To size a breaker, match it to the lowest ampacity rating in your circuit—usually the wire insulation or the terminal lug. For standard 120V/240V residential branch circuits using NM-B (Romex) cable, you must use the 60°C column of the NEC ampacity table. For THHN wire in conduit terminating in modern 75°C rated panels, use the 75°C column. Never size a breaker higher than the wire's ampacity, and always round down to the nearest standard breaker size (15, 20, 30, 40, 50, 60A) if your exact calculated load doesn't match a standard size.

The Master Circuit Breaker Amperage Chart (NEC 310.16 & 240.6)

This chart synthesizes the ampacity limits from National Fire Protection Association (NFPA) NEC Table 310.16 with the standard overcurrent device ratings from NEC 240.6. It applies strictly to copper conductors in an ambient temperature of 30°C (86°F). The most queried residential sizes (14 through 6 AWG) are highlighted as quick-jump rows for bench and jobsite reference.

Wire Size (AWG) 60°C Ampacity (NM-B / Romex) 75°C Ampacity (THHN in Conduit) 90°C Ampacity (Derating Baseline) Max Standard Breaker Size
14 AWG * 15A 20A 25A 15A
12 AWG * 20A 25A 30A 20A
10 AWG * 30A 35A 40A 30A
8 AWG 40A 50A 55A 40A (or 50A if 75°C)
6 AWG 55A 65A 75A 60A
4 AWG 70A 85A 95A 70A (or 80A)
3 AWG 85A 100A 110A 100A
2 AWG 95A 115A 130A 100A (or 125A)
* NEC 240.4(D) Small Conductor Rule: Even though 14, 12, and 10 AWG copper have higher ampacities at 75°C and 90°C, the NEC strictly caps their overcurrent protection at 15A, 20A, and 30A respectively for standard branch circuits. You cannot put a 12 AWG wire on a 25A breaker, even if the terminal lugs are rated 75°C.

Which Temperature Column Applies to Your Installation?

The most common mistake DIYers make is reading the 90°C column because the wire jacket (like THHN) is stamped with a 90°C rating. You must size the breaker based on the weakest link in the circuit's temperature chain.

  • The 60°C Column (NM-B / Romex): Use this for almost all standard interior residential branch circuits. Even if your NM-B cable contains THHN wires rated for 90°C, NEC 334.80 mandates that NM-B ampacity is limited to the 60°C column. Furthermore, standard 15A and 20A receptacles and switches are typically only rated for 60°C terminations.
  • The 75°C Column (THHN in Conduit): Use this when pulling individual THHN/THWN-2 conductors through PVC or EMT conduit, provided the panel lugs and breakers are explicitly marked 75°C (standard on almost all modern Square D, Eaton, and Siemens load centers). This is your column for subpanel feeders and large appliance circuits.
  • The 90°C Column (Derating Only): This column is never used to determine the final breaker size in a residential setting. It is used exclusively as the starting baseline when you must apply derating factors for high ambient temperatures or bundling wires in a conduit.

Derating: When the Chart Values Drop

The ampacities in the chart assume two things: an ambient temperature of 30°C (86°F) and no more than three current-carrying conductors in a raceway. When you violate either condition, you must derate the wire using the 90°C column as your baseline, per Copper Development Association (CDA) guidelines and NEC 310.15.

Worked Example: Bundling in Conduit
You are pulling four current-carrying conductors (two 240V circuits sharing a neutral, or two multi-wire branch circuits) through a single 3/4-inch EMT conduit using 10 AWG THHN.

  1. Find Baseline: 10 AWG THHN at 90°C is 40A.
  2. Apply Adjustment Factor: NEC Table 310.15(C)(1) states that 4 to 6 current-carrying conductors require an 80% adjustment factor.
  3. Calculate Derated Ampacity: 40A × 0.80 = 32A.
  4. Select Breaker: The derated ampacity (32A) is now the maximum allowable current. Per NEC 240.4(B), you can round up to the next standard breaker size, which is 35A. However, because of the 240.4(D) small conductor rule mentioned above, 10 AWG is hard-capped at a 30A breaker. If you need a 35A or 40A circuit, you must upsize to 8 AWG THHN.

Decision Path: Sizing Your Breaker and Wire

Use this if-then decision tree to terminate your sizing process with a concrete part pick.

  • IF you are wiring standard 120V general lighting and receptacles in a living room or bedroom:
    • THEN use 14 AWG NM-B on a 15A single-pole breaker (or 12 AWG NM-B on a 20A breaker for future-proofing).
  • IF you are wiring kitchen countertop receptacles, bathroom GFCIs, or a dedicated microwave circuit:
    • THEN use 12 AWG NM-B on a 20A single-pole breaker.
  • IF you are wiring a standard electric dryer (240V, ~5000W heating element):
    • THEN use 10 AWG THHN in conduit (or 10/3 NM-B) on a 30A double-pole breaker.
  • IF you are wiring an electric range/oven or a 60A subpanel feeder:
    • THEN use 6 AWG THHN in conduit (or 4 AWG aluminum SER) on a 60A double-pole breaker. (Note: 6 AWG NM-B is only rated 55A and cannot be used on a 60A breaker; you must use THHN or bump to 4 AWG NM-B).

What This Chart Cannot Tell You (And the Default Fixes)

An ampacity chart only prevents the wire from melting. It does not account for continuous thermal stress or voltage starvation. Here is what the table hides, and the concrete rules to fix it.

1. Continuous Loads (The 125% Rule)
NEC 210.20 requires that if a load will run continuously for 3 hours or more (like an EV charger, a baseboard heater, or a hardwired server rack), the breaker and wire must be sized at 125% of the actual load. Fix: If your EV charger draws a continuous 32A, multiply by 1.25 to get 40A. You must use 8 AWG THHN and a 40A breaker, not a 30A setup.

2. Voltage Drop Over Distance
A 12 AWG wire on a 20A breaker is perfectly legal at 150 feet, but your power tools will bog down and smart home hubs will brownout due to a 5%+ voltage drop. The NEC treats voltage drop as an informational note (NEC 210.19(A) Exception), but best practice demands keeping it under 3% for branch circuits.

Default Recommendation: For any 120V branch circuit exceeding 75 feet, or any 240V feeder exceeding 150 feet, upsizing the wire by exactly one AWG step is the default fix. Keep the breaker sized to the original wire's ampacity to maintain proper short-circuit protection, but use the larger wire solely to mitigate voltage drop. For example, on a 100-foot run to a 20A garage receptacle, pull 10 AWG THHN but terminate it on a standard 20A breaker.