Romex (NM-B cable) sizing for a specific breaker refers to selecting the correct American Wire Gauge (AWG) that can safely carry the breaker's maximum continuous current without overheating the conductor or the termination points. If you are asking what size Romex for 20 amp breakers, the direct, code-compliant answer is 12 AWG copper.
The Short Answer: 12 AWG NM-B Copper
For a standard residential 120V or 240V 20-amp circuit, you must use 12 AWG copper NM-B (commonly known by the brand name Romex). In big-box stores, 12 AWG NM-B is universally identified by its yellow outer jacket. You will typically buy this as 12/2 (two current-carrying conductors plus a ground) for standard receptacle and lighting circuits, or 12/3 (three current-carrying conductors plus a ground) for multi-wire branch circuits or 240V appliances like window AC units.
Why 12 AWG? The NEC Ampacity Math
To understand why 12 AWG is the mandated size, we have to look at how the National Electrical Code (NEC) rates wire insulation versus termination limits. If you read the printing on a yellow jacket of 12/2 NM-B, you will see it rated for 90°C (194°F). Looking at NEC Table 310.16, 12 AWG copper in the 90°C column is rated for 30 amps. So why can't we put a 30-amp breaker on it?
Two code rules override that 90°C insulation rating:
- NEC 110.14(C) (Termination Temperatures): Standard residential breakers and receptacles are only tested and rated for 60°C or 75°C terminations. You must size the wire based on the weakest link in the chain, which is usually the 60°C column for standard 15A/20A devices.
- NEC 240.4(D) (Small Conductor Rule): This specific article acts as a hard cap for small wires. It explicitly states that the overcurrent protection for 12 AWG copper shall not exceed 20 amps, regardless of the insulation temperature rating.
In the 60°C column of Table 310.16, 12 AWG copper is rated for exactly 20 amps. This perfectly matches the breaker size, ensuring the breaker will trip before the wire or the receptacle screws melt.
Where You Meet This in Practice
You will encounter 20-amp circuits (and therefore 12 AWG yellow Romex) in specific high-draw areas of a home where the NEC mandates extra capacity. According to Electrical Contractor Magazine and NEC Article 210, these mandatory 20-amp zones include:
- Kitchen Small Appliance Circuits: You need at least two dedicated 20-amp circuits for countertop receptacles (microwaves, toasters, blenders).
- Bathroom Receptacles: At least one 20-amp circuit is required for bathroom GFCI outlets to handle hair dryers and space heaters.
- Garage and Outdoor Receptacles: General use outlets in garages, sheds, and exterior walls must be 20-amp GFCI protected to handle power tools and holiday lighting.
- Laundry Rooms: A dedicated 20-amp circuit is required for the washing machine receptacle.
Decision Tree: Sizing for Distance and Load
While 12 AWG is the baseline, wire acts like a long resistor. Over extended distances, voltage drop becomes a factor. The Copper Development Association recommends keeping voltage drop under 3% for branch circuits. Here is the exact decision path for sizing your 20-amp Romex run:
| Circuit Length (One-Way) | Expected Continuous Load | Calculated Voltage Drop (120V) | Required Wire Size |
|---|---|---|---|
| Under 50 feet | Up to 16A (80% of 20A) | < 2.0% | 12 AWG NM-B |
| 50 to 85 feet | Up to 16A | 2.0% - 3.4% | 12 AWG NM-B (Acceptable up to 3%) |
| 85 to 130 feet | 12A to 16A | > 3.0% | 10 AWG NM-B (Bump up one size) |
| Over 130 feet | 16A | > 5.0% | 8 AWG THHN in conduit (NM-B not recommended) |
Worked Numeric Example: Let's calculate the voltage drop for a 12 AWG copper run that is 100 feet long, powering a 16-amp continuous load (like a portable heater) on a 120V circuit.
Formula: VD = (2 × K × I × L) / CM
Where K (copper resistance) = 12.9, I (current) = 16A, L (length) = 100 ft, and CM (circular mils for 12 AWG) = 6,530.
VD = (2 × 12.9 × 16 × 100) / 6530 = 6.32 Volts.
Percentage drop: (6.32 / 120) × 100 = 5.26%.
Because 5.26% exceeds the 3% recommendation, a 100-foot run at full 16-amp load requires bumping up to 10 AWG to prevent motor burnout or dimming lights.
Common Confusions and Wiring Mistakes
When sizing Romex for a 20-amp breaker, DIYers frequently fall into three traps:
- Confusing Aluminum with Copper: Romex (NM-B) is exclusively copper. If you are looking at aluminum wire (like SER cable for a subpanel), the sizing changes entirely. 12 AWG aluminum does not exist in standard residential branch circuits; you would need 10 AWG aluminum for 20 amps, but aluminum is generally prohibited for 15A and 20A branch circuits under NEC 310.106.
- Mixing 14 AWG and 12 AWG on the Same Circuit: If your home run from the panel is 12 AWG, every single extension and pigtail on that 20-amp circuit must also be 12 AWG. Dropping down to 14 AWG for a switch leg or a single outlet creates a bottleneck that the 20-amp breaker cannot protect.
- Ignoring the 80% Continuous Load Rule: If a device runs for 3 hours or more (like a grow light or a space heater left on overnight), NEC 210.20(A) requires the breaker to be sized at 125% of the load. A 16A continuous load requires a 20A breaker, and therefore 12 AWG wire. You cannot put a 16A continuous load on a 15A breaker/14 AWG wire.
FAQ: 20 Amp Circuit Edge Cases
Can I use 12 AWG Romex on a 15-amp breaker?
Yes, absolutely. It is perfectly legal and safe to use a larger wire on a smaller breaker. Many professional electricians use 12 AWG for all general lighting and receptacle circuits, even on 15-amp breakers, to minimize voltage drop and future-proof the installation. The only downside is that 12 AWG is stiffer and slightly harder to fold into crowded junction boxes.
Does the ground wire in 12/2 Romex need to be 12 AWG?
No. In standard 12/2 NM-B cable, the two insulated conductors (black and white) are 12 AWG, but the bare copper equipment grounding conductor is typically 14 AWG. This is code-compliant per NEC Table 250.122, which allows a 14 AWG ground for a 20-amp circuit.
What if I am wiring a 240V baseboard heater on a 20-amp breaker?
You still use 12 AWG. However, you will use 12/2 NM-B and mark the white wire with black or red electrical tape at both ends to indicate it is being used as a hot conductor, not a neutral. 240V baseboard heaters do not require a neutral wire.
For 95% of residential 20-amp runs under 85 feet, buy 12/2 NM-B copper (yellow jacket), terminate it on 20-amp rated devices, and torque the screws to the manufacturer's specification. Stop overthinking the 90°C column, respect the 60°C termination limits, and your installation will be safe, legal, and inspection-ready.
