% derating to 90°C column (30A × 0.5 = 15A). 12 AWG fails. | 10 AWG Copper THHN (40A × 0.5 = 20A) |
| Using Aluminum wire (e.g., SER cable or feeders). | Aluminum has higher resistance; requires larger gauge. | 10 AWG Aluminum (Min. 75°C rated) |
What Changes the Answer: Derating, Bundling, and Aluminum
The baseline 12 AWG answer assumes a clean, simple installation. Real-world jobsites introduce variables that force you to upsize.
Conduit Bundling and Derating
When you bundle multiple circuits in a single conduit, the wires heat each other up. Per NEC 310.15(C)(1), if you have more than three current-carrying conductors in a raceway, you must apply an adjustment factor. If you pull four 12 AWG THHN circuits (8 current-carrying conductors) through a single PVC conduit, you apply an 80% adjustment factor to the 90°C ampacity (30A × 0.80 = 24A). Since 24A is still above your 20A breaker, 12 AWG survives. But if you pack 12 current-carrying conductors into that pipe, the 50% derating factor drops 12 AWG to 15A. You must jump to 10 AWG.
Aluminum Conductors
Never treat aluminum and copper interchangeably. Aluminum has higher electrical resistance and expands/contracts more under thermal load. For a 20-amp circuit using AA-8000 series aluminum alloy, the minimum size is 10 AWG (rated 30A at 75°C). However, verify your breaker terminals are explicitly marked 'AL/CU'. Many standard 20-amp residential breakers are only listed for copper, or require a minimum of 8 AWG aluminum for physical termination strength. When in doubt, stick to copper for branch circuits under 30 amps.
When an Engineer or the AHJ Must Confirm
While 12 AWG copper covers 95% of 20-amp residential receptacle circuits, specific load profiles require formal verification by a licensed engineer or your local Authority Having Jurisdiction (AHJ).
- Continuous Loads: NEC Article 210.20(A) dictates that if a load will run for 3 hours or more (like commercial lighting or heavy server racks), the circuit must be sized at 125% of the continuous load. If your continuous load is exactly 16 amps, 16 × 1.25 = 20 amps. A 20-amp breaker and 12 AWG wire is the absolute limit. If the load is 17 amps continuous, you must step up to a 25-amp breaker and 10 AWG wire.
- High Ambient Temperatures: If your conduit runs through an attic that regularly exceeds 113°F (45°C), the ampacity of your wire drops. You must apply the temperature correction factors in NEC Table 310.15(B)(1). In a 50°C attic, 12 AWG THHN loses nearly 20% of its capacity, forcing an upgrade to 10 AWG.
- Specific Appliance Nameplates: Always defer to the manufacturer's Minimum Circuit Ampacity (MCA) printed on the appliance nameplate. If a 20-amp rated HVAC compressor specifies a 15-amp breaker and 14 AWG wire, follow the nameplate (which is UL-listed). If it specifies 10 AWG, use 10 AWG, regardless of general NEC branch circuit rules.
For any standard kitchen, bathroom, or garage receptacle circuit in a climate-controlled home under 50 feet from the panel, terminate your 12 AWG copper on a 20-amp breaker and move to the next box.
