For a standard residential branch circuit, the direct answer to what size wire for 20 amp circuit breakers is 12 AWG copper wire paired with a 20-amp breaker. This applies to typical 120V or 240V circuits using NM-B (Romex) or THHN in conduit, assuming a maximum continuous load of 16 amps and standard ambient temperatures.
- Material: Copper (Aluminum requires different sizing; see below)
- Temperature Column: 60°C for NM-B cable; 75°C for THHN/THWN in conduit (standard termination ratings)
- Ambient Temperature: 30°C (86°F) or lower
- Raceway/Conduit: No more than 3 current-carrying conductors bundled together
Note: NEC-style guidance is provided here; your local Authority Having Jurisdiction (AHJ) has final authority on all installations.
The Core Sizing Rule: Why 12 AWG and Not 14 AWG?
It is a common misconception that wire sizing is based purely on the melting point or thermal limits of the copper itself. If you look at the 90°C column in the NEC ampacity tables, 14 AWG copper can technically handle 25 amps without the insulation melting. So why can't you put it on a 20-amp breaker?
The answer lies in NEC Article 240.4(D), which establishes specific overcurrent protective device (OCPD) limits for small conductors. This rule exists to protect the wire from slow, smoldering overloads that might not trip a larger breaker but will degrade the insulation over time. Under 240.4(D):
- 14 AWG copper is strictly limited to a maximum 15-amp breaker.
- 12 AWG copper is strictly limited to a maximum 20-amp breaker.
- 10 AWG copper is strictly limited to a maximum 30-amp breaker.
If your 20-amp circuit will power a continuous load (defined by the NEC as a load expected to run for 3 hours or more, like a space heater, server rack, or commercial lighting), you must derate the breaker capacity by 80%. A 20-amp breaker can only safely handle 16 amps of continuous current. If your continuous load exceeds 16A, you must upsize to a 25-amp or 30-amp breaker and use 10 AWG wire.
Ampacity Tables and Temperature Columns
When sizing wire, electricians reference NEC Table 310.16. However, the most frequent mistake DIYers make is reading the wrong temperature column. You must use the column that matches the lowest temperature rating of any connected component, which is almost always the breaker or receptacle terminals.
| AWG Size | 60°C Column (NM-B / Romex) | 75°C Column (THHN in Conduit) | 90°C Column (Derating Only) |
|---|---|---|---|
| 14 AWG | 15A | 20A | 25A |
| 12 AWG | 20A | 25A | 30A |
| 10 AWG | 30A | 35A | 40A |
The "Weakest Link" Rule: Even if you pull 12 AWG THHN wire (rated for 90°C) through conduit, the breaker lugs are typically rated for 75°C. Therefore, your final allowable ampacity is capped at the 75°C column (25A). Furthermore, NEC 240.4(D) overrides this and caps the breaker at 20A for 12 AWG. The 90°C column is strictly reserved for calculating derating adjustments before applying the terminal limits.
When the Answer Changes: Distance, Bundling, and Material
The "12 AWG for 20 amps" rule assumes a short run in a cool environment. Real-world jobsite conditions frequently force you to upsize to 10 AWG or larger.
1. Voltage Drop and Distance Limits
The NEC recommends a maximum voltage drop of 3% for branch circuits to ensure equipment operates efficiently and motors don't overheat. For a 120V circuit, a 3% drop is 3.6 volts. Using the standard voltage drop formula ($VD = \frac{2 \times K \times I \times D}{CM}$), we can find the maximum run length for 12 AWG copper at a full 20A load:
- K (Copper resistivity): 12.9
- I (Current): 20A
- CM (Circular mils for 12 AWG): 6,530
- Maximum Distance (D): ~46 feet
If your 120V, 20-amp circuit run exceeds 46 feet from the panel to the furthest receptacle, 12 AWG will result in unacceptable voltage drop under full load. You must upsize to 10 AWG (which extends the 3% limit to roughly 73 feet) or 8 AWG for longer runs. For 240V circuits, the distance limits double because the baseline voltage is higher.
2. Conduit Bundling and Derating
When you pull multiple wires through a single conduit, they heat each other up. NEC Table 310.15(C)(1) requires you to reduce the ampacity of the wires based on the number of current-carrying conductors (CCC).
Suppose you have 5 CCCs in a conduit (requiring an 80% derating factor). You must use the 90°C column of 12 AWG (30A) for this math: 30A × 0.80 = 24A. Because 24A is greater than your 20A breaker, 12 AWG is still legally permitted. However, if you have 7 to 9 CCCs (70% factor), the math becomes 30A × 0.70 = 21A. While 21A technically passes a 20A non-continuous load, it leaves zero margin for error or continuous load requirements. In any bundle exceeding 6 CCCs, step up to 10 AWG.
3. Aluminum vs. Copper
Aluminum and copper are not interchangeable. Aluminum has higher resistance and expands/contracts more under heat. For a 20-amp circuit, you cannot use 12 AWG aluminum. The practical minimum for a 20-amp aluminum branch circuit or feeder is 10 AWG aluminum (rated 30A at 60°C), though many electricians prefer to use 8 AWG aluminum to ensure mechanical strength at the termination lugs and to mitigate voltage drop. Always use anti-oxidant paste (like Noalox) on aluminum terminations.
When to Call an Engineer or the AHJ
While the guidelines above cover 95% of residential and light-commercial branch circuits, certain scenarios require stamped engineering drawings or a direct consultation with your local electrical inspector:
- High Ambient Temperatures: If the conduit runs through an attic or boiler room where temperatures routinely exceed 113°F (45°C), Table 310.16 requires temperature correction factors that will severely derate 12 AWG wire.
- Motor and HVAC Loads: Air conditioners and large shop motors fall under NEC Article 430. Motor circuits require sizing the wire at 125% of the Full Load Amps (FLA), but the breaker is sized based on the motor's Locked Rotor Current (LRC) to allow for startup surges. Standard branch circuit rules do not apply here.
- Multi-Wire Branch Circuits (MWBC): If you are sharing a neutral between two 20-amp legs, the neutral counts as a current-carrying conductor under specific non-linear load conditions, altering your bundling math.
Frequently Asked Questions
Can I use 10 AWG wire on a 20 amp breaker?
Yes. The NEC dictates the minimum wire size for a given breaker, but you are always allowed to use a larger wire (lower AWG number). Using 10 AWG on a 20-amp breaker is an excellent strategy for long runs to mitigate voltage drop. The only physical challenge is that 10 AWG wire is stiffer and may be difficult to fold into standard single-gang receptacle boxes, and some cheap receptacles cannot physically accept 10 AWG under their screw terminals. In those cases, you must pigtail the 10 AWG to a short 12 AWG jumper using a wire nut or Wago connector.
What size wire for a 20 amp 240V circuit?
You still use 12 AWG copper wire, but you will route it through a 2-pole, 20-amp breaker. For a dedicated 240V appliance (like a baseboard heater or small compressor), you typically use 12/2 NM-B cable (black, white, bare ground). The white wire must be re-identified with black or red electrical tape at both ends to indicate it is a hot leg, not a neutral. Because the voltage is 240V, your allowable run length before hitting a 3% voltage drop doubles to roughly 92 feet compared to a 120V circuit.
Is it safe to mix 12 AWG and 14 AWG on a 20 amp circuit?
Absolutely not. This is a severe fire hazard. If you have a 20-amp breaker protecting a circuit, every single inch of wire on that circuit must be rated for at least 20 amps (12 AWG or larger). If you accidentally splice a piece of 14 AWG wire into a 12 AWG run—perhaps behind a wall or in a junction box—that 14 AWG segment becomes a bottleneck. If the circuit draws 18 amps, the 20-amp breaker will not trip, but the 14 AWG wire will overheat, potentially melting its insulation and starting a fire inside the wall cavity.
How do I size wire for a 20 amp circuit with USB outlets?
Standard 15A or 20A receptacles with built-in USB charging ports do not change the branch circuit wiring requirements. You still use 12 AWG copper wire on a 20-amp breaker. However, be aware that the internal electronics of USB outlets generate their own heat. If you are installing them in a tightly packed multi-gang box, ensure you are using deep boxes (at least 22 cubic inches) to allow for heat dissipation and to accommodate the bulky physical size of the USB receptacle chassis.
