If you need the direct answer for standard residential copper wiring (NM-B/Romex), here are the three most common matches: 14 AWG wire gets a 15A breaker, 12 AWG wire gets a 20A breaker, and 10 AWG wire gets a 30A breaker. These limits are strictly enforced by NEC Article 240.4(D) for small conductors, regardless of the insulation temperature rating printed on the wire jacket.

However, once you move past 10 AWG into larger feeder wires, or switch to THHN in conduit, the rules shift to the base ampacity tables. Sizing a breaker is not just about matching a wire gauge to a number; it requires understanding termination temperature limits, continuous load multipliers, and conduit derating. This reference guide breaks down the exact National Electrical Code (NEC) standards you need to size breakers and wire safely and legally.

The Master Circuit Breaker Sizing Chart (NEC Table 310.16)

How to read this table: This chart synthesizes data from NEC Table 310.16 and the small conductor overrides in Article 240.4(D). The columns represent the temperature rating of the wire insulation and the termination points. For residential work, you will almost exclusively use the 60°C and 75°C columns. The 'Max Standard Breaker' column reflects the standard overcurrent device sizes listed in NEC 240.6. Bookmark the quick-jump links below for the most queried residential sizes.

Quick-Jump Common Sizes: 15A (14 AWG) | 20A (12 AWG) | 30A (10 AWG) | 60A (6 AWG / 4 AWG)

Wire Size (AWG/kcmil) Copper 60°C (NM-B) Copper 75°C (THWN) Copper 90°C (THHN) Aluminum 75°C (XHHW) Max Standard Breaker (Res)
14 AWG 15A 20A 25A - 15A (240.4D Limit)
12 AWG 20A 25A 30A 15A 20A (240.4D Limit)
10 AWG 30A 35A 40A 25A 30A (240.4D Limit)
8 AWG 40A 50A 55A 40A 40A or 50A
6 AWG 55A 65A 75A 50A 60A (Next size up rule)
4 AWG 70A 85A 95A 65A 70A or 80A
3 AWG 85A 100A 110A 75A 100A
2 AWG 95A 115A 130A 90A 110A or 125A
1 AWG 110A 130A 145A 100A 125A or 150A
1/0 AWG 125A 150A 170A 120A 150A
Code Caveat: NEC Article 240.4(D) strictly limits 14, 12, and 10 AWG copper conductors to 15, 20, and 30 amps respectively. Even if you pull 12 AWG THHN (rated 30A at 90°C) through conduit, you cannot put it on a 30A breaker for a standard receptacle circuit. The 20A limit holds.

Which Column Applies to Your Installation?

The most common mistake DIYers and junior apprentices make is looking at the 90°C column because modern THHN wire is rated for it. You cannot use the 90°C column for final breaker sizing. Here is how to determine which column governs your installation based on NEC 110.14(C):

The 60°C Column (The Residential Default)

If you are using Nonmetallic-Sheathed Cable (NM-B / Romex), NEC 334.80 mandates that the ampacity must be determined using the 60°C column, regardless of the fact that the individual wires inside the jacket might have 90°C THHN insulation. Furthermore, for circuits rated 100 amps or less, standard breakers, receptacles, and switches are assumed to have 60°C termination ratings unless explicitly marked otherwise. Always use the 60°C column for standard 15A, 20A, and 30A branch circuits.

The 75°C Column (Subpanels and Feeders)

You can use the 75°C column if two conditions are met: (1) The wire insulation is rated for at least 75°C (e.g., THWN, XHHW), and (2) both the starting and ending termination points (lugs, breakers, terminal blocks) are rated for 75°C. Modern main lugs and subpanel feeders often carry the 75°C rating, allowing you to use 4 AWG copper for a 85A load (protected by a 90A or 100A breaker depending on continuous load rules), or 2 AWG aluminum for a 90A load.

The 90°C Column (Derating Only)

The 90°C column is almost exclusively used as a mathematical starting point for derating calculations when you have multiple current-carrying conductors bundled in a single raceway. The final calculated ampacity must still be capped at the 60°C or 75°C rating of your terminations.

Derating, Continuous Loads, and What the Chart Cannot Tell You

The table above provides the base thermal limits of the wire in a vacuum (specifically, at an ambient temperature of 30°C / 86°F with no more than three current-carrying conductors in a raceway). Real-world installations require adjustments.

Continuous Loads (The 125% Rule)

NEC 210.20 defines a continuous load as one where the maximum current is expected to continue for 3 hours or more. This includes hardwired lighting, EV chargers, and baseboard heaters. For continuous loads, the breaker must be sized at 125% of the continuous load.
Example: A 16A continuous EV charger requires a breaker rated for at least 20A (16 x 1.25 = 20). The wire must also be sized to carry 20A continuously.

Conduit Derating (NEC 310.15(C)(1))

When you pull more than three current-carrying conductors through a single conduit, they heat each other up. You must multiply the wire's base ampacity by a derating factor.
Example: You pull four 12 AWG THHN conductors in a conduit. The 90°C base ampacity is 30A. For 4-6 conductors, the derating factor is 80%.
30A x 0.80 = 24A.
However, because your breaker terminations are likely 75°C (25A) or 60°C (20A), your final usable ampacity is capped by the termination limit. If it's a standard 20A receptacle circuit, you are still capped at 20A.

Pro-Tip on Neutral Counting: In a standard multi-wire branch circuit (MWBC) or single-phase feeder, the neutral carries only the unbalanced load and is not counted as a current-carrying conductor for derating. However, if you are running 3-phase circuits or non-linear loads (like LED drivers or computers) that generate harmonic triplen currents, the neutral must be counted, which drastically changes your derating math.

What the Chart Cannot Tell You

While NEC Table 310.16 is the bible for thermal ampacity, it is blind to three critical installation variables:

  • Voltage Drop: The chart assumes a short run. If you are running a 50A subpanel 150 feet away, 6 AWG copper will safely handle the heat, but you will experience severe voltage drop. To maintain the NEC-recommended 3% drop on branch circuits (and 5% total for feeder + branch), you must consult Chapter 9, Table 8 for circular mil area and upsized the wire, often by two or three gauges. Tools like the Mike Holt Enterprises voltage drop calculators are essential here.
  • Short-Circuit Interrupting Capacity (AIC): The chart tells you nothing about fault currents. A standard residential breaker has an Ampere Interrupting Capacity (AIC) of 10,000 amps (10kA). If your home is located very close to a utility transformer, the available fault current might exceed 22kA. In that scenario, you must buy breakers with a higher AIC rating (e.g., 22kA or 65kA), or the breaker could physically explode during a dead short, regardless of the wire size.
  • Specific Appliance Rules (NEC 422, 430, 440): HVAC units and large motors have their own sizing rules based on Minimum Circuit Ampacity (MCA) and Maximum Overcurrent Protection (MOCP). It is entirely legal and common to see an HVAC condenser that specifies a 40A MOCP breaker but only requires 10 AWG or 8 AWG wire. In these cases, the breaker is protecting the motor's internal thermal overloads, not just the wire. Always defer to the manufacturer's nameplate data over the general ampacity chart for these specific loads.