Standard Breaker Sizes Chart: AWG Wire, Ampacity, and NEC Sizing Rules

If you need the direct answer for standard residential branch circuits: a 15-amp breaker requires 14 AWG copper wire, a 20-amp breaker requires 12 AWG, a 30-amp breaker requires 10 AWG, a 40-amp breaker requires 8 AWG, and a 50-amp breaker requires 6 AWG. However, sizing a breaker is never just about matching a wire gauge to an overcurrent device. The National Electrical Code (NEC) enforces strict terminal temperature limits, continuous load multipliers, and conductor derating rules that frequently force you to upsize your wire or downsize your load.

This reference guide provides the complete breaker sizes chart based on NEC Table 310.16 and 240.4(D), explains exactly which temperature column applies to your installation, and highlights the edge cases that cause DIYers and apprentices to fail electrical inspections.

How to Read This Breaker Sizes Chart

Before looking at the numbers, you must understand the three temperature columns in the NEC ampacity tables: 60°C, 75°C, and 90°C. Modern wire insulation (like THHN) is rated for 90°C, but your breaker and receptacle terminals are rarely rated that high.

The Standard Max Breaker Size column in the chart below reflects NEC 240.4(D), which places hard caps on overcurrent protection for small conductors. Even if a 12 AWG THHN wire has an ampacity of 30A in the 90°C column, NEC 240.4(D) strictly limits it to a 20-amp breaker.

Standard Breaker Sizes Chart (NEC Table 310.16 & 240.4(D))

The following table assumes copper conductors in an ambient temperature of 30°C (86°F). Bookmark this section for quick reference on the most common residential and light commercial feeder sizes.

*Note on 6 AWG: While 6 AWG is rated 55A at 60°C, NEC 240.4(B) allows you to round up to the next standard breaker size (60A) if the calculated load does not exceed 55A. However, for continuous loads, you cannot use the round-up rule.

When Derating Modifies Your Base Breaker Size

The ampacities in the chart above assume you have no more than three current-carrying conductors in a raceway and an ambient temperature of 30°C (86°F). When you bundle wires or run them through hot attics, you must apply derating factors from NEC Table 310.15(B)(1) (formerly 310.15(B)(3)(a)).

Derating is calculated using the 90°C column, but the final derated ampacity must still be protected by a breaker that does not exceed the wire's capacity.

Worked Example: You are pulling four 12 AWG THHN current-carrying conductors through a single conduit to feed a multi-wire branch circuit.

• Base 90°C Ampacity: 30A
• Derating Factor (4-6 conductors): 80%
• Derated Ampacity: 30A × 0.80 = 24A

Because 24A is greater than the 20A maximum breaker size mandated by NEC 240.4(D) for 12 AWG wire, you can still safely use a 20-amp breaker. However, if you pulled seven conductors (70% derating factor), the math becomes 30A × 0.70 = 21A. You are now dangerously close to the 20A breaker limit, and any ambient heat will push the wire past its safe operating threshold, requiring you to upsize to 10 AWG.

What This Breaker Sizes Chart Cannot Tell You

A standard ampacity chart is only half the equation. It will not account for these three critical installation variables:

1. Voltage Drop: The NEC recommends (and many local AHJs mandate) a maximum 3% voltage drop on branch circuits. If you are running a 240V circuit to a detached garage 150 feet away, 10 AWG wire on a 30A breaker will suffer unacceptable voltage drop under load. You must upsize the wire to 8 AWG or 6 AWG to compensate for resistance over distance, even though the breaker remains 30A.
2. Continuous Loads (The 125% Rule): NEC Article 210.20 defines a continuous load as any load expected to run for 3 hours or more. You must multiply the continuous load by 1.25 to size the breaker. A 16-amp continuous space heater requires a 20-amp breaker (16 × 1.25 = 20).
3. Motor Starting Currents: Motors draw massive Locked Rotor Amps (LRA) when starting. NEC Article 430 allows you to size motor branch circuit breakers up to 250% of the motor's Full Load Amps (FLA) to prevent nuisance tripping during startup, completely bypassing the standard wire-to-breaker ratio in the chart above.

Frequently Asked Questions

What size breaker and wire do I need for a 48-amp EV charger?

An EV charger is a continuous load, meaning you must apply the 125% rule: 48A × 1.25 = 60A. You need a 60-amp breaker. Because the circuit is 100A or less, NEC 110.14(C) forces you to use the 60°C column for wire sizing. 6 AWG copper in the 60°C column is only rated for 55A, which is insufficient for a 60A breaker. Therefore, you must use 4 AWG copper wire (rated 70A at 60°C). If you only have 6 AWG wire available, you must configure the charger's internal dip-switches to draw a maximum of 40A, allowing you to use a 50-amp breaker. For more on EV infrastructure, refer to the Department of Energy's home charging guidelines.

Can I use a 20-amp breaker on 14 AWG wire if my load is only 12 amps?

No. NEC 240.4(D) explicitly restricts 14 AWG copper wire to a maximum 15-amp overcurrent device, regardless of the actual connected load. The breaker protects the wire inside the walls, not just the appliance plugged into it. If someone later plugs in a high-draw device or taps into that circuit to add another receptacle, the 14 AWG wire could overheat and melt before the 20-amp breaker ever trips.

How do I size a breaker and wire for a 100-amp subpanel feeder?

For a 100-amp subpanel, you need a 100-amp breaker. Because this is exactly at the 100A threshold, you are permitted to use the 75°C column if your breaker and panel lugs are rated for it (most modern ones are). In the 75°C column, 4 AWG copper is rated for 85A (too small), so you must step up to 3 AWG copper (rated 100A) or 1 AWG aluminum (rated 100A). If you are using SER cable in thermal insulation, consult the specific derating requirements for bundled cable assemblies.