SAFETY WARNING: Working inside an electrical panel exposes you to lethal mains voltage. Always de-energize the panel, verify the bus bars are dead with a tested non-contact voltage meter and a multimeter, and wear appropriate PPE. Local codes may require a licensed electrician for panel work. NEC-style guidance provided here does not override your local Authority Having Jurisdiction (AHJ).

If you need the direct answer right now: for standard residential copper wiring, match 14 AWG to a 15A breaker, 12 AWG to a 20A breaker, 10 AWG to a 30A breaker, and 8 AWG to a 40A breaker. These pairings satisfy National Electrical Code (NEC) requirements for standard branch circuits using NM-B (Romex) cable. However, sizing wire isn't just about memorizing four numbers. The exact ampacity depends on your insulation type, termination ratings, and installation environment.

The Master Wire Size Circuit Breaker Chart (NEC Table 310.16)

This chart is derived directly from NEC Table 310.16 (formerly 310.15(B)(16)). It dictates the allowable ampacity for copper conductors based on their temperature rating.

Wire Size (AWG/kcmil) 60°C Column (NM-B / UF-B) 75°C Column (THWN / Terminations) 90°C Column (THHN / XHHW) Max Standard Breaker Size
14 AWG15A20A25A15A (NEC 240.4(D) limits)
12 AWG20A25A30A20A (NEC 240.4(D) limits)
10 AWG30A35A40A30A
8 AWG40A50A55A40A (or 50A if 75°C rated)
6 AWG55A65A75A60A (Next size up per 240.4(B))
4 AWG70A85A95A70A or 80A
3 AWG85A100A110A100A
2 AWG95A115A130A100A or 125A
1 AWG110A130A145A125A
1/0 AWG125A150A170A150A
2/0 AWG145A175A195A175A
3/0 AWG165A200A225A200A
4/0 AWG195A230A260A225A or 250A

How to read this table: The ampacity values represent the maximum continuous current the wire can handle before the insulation degrades. The 'Max Standard Breaker Size' column reflects standard breaker increments (15, 20, 30, 40, 50, 60, etc.) and incorporates NEC 240.4(B), which allows you to round up to the next standard breaker size if the exact ampacity doesn't match a standard breaker, provided the load isn't continuous and the wire is under 800A.

Quick-Jump Rows for the Most Common DIY Circuits

Bookmark this section for your most frequent trips to the hardware store. These assume standard residential copper wiring in a typical ambient temperature (86°F / 30°C).

  • 15-Amp Lighting/Receptacle Circuit: Use 14 AWG (NM-B) or 12 AWG. Breaker: 15A single-pole.
  • 20-Amp Kitchen/Bath/Garage Circuit: Use 12 AWG (NM-B). Breaker: 20A single-pole (often GFCI/AFCI).
  • 30-Amp Dryer / RV Outlet: Use 10 AWG (NM-B or THHN in conduit). Breaker: 30A double-pole.
  • 40-Amp Range / Welder: Use 8 AWG (NM-B) or 8 AWG THHN. Breaker: 40A double-pole.
  • 50-Amp Range / Hot Tub / Subpanel: Use 6 AWG (NM-B) or 6 AWG THHN. Breaker: 50A double-pole.
  • 100-Amp Subpanel Feeder: Use 3 AWG (THHN in conduit) or 2 AWG aluminum (SER cable). Breaker: 100A.

Which Temperature Column Actually Applies to Your Install?

The most common mistake DIYers make is looking at the 90°C column for THHN wire and assuming they can push 40A through 10 AWG. You cannot. The NEC requires you to use the lowest temperature rating of any component in the entire circuit.

In residential construction, standard circuit breakers and receptacles are typically rated for 75°C terminations, while older devices or specific lighting fixtures might only be rated for 60°C. Furthermore, NEC 110.14(C) strictly mandates that for circuits rated 100A or less, you must use the 60°C column unless the equipment is explicitly listed and marked for 75°C.

The Golden Rule: If you are pulling NM-B (Romex) cable, the cable itself is rated for 60°C. Even if you strip back the jacket and expose the 90°C THHN individual conductors inside the panel, the circuit's ampacity is permanently bottlenecked by the NM-B jacket's 60°C rating. Always use the 60°C column for NM-B.

How Derating Modifies Your Base Ampacity

The chart above assumes ideal conditions: an ambient temperature of 86°F (30°C) 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).

1. Bundling (Conduit Fill): If you pull wire through conduit and have 4 to 6 current-carrying conductors (e.g., two 120V circuits sharing a single conduit, meaning 2 hots and 2 neutrals), you must derate the 90°C ampacity to 80%. If you have 7 to 9 conductors, you derate to 70%. Note: Ground wires do not count as current-carrying.

2. Ambient Temperature: If your conduit runs through an attic in Texas where temperatures hit 113°F (45°C), you must apply a temperature correction factor. For 90°C wire at 113°F, the correction factor is 0.87.

Worked Example: You need to run a 30A circuit through an attic conduit with 4 current-carrying conductors using 10 AWG THHN (90°C base = 40A).
- Bundling derate (80%): 40A × 0.80 = 32A.
- Temp derate (0.87): 32A × 0.87 = 27.8A.
Result: 10 AWG is now undersized for a 30A breaker. You must step up to 8 AWG THHN.

Decision Path: Pick Your Exact Wire and Breaker

Use this decision matrix to lock in your final materials list. Follow the rows from top to bottom.

Condition / Scenario Action / Rule Final Concrete Pick
Standard 120V wall outlets & lights NEC 210.11 requires 20A for small appliance/bath; 15A general. 12 AWG NM-B on 20A (Best practice) or 14 AWG on 15A.
Continuous load (runs 3+ hours, e.g., EV charger, baseboard heat) NEC 210.20(A): Breaker must be rated 125% of continuous load. For a 32A EV charger: 32 × 1.25 = 40A. Use 8 AWG THHN on 40A.
Wire running in conduit with 4+ current-carrying wires Calculate derated ampacity using 90°C column, then size breaker. Upsize wire one step (e.g., use 8 AWG instead of 10 AWG for 30A).
100A Subpanel feeder in conduit Use 75°C column for THHN/XHHW terminations. 3 AWG Copper THHN or 1/0 AWG Aluminum on 100A breaker.

What This Chart Cannot Tell You (Voltage Drop & Local Code)

Ampacity charts only tell you the point at which the wire's insulation will melt or degrade. They do not account for voltage drop. If you are running a 12 AWG wire to a shed 150 feet away to power a 15A table saw, the wire won't melt, but the voltage at the saw might drop below 110V, causing the motor to overheat and burn out.

As a rule of thumb supported by industry experts like Mike Holt, you should limit voltage drop to 3% for branch circuits and 5% overall. If your one-way wire run exceeds 100 feet for a heavily loaded circuit, upsize your wire by one AWG step (e.g., use 10 AWG instead of 12 AWG for a 20A circuit) purely to maintain voltage, even if the breaker remains 20A.

Finally, never treat this chart as a substitute for your local building codes. Some municipalities have strict amendments—such as banning 14 AWG entirely and requiring 12 AWG minimum for all residential branch circuits, or requiring aluminum feeders to be upsized. Always pull a permit and consult your local inspector before closing up the panel.