For a standard 20-amp outlet, use 12 AWG copper wire protected by a 20-amp breaker. This applies to both NM-B (Romex) and THHN in conduit under standard residential conditions. Never use 14 AWG on a 20A breaker, as it creates a severe fire hazard.
- Material: Copper (unless explicitly noted as aluminum)
- Temperature Column: 60°C for NM-B cable; 75°C for THHN/THWN-2 in conduit
- Ambient Temperature: 30°C (86°F) or lower
- Conduit Type: Standard EMT, PVC, or non-metallic sheath (NM-B), with a maximum of 3 current-carrying conductors
The Baseline: 12 AWG Copper and the 60°C/75°C Rule
When determining what size wire for a 20 amp outlet is required, the National Electrical Code (NEC) draws a hard line based on the physical limits of the conductor and the overcurrent protective device. The direct answer is 12 AWG copper, but understanding why requires looking at NEC Article 240.4(D), which governs small conductors.
You might look at a wire spool and see that 14 AWG THHN insulation is rated for 90°C, which theoretically allows it to carry more current. However, NEC 240.4(D)(4) strictly limits the overcurrent protection for 14 AWG copper to 15 amps. If you wire a 20-amp outlet with 14 AWG wire and protect it with a 20-amp breaker, a 19-amp load will not trip the breaker, but it will overheat the 14 AWG wire, potentially melting the insulation and starting a fire inside the wall cavity.
By stepping up to 12 AWG copper, you safely cross the 20-amp threshold. The breaker will trip before the wire reaches its thermal failure point. This is a non-negotiable baseline for any 20A branch circuit, whether you are wiring a kitchen countertop receptacle, a garage workbench, or a dedicated window air conditioner circuit.
Ampacity Table: THHN vs. NM-B Insulation
Not all 12 AWG wire is treated equally by the NEC. The allowable ampacity depends heavily on the insulation type and how the cable is installed. According to NFPA 70 (NEC) Table 310.16, we must select the correct temperature column based on the cable type and termination ratings.
| Wire Type | Insulation Rating | NEC Temp Column Used | 12 AWG Ampacity | 10 AWG Ampacity |
|---|---|---|---|---|
| NM-B (Romex) | 90°C | 60°C (per NEC 334.80) | 20A | 30A |
| THHN / THWN-2 | 90°C | 75°C (Termination limit) | 25A | 35A |
| XHHW-2 | 90°C | 75°C (Termination limit) | 25A | 35A |
Why the 60°C column for NM-B? Even though the individual conductors inside a standard Romex (NM-B) cable have 90°C insulation, NEC 334.80 mandates that the ampacity of NM cable must be determined using the 60°C column. This is due to the heat dissipation characteristics of the bundled cable and the plastic sheathing. Fortunately, 12 AWG at 60°C is exactly 20 amps, making it a perfect match for a 20A breaker.
Why the 75°C column for THHN? When you pull individual THHN wires through conduit, the heat dissipation is better. While the wire is rated for 90°C, standard breakers and receptacles are typically rated for 75°C terminations. NEC 110.14(C) requires you to use the 75°C column for termination limits, which grants 12 AWG a 25-amp capacity. We still use a 20A breaker to protect the specific 20A receptacle, but the wire itself is running well within its safe thermal limits.
Voltage Drop: When 12 AWG Isn't Enough
Ampacity tells you if the wire will melt; voltage drop tells you if your equipment will actually run correctly. The NEC recommends (via Informational Notes in Article 210.19) that branch circuit voltage drop should not exceed 3%. For a 120V circuit, that means a maximum allowable drop of 3.6 volts.
Copper wire has inherent resistance. For 12 AWG copper, the resistance is approximately 1.93 ohms per 1,000 feet. Let us run a voltage drop check at a stated distance of 50 feet (100 feet total round-trip wire length) carrying a full 20-amp load.
Formula: VD = (2 × Length × Current × Resistance per ft)
VD = 2 × 50 × 20 × 0.00193
VD = 3.86 Volts
Percentage Drop = (3.86 / 120) × 100 = 3.21%
At 50 feet, a 12 AWG wire exceeds the recommended 3% drop under a maximum 20A load. If you are running a circuit to a detached garage, a long driveway, or a large workshop where the outlet is more than 45 feet from the panel, you must bump the wire size up to 10 AWG copper to maintain power quality. Tools like the Southwire Voltage Drop Calculator are excellent for verifying these numbers on the jobsite before you pull wire.
At 60 feet with 10 AWG copper (resistance ~1.21 ohms/1000ft), the drop is 2.9 volts (2.4%), which safely clears the 3% threshold.
Decision Tree: Finalizing Your Wire and Breaker Pick
Use this decision path to lock in your exact materials for the job. Follow the conditions from top to bottom until you hit your specific scenario.
| Condition / Scenario | Wire Size & Material | Breaker Size |
|---|---|---|
| Standard indoor run, under 45 feet, max 3 conductors in conduit/cable | 12 AWG Copper (NM-B or THHN) | 20A (Single Pole) |
| Long run between 45 and 75 feet at full 20A load | 10 AWG Copper (THHN preferred for conduit fill) | 20A (Single Pole) |
| Running Aluminum wire instead of Copper (e.g., SER cable feed to a subpanel feeding the outlet) | 10 AWG Aluminum (Minimum), 8 AWG preferred for 60°C terminations | 20A or 30A (depending on subpanel feed rules) |
| Conduit contains 4 to 6 current-carrying conductors (requires 80% derating) | 10 AWG Copper (Derates to 28A at 75°C, safely above 20A) | 20A (Single Pole) |
| Default Residential Pick (Standard Conditions) | 12 AWG Copper NM-B or THHN | 20A Breaker |
What Changes the Answer: Bundling, Aluminum, and High Heat
The 12 AWG baseline assumes standard installation conditions. Three main variables will force you to change your wire size:
1. Conductor Bundling (Derating)
If you pull multiple circuits through the same conduit, the wires heat each other up. NEC Table 310.15(C)(1) requires ampacity derating when you have more than three current-carrying conductors in a single raceway. If you have 4 to 6 conductors, you must derate to 80%. A 12 AWG THHN wire (25A at 75°C) derated to 80% yields 20A. While this technically still supports a 20A breaker, best practice and future-proofing dictate stepping up to 10 AWG when bundling to prevent thermal buildup and make pulling the wire physically easier.
2. Aluminum Conductors
Aluminum has higher resistance and different thermal expansion properties than copper. You cannot use the same AWG size for aluminum as you do for copper. For a 20-amp circuit using aluminum, the absolute minimum is 10 AWG (rated 30A at 75°C). However, because many standard 20A receptacles are only rated for 60°C terminations, and 10 AWG aluminum is not rated in the 60°C column for standard branch circuits, you should use 8 AWG aluminum to ensure safe terminations. Always apply an antioxidant compound (like Noalox) to aluminum connections to prevent galvanic corrosion.
3. High Ambient Temperatures
If your conduit runs through an attic that reaches 110°F (43°C) or higher, or runs along a hot roof deck, you must apply ambient temperature correction factors from NEC Table 310.15(B)(1). At 113°F, a 90°C THHN wire must be derated to 87% of its base ampacity. While 12 AWG can usually absorb this slight hit, running 10 AWG provides a necessary safety margin in extreme heat environments.
When to Call an Engineer or the AHJ
While the rules above cover 95% of residential and light commercial 20A outlet installations, specific scenarios require formal verification from a licensed Professional Engineer (PE) or your local Authority Having Jurisdiction (AHJ / electrical inspector).
Continuous Loads: NEC Article 210.20(A) dictates that if a 20-amp outlet will power a continuous load (defined as a load expected to run for 3 hours or more, like a commercial heater, a server rack, or heavy aquarium lighting), the branch circuit must be sized at 125% of the load. A 20A continuous load requires a circuit rated for 25A. This means you must use 10 AWG copper wire and a 25A or 30A breaker (with a properly rated 30A receptacle, or hardwired). You cannot use a standard 20A receptacle for a 20A continuous load.
Local Code Amendments: The NEC is a model code. Your local municipality may have amendments that override standard practices. For example, some jurisdictions require 12 AWG wire for all 15A and 20A lighting and receptacle circuits universally to reduce voltage drop and line losses across the grid, while others may mandate AFCI/GFCI combination breakers that have specific neutral-pigtail wiring requirements. Always check with your local building department or a licensed electrical professional before closing up walls.
