If you are wiring a standard 120V or 240V residential branch circuit using copper wire, the baseline pairings are straightforward: 14 AWG for 15A, 12 AWG for 20A, 10 AWG for 30A, 8 AWG for 40A, and 6 AWG for 50A. These pairings assume standard conditions: copper conductors, an ambient temperature of 86°F (30°C) or less, and no more than three current-carrying conductors in a single conduit or cable.

Safety First: Working inside an electrical panel exposes you to lethal mains voltage. Always de-energize the main breaker, lock it out, and verify the bus bars are dead with a tested non-contact voltage tester and a multimeter before touching any terminals. NEC-style guidance is provided here for educational purposes; your local Authority Having Jurisdiction (AHJ) has final authority, and local code may require a licensed electrician for panel work.

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

To use this chart correctly, you must understand the temperature columns. Wire insulation can handle more heat than the breaker terminals it connects to. The 60°C column applies to NM-B (Romex) cable and circuits rated 100A or less where terminal ratings are unmarked. The 75°C column applies to THHN/THWN-2 wires in conduit connected to 75°C-rated terminals (common on modern breakers and subpanel lugs). The breaker sizes listed are the maximum standard overcurrent protection devices permitted by NEC 240.4, factoring in the strict limits for smaller wires under NEC 240.4(D).

Wire Size (AWG/kcmil) 60°C Column (NM-B / Romex) 75°C Column (THHN in Conduit) Max Standard Breaker Size
14 AWG 15A 20A 15A
12 AWG 20A 25A 20A
10 AWG 30A 35A 30A
8 AWG 40A 50A 40A
6 AWG 55A 65A 60A
4 AWG 70A 85A 70A
3 AWG 85A 100A 100A
2 AWG 95A 115A 125A
1/0 AWG 125A 150A 150A
Bookmark Quick-Jump (Most Queried DIY Circuits):
  • 15A Lighting/Receptacles: 14 AWG Copper (15A breaker)
  • 20A Kitchen/Bath/Garage: 12 AWG Copper (20A breaker)
  • 30A Dryer/RV Outlet: 10 AWG Copper (30A breaker)
  • 40A Oven/Welder: 8 AWG Copper (40A breaker)
  • 50A EV Charger/Range: 6 AWG Copper (50A breaker)

Which Column Applies and How Derating Modifies Base Values

A common mistake DIYers make is looking at the 75°C or 90°C ampacity of THHN wire and sizing the breaker to that higher number. According to NEC 110.14(C) termination rules, you are limited by the lowest temperature rating of any connected component. Since most residential breakers and receptacles are rated for 75°C, you cannot use the 90°C column for final breaker sizing, even if the wire insulation is rated for 90°C.

Furthermore, if you are pulling NM-B (Romex) through bored holes in studs, you are strictly bound to the 60°C column, regardless of the breaker's terminal rating. NM-B cable is manufactured with 90°C insulation, but the National Electrical Code mandates it be treated as 60°C for ampacity purposes due to historical installation practices and heat trapping in insulated walls.

Applying Derating Factors

The base values in the chart above assume optimal conditions. Real-world jobsites require derating, which modifies the base ampacity downward. Derating happens for two main reasons:

  1. Ambient Temperature: If you run conduit through an attic that reaches 110°F (43°C), you must multiply the wire's ampacity by 0.87 (per NEC Table 310.15(C)(1)).
  2. Conduit Fill (Current-Carrying Conductors): If you pull more than three current-carrying conductors in a single raceway, the wires heat each other up. Four to six conductors require an 80% derating factor; seven to nine require 70%.

Crucial Derating Rule: When calculating derating for THHN/THWN-2 in conduit, you apply the derating multiplier to the 90°C column value, not the 75°C value. You then compare that derated number to the termination limit (75°C) and use whichever is lower.

Worked Example: You are pulling 10 AWG THHN in a conduit with six current-carrying conductors (50% derating factor). The 90°C ampacity for 10 AWG is 40A. Multiplying 40A by 0.50 gives you a derated ampacity of 20A. Even though your breaker terminals are rated 75°C (which normally allows 35A for 10 AWG), the conduit fill limits the wire to 20A. You must protect this circuit with a 20A breaker, effectively losing the 30A capacity you expected from the base chart.

What This Chart Cannot Tell You: Voltage Drop and Long Runs

The wire size and breaker size chart dictates ampacity—the maximum current a wire can carry before its insulation degrades or it becomes a fire hazard. What it completely ignores is voltage drop.

Ampacity is a thermal limit; voltage drop is a performance limit. If you run a 50A EV charger using 6 AWG copper wire (the correct ampacity pairing from the chart), the wire will not melt. However, if that run is 150 feet long, the resistance of the 6 AWG wire will cause the voltage to drop by more than 5%. This results in slow charging, overheating motors, and premature failure of sensitive electronics. The National Fire Protection Association (NFPA) recommends a maximum 3% voltage drop for branch circuits and a combined 5% for feeder and branch circuits.

The Voltage Drop Upsize Rule:
For every 100 feet of one-way run beyond 50 feet, consider upsizing your wire by one AWG step to maintain a tight 3% voltage drop threshold.
  • 20A Circuit at 120V: 12 AWG is fine up to 60 feet. Upsize to 10 AWG for runs up to 100 feet.
  • 50A Circuit at 240V: 6 AWG is fine up to 110 feet. Upsize to 4 AWG for runs up to 140 feet, and 3 AWG for runs up to 175 feet.

Always calculate voltage drop based on the actual continuous load of the appliance, not the breaker size. A 50A breaker protecting a 40A continuous EV charger means you calculate voltage drop using 40A, which slightly extends your maximum distance before upsizing is required. When in doubt, or when running feeders to a detached garage or subpanel, use a dedicated voltage drop calculator and upsize the wire while keeping the breaker sized to the original load requirements.