For standard residential branch circuits, the correct wire size for a 20-amp breaker is 12 AWG copper, and for a 15-amp breaker, it is 14 AWG copper. This baseline assumes standard NM-B (Romex) cable and is derived directly from the 60°C ampacity column of NEC Table 310.16. However, simply matching a breaker to a wire gauge without considering terminal ratings, conduit fill, or continuous loads is a common path to tripped breakers or melted insulation.

Safety Warning: Any work inside a breaker panel involves lethal mains voltage. De-energize the main breaker, verify the bus bars are dead with a tested non-contact voltage tester and a multimeter, and consult your local Authority Having Jurisdiction (AHJ). NEC-style guidance provided here is for educational purposes; local code always has final authority.

How to Read the Wire Size for Breaker Chart

The table below is a streamlined version of NEC Table 310.16 (formerly 310.15(B)(16)), which dictates the allowable ampacities for insulated conductors. To use this chart, locate your wire gauge (AWG or kcmil) in the left column, then read across to the appropriate material and temperature column.

How to read the columns: The temperature headings (60°C, 75°C, 90°C) refer to the insulation rating of the wire (e.g., THHN is 90°C, THWN is 75°C, NM-B is 60°C). The values represent the maximum continuous current the wire can carry under standard conditions (ambient temperature of 30°C / 86°F, with no more than three current-carrying conductors in a raceway). Bookmark the quick-jump rows below for the most common residential queries.

Wire Size (AWG/kcmil)Copper 60°C (NM-B)Copper 75°C (THWN)Copper 90°C (THHN)
14 AWG15A20A25A
12 AWG20A25A30A
10 AWG30A35A40A
8 AWG40A50A55A
6 AWG55A65A75A
4 AWG70A85A95A
3 AWG85A100A110A
2 AWG95A115A130A
1 AWG110A130A145A
1/0 AWG125A150A170A
2/0 AWG145A175A195A
3/0 AWG165A200A225A
4/0 AWG195A230A260A

Which Temperature Column Applies to Your Installation

A frequent mistake on the jobsite is looking at a spool of 90°C THHN wire, seeing that 12 AWG is rated for 30A in the 90°C column, and attempting to protect it with a 30-amp breaker. This violates NEC 110.14(C).

The rule is simple: your ampacity is limited by the lowest temperature rating of any connected component in the circuit. Most standard residential breakers, receptacles, and switches are rated for 75°C terminals. However, NEC 110.14(C)(1)(a) explicitly states that for circuits rated 100 amps or less, or for wire sizes 14 AWG through 1 AWG, you must use the 60°C column to determine ampacity, regardless of the wire's actual insulation rating.

Pro Tip: The 90°C column is not useless. You use the 90°C column to calculate derating (adjusting for heat or conduit fill), but the final derated ampacity must still be equal to or greater than the 60°C baseline ampacity to be protected by that specific breaker size.

Derating Factors: How Environment Modifies the Base Value

The wire size for breaker chart assumes ideal conditions: an ambient temperature of 30°C (86°F) and no more than three current-carrying conductors bundled together. When reality deviates from this, you must apply derating factors from NEC Table 310.15(C)(1).

  • Conduit Fill (Bundling): If you pull four to six current-carrying conductors through a single conduit, the heat generated by the wires cannot dissipate. You must multiply the base ampacity (from the 90°C column) by 80%. For example, 10 AWG THHN (40A at 90°C) derates to 32A. Since 32A is still higher than the 60°C baseline of 30A, you can still use a 30A breaker. But if you bundle 7 to 9 conductors, the multiplier drops to 70% (40A x 0.70 = 28A), forcing you to upsize to 8 AWG wire.
  • Ambient Temperature: If your conduit runs through a hot attic that reaches 50°C (122°F), you must apply an ambient temperature correction factor. For 90°C wire at 50°C, the multiplier is 0.82.

Always multiply the derating factors together if both conditions exist. A comprehensive wire sizing guide from the Copper Development Association provides detailed worksheets for these combined calculations.

What This Chart Cannot Tell You

While the ampacity chart is the foundation of wire sizing, it is blind to three critical real-world variables:

  1. Voltage Drop: The NEC table assumes the wire is short enough that resistance won't cause a significant voltage drop. For long runs (typically over 50 feet for 120V circuits), a 12 AWG wire might safely handle 20A thermally, but the voltage at the receptacle could drop below 114V, damaging sensitive electronics. Aim for a maximum 3% voltage drop on branch circuits by upsizing the wire for distance.
  2. Continuous Loads: If a load will run for 3 hours or more (like a hardwired heater or commercial lighting), NEC 210.20(A) requires you to size the breaker at 125% of the continuous load. A 16-amp continuous heater requires a 20-amp breaker (16 x 1.25 = 20), which in turn requires 12 AWG wire, not 14 AWG.
  3. Motor Full Load Current (FLC): Motors draw massive inrush currents. You cannot size a motor circuit breaker using the standard chart alone; you must follow NEC Article 430, which allows breakers to be sized up to 250% of the motor's FLC to prevent nuisance tripping during startup, while relying on the motor's internal overload protection to protect the wire.

Frequently Asked Questions

What wire size for a 30 amp breaker?

For a standard 30-amp breaker, you must use a minimum of 10 AWG copper wire. This applies whether you are using NM-B (Romex) or THHN in conduit, as the 60°C baseline for 10 AWG is exactly 30 amps. If you are using aluminum wire (like SER cable for an outdoor subpanel), you must step up to 8 AWG aluminum, as aluminum has a lower ampacity per gauge than copper.

Can I use 10 AWG wire on a 20 amp breaker?

Yes, absolutely. It is always safe and code-compliant to use a wire that is larger (thicker) than the minimum required for the breaker. A 10 AWG wire on a 20-amp breaker will run cooler and experience less voltage drop over long distances. The only drawback is the physical difficulty of terminating 10 AWG solid copper onto the smaller terminal screws of standard 15A/20A receptacles; you may need to pigtail it down to 12 AWG inside the junction box.

What size wire for a 50 amp breaker?

A 50-amp breaker requires a minimum of 6 AWG copper wire or 4 AWG aluminum wire. This is the standard configuration for a 50A RV receptacle (NEMA 14-50) or a heavy-duty workshop welder circuit. Ensure the receptacle itself is rated for 75°C, which allows you to use the 75°C column for termination verification, though the 60°C column still governs standard branch circuit sizing for wires under 1 AWG.

Does the ground wire need to be the same size as the hot wire?

Not necessarily. The equipment grounding conductor (EGC) is sized based on the rating of the overcurrent device (the breaker), not the hot wire size. According to NEC Table 250.122, a 20A breaker requires a 12 AWG copper ground, and a 30A breaker requires a 10 AWG copper ground. However, if you upsized your hot wires to compensate for voltage drop over a long distance, NEC 250.122(B) requires you to proportionally increase the ground wire size as well to maintain a low-impedance fault path.