For standard 120V/240V residential branch circuits using copper wire, the baseline rule is: 15A breaker = 14 AWG, 20A breaker = 12 AWG, 30A breaker = 10 AWG, 40A/50A breaker = 8 AWG, and 60A breaker = 6 AWG. However, relying on memory fails when you hit subpanel feeders, EV chargers, or long runs. To size a circuit correctly, you need the master breaker and wire size chart derived from NEC Table 310.16, paired with a strict understanding of temperature ratings and derating factors.

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

The following table is based on the NFPA 70 (National Electrical Code) Table 310.16 for copper conductors.

How to read this table: The ampacity columns represent the maximum continuous current the wire can handle before its insulation degrades. The "Standard Breaker" column assumes standard termination rules (NEC 110.14(C)), meaning circuits 100A or under default to the 60°C column unless the equipment is explicitly rated and marked for 75°C. The breaker size listed is the maximum standard overcurrent device permitted for that wire under baseline conditions.

Wire Size (AWG/kcmil) 60°C Copper (NM-B) 75°C Copper (THHN/THWN) 90°C Copper (Derating Base) Max Standard Breaker
14 AWG15A20A25A15A
12 AWG20A25A30A20A
10 AWG30A35A40A30A
8 AWG40A50A55A40A*
6 AWG55A65A75A60A
4 AWG70A85A95A70A
3 AWG85A100A110A100A
2 AWG95A115A130A100A
1 AWG110A130A145A125A
1/0 AWG125A150A170A150A
2/0 AWG145A175A195A175A
3/0 AWG165A200A225A200A
4/0 AWG195A230A260A200A

*Note: 8 AWG copper is rated 40A at 60°C. If your terminals are rated 75°C and you are using THHN in conduit, 8 AWG can be used on a 50A breaker. Always verify terminal markings.

Bookmark Quick-Jump Rows (Most Queried):
  • 20A Kitchen/Bath Receptacles: 12 AWG wire, 20A breaker.
  • 30A Dryer / RV Outlet: 10 AWG wire, 30A breaker.
  • 50A Range / EV Charger: 6 AWG wire (for 60°C/NM-B) or 8 AWG (if 75°C THHN in conduit), 50A breaker.
  • 100A Subpanel Feeder: 3 AWG copper or 1 AWG aluminum, 100A breaker.

Which Temperature Column Applies to Your Installation?

The most common mistake DIYers make is looking at the 90°C column because the wire jacket (like THHN) is printed with "90°C". You almost never use the 90°C column for final ampacity. Here is how to pick the right column based on NEC 110.14(C) termination rules:

  • The 60°C Column: Use this for all NM-B (Romex) cable, regardless of the fact that the internal wires are 90°C rated. You must also use this column for any circuit rated 100A or less, or wire sizes 14 AWG through 1 AWG, unless the breaker and equipment terminals are explicitly marked "75°C".
  • The 75°C Column: Use this when pulling individual THHN/THWN-2 conductors in conduit, provided both the breaker terminals and the load terminals (like a subpanel lug or HVAC disconnect) are rated for 75°C. Most modern Square D, Eaton, and Siemens breakers are 75°C rated.
  • The 90°C Column: This column is strictly used as the starting point for applying derating factors (ambient temperature or conduit fill). You calculate the derated ampacity using the 90°C column, but the final result cannot exceed the 60°C or 75°C termination limits.

Derating Factors: When the Base Chart Isn't Enough

The chart above assumes an ambient temperature of 30°C (86°F) and no more than three current-carrying conductors in a single raceway. When you bundle wires or run them through hot attics, the wire cannot dissipate heat, and you must reduce (derate) its ampacity.

Conduit Fill Derating (NEC Table 310.15(C)(1)):
If you have 4 to 6 current-carrying conductors in a conduit, multiply the 90°C ampacity by 80%. If you have 7 to 9 conductors, multiply by 70%.

Real-World Jobsite Example:
You are running a multi-wire branch circuit and a dedicated smart-home neutral return through a single 3/4" EMT conduit, resulting in 8 current-carrying 12 AWG THHN conductors.

  1. Base 90°C ampacity for 12 AWG = 30A.
  2. Derating factor for 7-9 conductors = 70%.
  3. 30A × 0.70 = 21A derated ampacity.
  4. Since 21A is greater than the 20A breaker protecting the circuit, 12 AWG is legally compliant. However, if you added two more wires (10 total, 50% derating), the ampacity drops to 15A, forcing you to upsize to 10 AWG wire to maintain a 20A circuit.

Decision Path: Sizing Your Next Circuit

Use this decision tree to terminate your sizing process with a single, concrete wire and breaker pick.

Step Condition / Question Action / Result
1 Calculate total expected load in Amps. Determine baseline amp requirement (e.g., 32A EV charger).
2 Is the load continuous (on for 3+ hours)? Yes: Multiply load by 1.25 (32A × 1.25 = 40A).
No: Keep baseline (32A).
3 What cable type are you using? NM-B (Romex): Use 60°C column.
THHN in Conduit: Use 75°C column.
4 Select wire that meets/exceeds Step 2 result. For 40A on 75°C column: 8 AWG Copper (rated 50A).
5 Select standard breaker size. Pick the standard size that protects the wire but handles the load. 50A Breaker (Standard size per NEC 240.6).

What This Chart Cannot Tell You (Edge Cases & Limits)

A breaker and wire size chart is a thermal limit guide, not a complete engineering tool. It will not warn you about the following critical failures:

  • Voltage Drop: The chart says 6 AWG copper is fine for a 60A subpanel. But if that subpanel is 180 feet away in a detached garage, pushing 60A through 6 AWG will result in a 7.5% voltage drop. Your 240V tools will see 222V, motors will overheat, and electronics will brownout. For runs over 100 feet, use a voltage drop calculator and typically upsize the wire by 1 to 2 AWG sizes to maintain a 3% maximum drop.
  • Aluminum vs. Copper: The chart above is for copper. If you are feeding a 200A main panel with SER cable (which is almost always aluminum to save money), you must use the aluminum columns. 4/0 AWG aluminum is rated for 180A at 75°C, which the NEC permits for a 200A residential service feeder via specific dwelling unit allowances (NEC 310.12).
  • Short-Circuit Interrupting Ratings: The chart sizes the wire for continuous thermal load. It does not tell you if your breaker has a high enough AIC (Ampere Interrupting Capacity) rating to safely stop a dead short. Standard residential breakers are 10kAIC; if your utility transformer is close to your meter, you may need 22kAIC breakers to prevent the panel from exploding during a fault.
The Default Recommendation: When in doubt on a standard residential branch circuit under 100A, default to the 60°C column and copper wire. It costs slightly more upfront than aluminum or pushing the 75°C limits, but it guarantees NEC compliance regardless of the terminal ratings inside your specific panel or receptacle. Always turn off the main breaker, verify dead with a non-contact voltage tester and a multimeter, and consult your local AHJ (Authority Having Jurisdiction) before energizing new feeder circuits.