The direct answer to "what gauge romex for 20 amp" is 12 AWG copper wire (typically sold as 12/2 or 12/3 NM-B). Romex is a brand name for Non-Metallic (NM-B) sheathed cable, and for a 20-amp branch circuit, the National Electrical Code (NEC) mandates a minimum wire size of 12 American Wire Gauge (AWG) to safely carry the load without overheating. Using the correct gauge dictates the circuit's maximum safe current capacity, limits voltage drop over distance, and ensures the breaker will trip before the wire insulation melts in a fault condition.
The Core Sizing Rule and Ampacity Table
Wire gauge is an inverse measurement system: the lower the AWG number, the thicker the copper conductor and the more current it can safely carry. For standard residential branch circuits operating at 120V or 240V, the NEC ties specific breaker sizes to minimum wire gauges to prevent thermal runaway. While you can always use a thicker wire (like 10 AWG) on a 20-amp breaker, you can never use a thinner wire (like 14 AWG).
Below is the standard ampacity chart for NM-B Romex cable based on the 60°C column of NEC Table 310.16. Note the jacket colors, which manufacturers use to help electricians and inspectors quickly identify wire sizes on the jobsite.
| Wire Gauge (AWG) | Max Ampacity (60°C) | Standard Breaker Size | Common NM-B Jacket Color | Typical Use Case |
|---|---|---|---|---|
| 14 AWG | 15 Amps | 15A | White | General lighting, bedroom receptacles |
| 12 AWG | 20 Amps | 20A | Yellow | Kitchen/bath receptacles, garage outlets |
| 10 AWG | 30 Amps | 30A | Orange | Electric dryers, RV receptacles, long runs |
| 8 AWG | 40 Amps | 40A | Black | Electric ranges, large HVAC air handlers |
Worked Example: Continuous Loads and the 80% Rule
To understand what wire gauge changes in a real installation, let's look at a continuous load scenario. The NEC defines a continuous load as one where the maximum current is expected to continue for three hours or more. For continuous loads, you must derate the circuit capacity to 80% of the breaker's rating.
The Scenario: You are wiring a dedicated 120V circuit for a basement server rack and networking equipment that will draw a steady 16 amps continuously.
- Step 1: Calculate the required circuit rating. 16 amps / 0.80 (the continuous load multiplier) = 20 amps. You need a 20-amp breaker.
- Step 2: Select the wire. A 20-amp breaker requires a minimum of 12 AWG Romex.
- Step 3: Verify the physics. 12 AWG copper has a resistance of roughly 1.588 ohms per 1,000 feet. On a 100-foot run (200 feet total for hot and neutral), the resistance is 0.317 ohms. At 16 amps, the voltage drop is V = I × R (16 × 0.317 = 5.07 volts). This leaves 114.9V at the load, well within the acceptable 5% drop limit for branch circuits.
If you had mistakenly used 14 AWG wire (rated for 15A), the 16A continuous load would exceed the wire's thermal limit. The insulation would slowly degrade, become brittle, and eventually short out, all while the 20A breaker remains completely unaware of the danger because 16A is below its 20A trip threshold.
Where You Meet This in Practice
In modern residential construction, 12 AWG Romex on 20-amp breakers is the standard for high-draw and moisture-prone areas. You will specifically encounter this requirement in the following locations:
- Kitchen Small Appliance Circuits: The NEC requires at least two 20-amp circuits to serve kitchen countertop receptacles. These must be wired with 12/2 NM-B to handle microwaves, toasters, and coffee makers running simultaneously.
- Bathroom Receptacles: Bathroom outlets require GFCI protection and must be on a 20-amp circuit to handle high-wattage hair dryers and space heaters. 12 AWG is mandatory here.
- Garage and Outdoor Outlets: Because these areas frequently power heavy-duty tools, air compressors, and holiday lighting, code requires 20-amp circuits utilizing 12 AWG wire.
- Laundry Rooms: A dedicated 20-amp receptacle for a washing machine must be wired with 12/2 Romex to handle the motor's startup surge and heating elements.
Common Confusions and Dangerous Mismatches
When sizing wire for a 20-amp circuit, DIYers and even some apprentice electricians frequently run into a few specific traps regarding insulation ratings and voltage drop.
Another common confusion involves long-run voltage drop. While 12 AWG is the code minimum for a 20-amp breaker, it is not always the best engineering choice. If you are running a 20-amp circuit to a detached garage 150 feet away, pushing a full 20-amp load will result in a voltage drop exceeding 5%, which can damage power tools and cause motors to overheat. In this scenario, you should upsize to 10 AWG Romex (orange jacket) for the long run, terminating it at a junction box and transitioning back to 12 AWG for the final drops to the receptacles.
Frequently Asked Questions
Can I use 12/3 Romex for a 20-amp circuit?
Yes. 12/3 Romex contains a black (hot), red (hot), white (neutral), and bare (ground) wire. It is perfectly rated for 20 amps and is typically used for 240V appliances, multi-wire branch circuits (MWBC), or 3-way switch travelers. Just ensure you cap the unused red wire with a wire nut if you are only wiring a standard 120V receptacle.
Do I need a torque screwdriver for 12 AWG terminations?
Yes. Since the 2017 NEC update, Article 110.14(D) requires that terminations be tightened to the manufacturer's specified torque. 12 AWG solid copper is stiff; if you just "crank it down" with a standard screwdriver, you risk stripping the brass terminal on a cheap receptacle or leaving it too loose, which causes high-resistance arcing and melted outlets under a 20-amp load. Use an insulated torque screwdriver set to the receptacle manufacturer's spec (usually between 12 to 18 in-lbs).
Is aluminum Romex ever used for 20-amp circuits?
No. NM-B Romex sold for residential branch circuits is exclusively copper. Aluminum wire requires different sizing (you would need 10 AWG aluminum to safely carry 20 amps) and specific anti-oxidant compounds at terminations. Stick to copper NM-B for all standard 120V/240V indoor branch circuits.
