For a standard 20 amp breaker, you must use 12 AWG copper wire (or 10 AWG aluminum). This is the absolute minimum size required by the NEC to safely handle 20 amps without overheating the insulation or tripping the breaker under continuous load conditions.
- Material: Copper (unless aluminum is explicitly stated)
- Temperature Column: 75°C (standard for most modern breakers and THHN/THWN-2 terminations)
- Ambient Temperature: 30°C (86°F)
- Conduit/Cable Type: Standard raceway (EMT) or NM-B cable with 3 or fewer current-carrying conductors
The Baseline: 12 AWG Copper and NEC Ampacity Rules
The direct answer to "what gauge wire for 20 amp breaker" is rooted in NFPA 70 (National Electrical Code), specifically Article 240.4(D). This article places strict limits on small conductors to prevent DIYers and even seasoned pros from over-fusing wires that cannot safely dissipate the heat generated by high current.
Why 12 AWG and not one size smaller? A 14 AWG copper wire is rated for a maximum of 15 amps in the 60°C column. If you were to pull 20 amps through 14 AWG wire, the conductor would heat up beyond its insulation rating. Over time, this thermal degradation causes the insulation to become brittle, crack, and eventually expose bare copper, leading to arc faults or short circuits. The breaker might not trip immediately because it is rated for 20 amps, but the wire is silently failing.
| Wire Size (AWG) | Insulation Type | Temp Rating | Max Ampacity | Max Breaker Size (NEC 240.4) |
|---|---|---|---|---|
| 14 AWG | THHN / NM-B | 60°C / 90°C | 15A (60°C col) | 15 Amps |
| 12 AWG | THHN / NM-B | 60°C / 90°C | 20A (60°C col) | 20 Amps |
| 10 AWG | THHN / NM-B | 60°C / 90°C | 30A (60°C col) | 30 Amps |
Note: While THHN wire is technically rated for 30A at 90°C for 12 AWG, NEC 240.4(D) overrides this and hard-limits the overcurrent protection for 12 AWG copper to 20 amps regardless of the insulation's higher thermal tolerance.
When 12 AWG Fails: Voltage Drop and Long Runs
Ampacity tables assume the wire can handle the heat, but they do not account for the resistance of the copper over long distances. As wire length increases, resistance increases, causing voltage to drop before it reaches the load. The NEC recommends a maximum voltage drop of 3% for branch circuits to ensure equipment operates efficiently and motors do not overheat from undervoltage.
Let's run a voltage drop check at a stated distance of 50 feet (one-way run length) on a 120V circuit pulling a full 20A load. Using the standard formula and referencing the Cerrowire voltage drop guidelines:
- Formula: VD = (2 × L × I × R) / 1000
- 12 AWG Copper Resistance (R): ~1.98 ohms per 1,000 ft
- Calculation: (2 × 50 × 20 × 1.98) / 1000 = 3.96 Volts dropped
- Percentage: (3.96 / 120) × 100 = 3.3%
At 50 feet, 12 AWG copper exceeds the 3% recommended limit. If your run from the panel to the furthest receptacle exceeds 45 feet on a 120V/20A circuit, you must upsize to 10 AWG copper to mitigate voltage drop. (10 AWG has a resistance of ~1.24 ohms/kft, dropping the loss to 2.48V, or 2.06%).
| Circuit Voltage | One-Way Run Distance | Required Copper Wire Size | Reasoning |
|---|---|---|---|
| 120V | Under 45 ft | 12 AWG | Meets ampacity & <3% VD |
| 120V | 45 ft to 75 ft | 10 AWG | Prevents >3% VD at 20A |
| 240V | Under 90 ft | 12 AWG | Higher voltage halves VD % |
| 240V | 90 ft to 150 ft | 10 AWG | Prevents >3% VD at 20A |
Derating Factors: What Changes the Wire Size Answer
The baseline answer of 12 AWG assumes ideal conditions. In the real world, bundling, ambient heat, and material changes force you to upsize. Here is what changes the answer and why.
1. Conductor Bundling (The NM-B Trap)
NEC 310.15(C)(1) requires you to derate the ampacity of conductors when you bundle more than three current-carrying wires in a single raceway or cable. If you pull four 12 AWG THHN wires in an EMT conduit (two circuits), you apply an 80% derating factor. Because THHN is rated in the 90°C column for derating purposes (30A × 0.80 = 24A), 12 AWG THHN is still perfectly legal for a 20A breaker.
However, if you are using NM-B (Romex) cable, you are legally restricted to the 60°C column. 12 AWG NM-B is rated 20A. If you bundle multiple NM-B cables through a single bored hole in a wood stud for more than 24 inches, you must apply the 80% derating factor: 20A × 0.80 = 16A. Because 16A is less than your 20A breaker, you must upsize to 10 AWG NM-B or switch to individual THHN wires in conduit.
2. Aluminum Conductors
Aluminum and copper are not interchangeable. Aluminum has higher resistance and expands/contracts more under thermal load. 12 AWG aluminum is only rated for 15 amps. To safely feed a 20 amp breaker with aluminum, you must use a minimum of 10 AWG aluminum (rated 30A at 75°C). Furthermore, standard 20A duplex receptacles are rarely rated for aluminum terminations. If you must use aluminum, you need CO/ALR rated devices and must apply an anti-oxidant compound like Noalox to the terminations to prevent galvanic corrosion and high-resistance arcing.
3. When an Engineer or AHJ Must Confirm
While standard residential branch circuits are straightforward, you must defer to a licensed professional engineer or your local AHJ/inspector under the following conditions:
- Ambient temperatures consistently exceed 40°C (104°F), such as in unventilated attic spaces in southern climates, which requires additional temperature correction factors.
- Complex bundling scenarios involving more than 6 current-carrying conductors in a single conduit.
- Runs exceeding 200 feet, where voltage drop calculations and fault-current clearing times require formal engineering validation.
- Local municipal amendments that ban NM-B cable entirely or mandate specific AFCI/GFCI combinations that alter panel fill calculations.
Frequently Asked Questions
Can I use 14 gauge wire on a 20 amp breaker if the actual load is only 10 amps?
No. The NEC does not allow you to size the breaker based on the "expected" load of a specific appliance on a general-purpose branch circuit. Article 240.4(D) strictly prohibits placing a 20A overcurrent device on 14 AWG copper wire. If a future homeowner or tenant plugs in a space heater, a vacuum, and a hair dryer simultaneously, the 14 AWG wire will overheat and potentially cause a fire before the 20A breaker ever trips. The breaker must protect the wire's physical limits, not just your current appliance list.
What gauge wire for a 20 amp breaker at 240 volts?
You still use 12 AWG copper as the baseline minimum. The ampacity of the wire is determined by the current (amps) and the heat it generates, not the voltage. However, because the voltage is doubled (240V vs 120V), the percentage of voltage drop is cut in half for the same distance. This means you can run a 240V/20A circuit (like a heavy-duty window AC unit or a baseboard heater) roughly twice as far as a 120V circuit before you are forced to upsize to 10 AWG for voltage drop mitigation.
Does the equipment grounding conductor (EGC) need to be 12 AWG on a 20 amp circuit?
Yes. According to NEC Table 250.122, the minimum size for an equipment grounding conductor on a 20A overcurrent device is 12 AWG copper. You cannot use a 14 AWG ground wire just because it only carries current during a fault condition. During a dead short, the ground wire must be robust enough to carry massive fault current instantaneously to trip the breaker without melting. If you upsize your current-carrying conductors to 10 AWG for voltage drop over a long run, you are also required by code to proportionally upsize your ground wire to 10 AWG.
Can I mix 12 AWG and 14 AWG wire on the same 20 amp breaker?
Absolutely not. A fundamental rule of electrical wiring is that the overcurrent protective device (the breaker) must be sized to protect the smallest wire in the entire circuit. If you have a 20A breaker feeding a 12 AWG home run that transitions to a 14 AWG pigtail or branch, the 14 AWG wire becomes a bottleneck. If the circuit draws 18 amps, the 14 AWG wire will overheat and melt while the 20A breaker remains completely closed. Every inch of wire on a 20A circuit must be 12 AWG or larger.
