If you need the numbers right now, here are the standard American wire gauge ampacity limits for copper wire in typical residential applications (based on the 60°C column for NM-B/Romex cable): 14 AWG = 15 Amps, 12 AWG = 20 Amps, 10 AWG = 30 Amps, 8 AWG = 40 Amps, and 6 AWG = 55 Amps. These are the hard limits for standard branch circuits before you must account for terminal temperature ratings, voltage drop, or conduit derating.

Safety & Code Caveat: This guide provides NEC-style guidance based on the National Electrical Code (NFPA 70). Your local Authority Having Jurisdiction (AHJ) or electrical inspector always has final authority. Always de-energize panels, lock out breakers, and verify dead with a tested multimeter before working on mains voltage.

The Master American Wire Gauge Ampacity Chart (NEC Table 310.16)

The following table reproduces the core values from NEC Table 310.16 (formerly 310.15(B)(16)) for copper conductors. This assumes an ambient temperature of 30°C (86°F) and not more than three current-carrying conductors in a raceway or cable.

AWG Size 60°C (140°F)
NM-B / Romex		 75°C (167°F)
THHN Terminations		 90°C (194°F)
Derating Baseline
14 AWG15A*20A*25A*
12 AWG20A*25A*30A*
10 AWG30A*35A*40A*
8 AWG40A50A55A
6 AWG55A65A75A
4 AWG70A85A95A
3 AWG85A100A110A
2 AWG95A115A130A
1 AWG110A130A145A
1/0 AWG125A150A170A
2/0 AWG145A175A195A
3/0 AWG165A200A225A
4/0 AWG195A230A260A

*Note on NEC 240.4(D): For small conductors (14, 12, and 10 AWG), the NEC strictly limits overcurrent protection to 15A, 20A, and 30A respectively, regardless of the higher ampacities listed in the 75°C or 90°C columns.

How to Read the Temperature Columns (60°C vs. 75°C vs. 90°C)

The most common mistake DIYers make is looking at the 90°C column because modern THHN wire is printed with '90°C' on the jacket. You cannot legally use that column for your final breaker sizing in most cases. Here is how to determine which column applies to your installation per manufacturer ampacity guidelines and NEC Article 110.14(C):

  • The 60°C Column: Use this if you are running NM-B (Romex) cable. NEC 334.80 explicitly limits NM-B ampacity to the 60°C column, even if the individual wires inside are rated 90°C. You also use this column if your breaker or receptacle terminals are marked '60°C' (common on older equipment) or if the terminal temperature is unmarked and the circuit is 100A or less.
  • The 75°C Column: Use this for THHN/THWN-2 wires pulled through conduit, provided the breaker lugs and termination devices (like receptacles or lugs) are rated for 75°C. Almost all modern residential breakers and panel lugs are rated 75°C.
  • The 90°C Column: You almost never use this column for your final ampacity. Its sole purpose is to serve as the baseline starting point for calculating derating factors (ambient temperature and conductor bundling) before you compare the result back to the 75°C or 60°C termination limits.

Derating Factors: When Base Ampacity Drops

The table above assumes ideal conditions: an ambient air temperature of 30°C (86°F) and no more than three current-carrying conductors bundled together. When conditions change, the wire cannot dissipate heat as efficiently, and you must derate the ampacity.

Bundling (More than 3 Current-Carrying Conductors)

If you pull four to six current-carrying conductors through a single conduit, you must multiply the 90°C column baseline by 80%. (Neutral conductors carrying only unbalanced current do not count; neutrals on a 3-phase wye circuit carrying harmonic loads do count).

Worked Example: You are pulling four 12 AWG THHN wires (two hots, one neutral, one ground) through a conduit to a multi-wire branch circuit. The ground does not count. You have three current-carrying conductors. No derating is required.

However, if you pull four current-carrying conductors (e.g., two 20A circuits sharing a conduit), you take the 12 AWG 90°C baseline (30A) and multiply by 0.80. 30A × 0.80 = 24A. Because 24A is still greater than the 20A breaker size, 12 AWG THHN is perfectly legal. If you had 8 current-carrying conductors (70% derating), 30A × 0.70 = 21A, which is still safe for a 20A breaker.

Ambient Temperature Corrections

If your conduit runs through an attic in a hot climate where ambient temperatures reach 50°C (122°F), you apply a correction factor. For 90°C wire at 50°C ambient, the correction factor is 0.82. A 10 AWG THHN wire (90°C baseline = 40A) derates to 32.8A. You then compare this to the 75°C termination limit (35A) and use the lower value (32.8A), which still safely supports a 30A breaker.

Decision Tree: Sizing Your Wire and Breaker

Use this decision path to terminate your wire sizing process with a concrete pick. Do not guess; follow the logic.

Installation Scenario Wire Type Column Used Concrete Pick (Wire & Breaker)
Standard 15A lighting/outlet branch circuit in walls NM-B (Romex) 60°C 14 AWG on a 15A breaker
Standard 20A kitchen/bath receptacle circuit NM-B (Romex) 60°C 12 AWG on a 20A breaker
30A electric dryer or RV outlet (short run < 50ft) NM-B (Romex) 60°C 10 AWG on a 30A breaker
50A hot tub or range (run in conduit) THHN/THWN-2 75°C 6 AWG Copper on a 50A breaker
100A subpanel feeder (run in conduit) THHN/THWN-2 75°C 3 AWG Copper (or 1/0 Aluminum) on a 100A breaker
200A main service entrance (conduit) THHN/THWN-2 75°C 2/0 AWG Copper (or 4/0 Aluminum) on a 200A breaker

What the Ampacity Table Cannot Tell You

The NEC ampacity tables assume the wire can handle the heat generated by the current. They do not account for voltage drop over distance. If you push 30 Amps through a 10 AWG wire for 200 feet to a detached garage, the wire won't melt (it's within ampacity limits), but the voltage at the far end will sag below 110V, causing motors to overheat and electronics to brown out.

The Voltage Drop Rule of Thumb: The NEC recommends (and many local codes mandate) a maximum 3% voltage drop on branch circuits and 5% total from the service entrance to the furthest outlet. For a 120V circuit, 3% is 3.6 Volts.

  • Scenario: 120V, 20A load, 150 feet one-way distance.
  • 12 AWG Copper: Yields a ~7.2V drop (6%). Fails.
  • 10 AWG Copper: Yields a ~4.5V drop (3.7%). Fails strict 3%.
  • 8 AWG Copper: Yields a ~2.8V drop (2.3%). Passes.

Default Recommendation: For any 120V branch circuit run exceeding 75 feet, or any 240V run exceeding 150 feet, step up one wire size from the minimum ampacity requirement purely to mitigate voltage drop. Always use a dedicated voltage drop calculator with your exact wire length, material (copper vs. aluminum), and expected continuous load before finalizing long feeder runs.