Breaker Amperage Chart: Wire Gauge Sizing & NEC 310.16 Guide

For standard residential copper wiring, a 15-amp breaker requires 14 AWG wire, a 20-amp breaker requires 12 AWG, and a 30-amp breaker requires 10 AWG. These baseline values are derived from the 75°C column of NEC Table 310.16, but the actual ampacity you can use depends on your insulation type, termination temperature ratings, and ambient heat. Below is the master reference chart, followed by the exact rules for selecting the right column and adjusting for real-world conditions.

The Master Breaker Amperage Chart (NEC Table 310.16)

How to read this table: This chart assumes copper conductors, an ambient temperature of 30°C (86°F), and standard residential branch circuits. The three temperature columns represent the thermal limits of the wire insulation and the equipment terminations. The "Max Breaker" column factors in NEC 240.4(D) restrictions for small conductors and standard breaker sizing increments.

Quick-jump to the most queried residential sizes:

• 15 Amp Breaker (14 AWG)
• 20 Amp Breaker (12 AWG)
• 30 Amp Breaker (10 AWG)
• 50 Amp Breaker (6 AWG)

*Asterisk denotes sizes where NEC 240.4(B) "next standard size up" rule applies if the calculated load does not exceed the wire's ampacity and standard breaker sizes (like 45A or 75A) are unavailable. Source: NFPA 70 (NEC) Table 310.16.

Which Temperature Column Applies to Your Installation?

The most common mistake DIYers make is looking exclusively at the 90°C column because it offers the highest ampacity. You can rarely use the 90°C column for your final breaker sizing. Here is how to select the correct column based on termination temperature rules (NEC 110.14):

• The 60°C Column: Use this for Non-Metallic Sheathed Cable (NM-B / Romex) in sizes 14, 12, and 10 AWG. Even though modern NM-B insulation is rated for 90°C, NEC 334.80 mandates that the ampacity must be determined using the 60°C column. You also use this column if your breaker or device lugs are explicitly marked "60°C" (common on older equipment or specific smart switches).
• The 75°C Column: This is your default for most residential branch circuits using individual THHN/THWN wires in conduit, or for NM-B cables sized 8 AWG and larger. Nearly all modern residential breakers and receptacles feature 75°C rated terminations.
• The 90°C Column: You only use this column as a starting point for calculating derating factors (ambient temperature or conductor bundling). After applying the derating math, the final adjusted ampacity must still be compared against the 75°C or 60°C termination limits.

How Derating and the 125% Rule Modify Base Values

The chart above assumes a perfect environment: 30°C (86°F) ambient temperature and no more than three current-carrying conductors (CCCs) in a single raceway. Real-world jobsites rarely cooperate.

Ambient Temperature and Bundling Derating

If you are running THHN wire through an attic in Texas where ambient temperatures hit 45°C (113°F), or if you are pulling four or more CCCs through a single conduit, you must apply correction factors from NEC Table 310.15(B)(1)(1) and 310.15(C)(1).

Worked Example: You are pulling four 12 AWG THHN current-carrying conductors through a conduit in a 40°C (104°F) attic to feed a multi-wire branch circuit.

1. Start with the 90°C column base for 12 AWG: 30A.
2. Apply the 40°C ambient correction factor (0.91): 30A × 0.91 = 27.3A.
3. Apply the bundling correction factor for 4 CCCs (0.80): 27.3A × 0.80 = 21.84A.
4. Check against termination limits: Your breaker lugs are 75°C (rated 25A for 12 AWG). 21.84A is less than 25A, so the wire is thermally protected.
5. Apply NEC 240.4(D): Because it is 12 AWG, the absolute maximum breaker size remains 20A. The wire is perfectly safe for this installation.

The 125% Continuous Load Rule

If a load is expected to run for 3 hours or more (e.g., EV chargers, baseboard heaters, commercial lighting), NEC Article 210.20 requires you to size the breaker and wire at 125% of the continuous load. A 16-amp continuous EV charger requires a circuit rated for at least 20 amps (16 × 1.25 = 20), meaning you must use a 20A breaker and 12 AWG wire. You cannot put a 16A continuous load on a 15A breaker, even if the breaker technically hasn't tripped yet.

What This Chart Cannot Tell You (Edge Cases & Voltage Drop)

An ampacity chart only solves for heat dissipation and fire prevention. It does not account for power quality or specialized equipment behavior. Before finalizing your wire and breaker size, check these three blind spots:

1. Voltage Drop Over Distance

NEC 310.16 does not penalize you for voltage drop, but NEC Informative Annex D recommends keeping branch circuit voltage drop under 3%. If you are running a 120V, 20-amp circuit to a detached workshop 150 feet away, 12 AWG copper will result in a voltage drop of roughly 9.7V (over 8%). Your tools will run hot and inefficiently. In this scenario, you must upsize to 8 AWG or 6 AWG copper to maintain voltage, even though a 20A breaker technically allows 12 AWG.

2. Motor Inrush Currents (LRA vs. FLA)

If you are wiring an air compressor or table saw, the motor's Full Load Amps (FLA) might be 12A, suggesting a 15A breaker. However, the Locked Rotor Amps (LRA) or inrush current can be 5 to 7 times higher for a fraction of a second. Standard thermal-magnetic breakers handle brief magnetic spikes, but if the breaker is sized too close to the FLA, nuisance tripping will occur on startup. Always size motor circuit breakers based on the manufacturer's nameplate maximum overcurrent protection, which often permits sizing up to 250% of the FLA per NEC Article 430.

3. Aluminum vs. Copper

This entire guide assumes copper conductors. If you are pulling SER cable for a subpanel or running overhead service drops using aluminum (like 2-2-2-4 Dyke or XHHW-2), the ampacities are significantly lower. For example, 2 AWG aluminum is only rated for 90A at 75°C, whereas 2 AWG copper is rated for 115A. Always verify the conductor material stamp on the wire jacket before referencing any chart.