When sizing wire for a circuit, guessing is how you start a fire. The definitive reference for wire ampacity in the United States is the AWG chart published in the National Electrical Code (NEC) as Table 310.16 (formerly 310.15(B)(16)). This chart dictates exactly how much continuous current a copper or aluminum conductor can carry before its insulation begins to thermally degrade.

Below is the complete, bookmark-friendly data table for copper conductors, followed by the exact rules for reading it, calculating derating, and a final decision tree to tell you exactly which wire to pull for your next project.

The Master AWG Chart (NEC Table 310.16)

How to read this table: This chart lists the allowable ampacities for insulated copper conductors rated 0 through 2000 volts. The three temperature columns (60°C, 75°C, and 90°C) correspond to the thermal rating of the wire's insulation. Common residential wires like NM-B (Romex) fall under the 60°C column limitations, while THHN/THWN-2 wires in conduit use the 75°C or 90°C columns. Source standard: NFPA 70 (NEC) Table 310.16.

AWG / kcmil 60°C (140°F)
NM-B, TW, UF		 75°C (167°F)
THW, THWN, XHHW		 90°C (194°F)
THHN, THWN-2, XHHW-2
14 AWG15A20A25A
12 AWG20A25A30A
10 AWG30A35A40A
8 AWG40A50A55A
6 AWG55A65A75A
4 AWG70A85A95A
3 AWG85A100A110A
2 AWG95A115A130A
1 AWG110A130A145A
1/0 AWG125A150A170A
2/0 AWG145A175A195A
3/0 AWG165A200A225A
4/0 AWG195A230A260A

For a deeper look into the physics of conductor resistance and international metric equivalents, refer to the comprehensive AWG to mm² conversion tables maintained by Electrical Technology, or consult the official NFPA 70 NEC codebook.

Which Temperature Column Actually Applies to Your Install?

The most common mistake DIYers and junior apprentices make is looking at the 90°C column because they bought THHN wire, and assuming they can push 90°C ampacities through the circuit. You cannot. The governing rule is the weakest link principle (NEC 110.14(C)).

The Termination Rule: Your circuit's allowable ampacity is limited by the lowest temperature rating of any connected device, termination, or conductor in the run.
  • NM-B (Romex): Even though the internal THHN wires are rated 90°C, NEC 334.80 legally restricts NM-B cable assemblies to the 60°C column due to the heat trapped inside the plastic outer jacket.
  • Standard Breakers & Receptacles: Most residential breakers and 15A/20A receptacles are rated for 75°C terminations. Therefore, if you pull individual THHN wires in conduit, you are generally capped at the 75°C column.
  • High-Temp Lugs: You can only use the 90°C column if every single lug, splice connector, and breaker terminal in the circuit is explicitly stamped "AL/CU 90°C".

Derating: How Bundling and Heat Shrink Your Ampacity

So, why does the 90°C column exist if we are capped at 75°C for terminations? The 90°C column is your starting point for derating calculations. When you bundle multiple current-carrying conductors in a single conduit, or run them through high-ambient-temperature attics, the wires heat each other up. The NEC requires you to reduce (derate) their ampacity.

Worked Numeric Example:
Imagine you are pulling eight 12 AWG THHN current-carrying conductors through a single EMT conduit to feed two 20A multi-wire branch circuits.

  1. Base Ampacity: Look at the 90°C column for 12 AWG. Base = 30A.
  2. Adjustment Factor: Per NEC Table 310.15(C)(1), 7 to 9 current-carrying conductors require an 80% adjustment factor (0.70 for 4-6 wires, 0.80 for 7-9).
  3. Derated Ampacity: 30A × 0.80 = 24A.
  4. Termination Check: The 75°C column for 12 AWG is 25A. Compare the derated value (24A) to the termination limit (25A). The lower number wins: 24A.
  5. Verdict: Since 24A is greater than your 20A breaker, 12 AWG THHN is perfectly legal. If you had 10 wires (50% factor), the math would yield 15A, forcing you to upsize to 10 AWG.

What This AWG Chart Cannot Tell You

Table 310.16 is the law for thermal limits, but it is not a complete engineering tool. Before you cut your wire, you must account for three blind spots:

  • Voltage Drop: The AWG chart does not calculate voltage drop over distance. A 14 AWG wire can legally carry 15A forever on a 50-foot run, but on a 150-foot run to a shed, the resistance (found in NEC Chapter 9, Table 8) will cause the voltage at the receptacle to drop below the 114V minimum, starving motors and tripping tool breakers. For runs over 100 feet, always upsize one or two AWG steps.
  • Physical Lug Fit: A 4/0 AWG copper wire has an outside diameter of roughly 0.52 inches. It will physically not fit into the set-screw lug of a standard 100A main breaker. If your math calls for a massive wire, check the manufacturer's lug data sheet; you may need a mechanical reducer or a specific breaker model with oversized lugs.
  • Short-Circuit Withstand: The chart assumes steady-state continuous load. Under a massive short-circuit fault, a 14 AWG wire can literally vaporize before a standard thermal-magnetic breaker trips. This physical limitation is why NEC 240.4(D) hard-caps 14 AWG at 15A, 12 AWG at 20A, and 10 AWG at 30A, regardless of what the higher temperature columns say.

Decision Tree: Pick Your Exact Wire Size Right Now

Stop second-guessing. Use this decision matrix to select your exact wire size for standard residential applications. These picks assume standard 75°C terminations, copper wire, and runs under 100 feet (no voltage drop upsize required).

Application Breaker / Load If Using NM-B (Romex) If Using THHN in Conduit Default Concrete Pick
Standard 120V Receptacles & Lighting 15A or 20A 14 AWG (15A)
12 AWG (20A)		 14 AWG (15A)
12 AWG (20A)		 12 AWG NM-B (Future-proofs for 20A upgrades and minimizes voltage drop on long runs).
Electric Dryer (Standard) 30A 10 AWG 10 AWG 10 AWG NM-B (or 10/3 with ground).
Electric Range / Oven / Spa 50A 6 AWG 6 AWG 6 AWG NM-B (or 6 AWG THHN in 3/4" conduit).
100A Subpanel Feeder 100A 3 AWG (Copper)
1 AWG (Aluminum)		 3 AWG (Copper)
1 AWG (Aluminum)		 1 AWG XHHW Aluminum (Copper is cost-prohibitive for feeders; 1 AWG Al is exactly 100A at 75°C).
200A Main Service Entrance 200A 3/0 AWG (Copper)
4/0 AWG (Aluminum)		 3/0 AWG (Copper)
4/0 AWG (Aluminum)		 4/0 AWG XHHW Aluminum (The undisputed industry standard for 200A residential services).

Final Rule of Thumb: If your calculated load lands exactly on the boundary of a wire's ampacity (e.g., a 55A load on a 6 AWG wire rated for 55A at 60°C), NEC 240.4(B) generally allows you to round up to the next standard breaker size (60A), provided the wire is not a small conductor restricted by 240.4(D). However, for continuous loads (running 3 hours or more), you must multiply the load by 125% before consulting this chart. When in doubt, upsize one AWG step; copper is cheap compared to a melted termination lug.